Why does the standard give two thicknesses, a minimum and a specified one?

Because they answer different questions. The MINIMUM is a property of the finished screed: below it the screed cannot do its job, and it has to be achieved at every point on the floor including the worst one. The SPECIFIED thickness is what goes on the drawing, and it has to be enough that the minimum is still met after the base turns out not to be flat. BS 8204-1's bonded figures show the arithmetic in the open: the minimum is 25 mm, the normal specification is 40 mm, and the tolerance is ±15 mm — and 40 minus 15 is 25 exactly. The nominal is the minimum plus an allowance for reality. That is why quoting a screed as "25 mm bonded" and then laying it at 25 mm nominal is wrong: half the floor ends up below the minimum, and the half that does is wherever the slab happens to be high.

Why add my base deviation on top instead of trusting the nominal?

Because the nominal's allowance is for a NORMAL base, and yours may not be one. The allowance built into a bonded screed's 40 mm nominal is 15 mm; if your slab is 25 mm out, that allowance is already spent and the screed goes below its minimum over the high spot. So this page takes the greater of the standard's nominal and the minimum plus your own measured deviation, and tells you which one governed. When the deviation governs, the cost is worth pausing on — it applies across the WHOLE floor, not just over the hump, so a slab a centimetre out can be a third more material on every square metre. That is usually the moment to price a morning with a floor grinder against the extra screed, because taking the high spot down is frequently the cheaper of the two and nothing in a quantity calculation will suggest it.

Why does a floating screed have to be so much thicker than a bonded one?

Because of what is underneath it. A bonded screed is glued to the slab and acts with it — the slab carries the load and the screed is a wearing and levelling layer, so 25 mm is enough. A floating screed sits on insulation, which is compressible by design, so it has to span between the points where a load pushes the insulation down. That makes it a slab, and a slab spanning something soft fails under concentrated load rather than under its own weight. A wheel, a rack leg or the foot of a heavy appliance drives force into a small area, and the screed either has the depth to spread it or it does not. That is also why BS 8204-1 splits floating screeds by duty category rather than giving one number: Category C at 65 mm is domestic foot traffic and furniture, while Categories A and B at 75 mm are medium and heavy duty, where the loads are not necessarily bigger overall but are far more concentrated.

What is curling, and why does thickness help?

Curling is the edges and corners of a screed lifting off what they were laid on, and it comes from drying unevenly. An unbonded or floating screed has a membrane or insulation underneath, so it can only dry upward: the top loses water and shrinks while the bottom stays wet and does not, and a layer that shrinks more on one face than the other bends. Depth resists it because a thicker section is stiffer against that bending, and because the moisture gradient through it is gentler. BS 8204-1 treats the risk as high enough that it recommends unbonded and floating levelling screeds be either reinforced across the joints or laid 100 mm or more thick — which is well above every minimum in the table, and worth knowing before a floor at its minimum is signed off as compliant. The symptom on a finished floor is drummy tiles near the edges of bays, cracked grout lines following the bay layout, and a hollow ring where a corner has lifted.

How long before a floor covering can go down?

Longer than most programmes allow, and it is a moisture question rather than a time question. The trade rule of thumb is about a day per millimetre for the first 50 mm and roughly half that rate beyond it, in still, heated conditions — so a 75 mm floating screed is around a hundred days, not a fortnight. Drying is slowed further by anything that stops evaporation: a membrane under an unbonded screed means it can only dry upward, cold weather slows it, and a sealed building with no air movement can effectively stop it. The only honest test before laying vinyl, wood or a resin is a hygrometer reading on the screed itself. Coverings that trap moisture fail by lifting, bubbling or debonding, and the screed is usually blamed for a fault that belongs to the programme.

Materials & Quantities

Floor Screed Thickness Calculator — Bonded, Unbonded and Floating

How thick a bonded, unbonded or floating sand and cement screed must be for its duty and a base out of level, with the sand, cement and bags it takes.

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SettingsSettings for this calculationUS

Market

Imperial · sales tax

Who is doing the work?

The standard allowance most suppliers and estimating guides assume for ordinary work.

Floor area

The area being screeded.

The finished floor area, room by room where the build-ups differ. A bathroom dropped to take a tray and a bedroom on the same slab are two areas with two thicknesses, and averaging them gives a screed that is too thin in one of them.

How the screed sits on the floor

Bonded to the slab, separated from it by a membrane, or floating on insulation.

The single biggest driver of thickness, and a decision about the build-up rather than a preference. Insulation under the screed forces a floating screed; a damp-proof membrane forces an unbonded one; only a screed laid directly onto a prepared, sound slab is bonded. For floating work the duty category then decides: Category C is domestic and light, A and B are medium and heavy.

How far the base is out of level

The difference between the highest and lowest point of the base.

Put a long straightedge or a laser across the floor and find the worst high spot, not the average. The minimum thickness has to be reached AT that high spot, so a deviation worse than the standard's built-in allowance drives the whole floor deeper. The standard's own bonded figures show the logic: a 40 mm nominal at ±15 mm tolerance is the 25 mm minimum with 15 mm of base allowance on top.

Waste allowance

Percentage added for overfill, spillage and part batches.

Ten per cent covers a normal floor laid in bays off a mixer on site. Raise it for small rooms, for pumped screed where the line has to be primed and blown through, and for any floor where the base deviation is uncertain, because the overrun on a screed is always thickness rather than area.

Screed thickness needed

2.95 in

Medium confidence

The base drives this one. BS 8204-1 asks for 2.56 in at every point for a screed spanning compressible insulation under foot traffic and furniture, but a base 0.39 in out needs 2.95 in to reach that minimum over the high spot. Edge insulation at every perimeter and upstand, or the screed bridges to the wall and cracks along it. Price grinding the high spot down against screeding up to it across the whole floor. At this depth the standard's curling guidance applies: reinforce across the joints, or go to 100 mm (about 4 in).

Minimum at any point: 2.56 in
Minimum plus this base's deviation: 2.95 in
Screed volume after waste: 175.71 ft³
Cement by weight: 4,387.8 lb
94 lb cement sacks: 47 sacks
Sharp sand by weight: 8.78 tons
Rough drying time before a covering: 99.81 days

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How this was calculated

Formula source(s)

BS 8204-1 gives a minimum thickness of 25 mm for a bonded levelling screed and 50 mm for an unbonded one, and splits floating screeds by duty category: 65 mm for light duty and domestic work (Category C), and 75 mm for medium and heavy duty (Categories A and B)
The standard distinguishes that MINIMUM from a SPECIFIED thickness for the drawing: a bonded screed is normally specified at 40 mm with a tolerance of ±15 mm, and an unbonded screed at a specified minimum of 70 mm. The ±15 mm on a 40 mm bonded screed is exactly the 25 mm minimum plus an allowance for the base not being flat, which is why this page takes the greater of the standard's nominal and the minimum plus your own measured deviation
BS 8204-1 flags a high risk of curling in unbonded and floating levelling screeds, and recommends they be either reinforced across the joints or made 100 mm or more thick
For heavy commercial traffic, or point loads above about 3.0 kN/m², published guidance on the standard adds a further 10 mm to 15 mm to a floating screed's depth over the Category A and B figure
Drying is the trade rule of thumb — roughly a day per millimetre for the first 50 mm and slower beyond it, in still, heated conditions — rather than a standard, and only a hygrometer reading on the floor settles it
Mineral Products Association, MPA Mortar Data Sheet 22, Screeds (Issue 3, October 2023), reporting BS 8204-1: nominal proportions by weight of cement to fine aggregate of 1:4, with a lower limit of 1:3 and an upper limit of 1:4.5 — the 1:4 by weight this page works the cement and sand at; a specification that names a ratio governs over it
Sika, SikaScreed-40 Binder product data sheet (Version 04.01, February 2026): 400 kg of binder per m³ of fresh mortar at 1:4 with the sand by weight (4 bags of 20 kg to about 320 kg of aggregate in a 200 litre mixer), 2,000 kg of material as batched in each cubic metre — the figure this page and the floor screed page both turn a volume of screed into cement and sand with
QUIKRETE Portland Cement (No. 1124) data sheet (revised August 2022): sold in 94 lb (42.6 kg) bags, among other sizes — the US sack this page counts

Inputs used

Floor area: 650 sq ft
How the screed sits on the floor: Floating on insulation — light duty, Category C (domestic)
How far the base is out of level: 0.39 in
Waste allowance: 10

Intermediate steps

Minimum at any point: 2.56 in
Minimum plus this base's deviation: 2.95 in
Screed volume after waste: 175.71 ft³
Cement by weight: 4,387.8 lb
94 lb cement sacks: 47 sacks
Sharp sand by weight: 8.78 tons
Rough drying time before a covering: 99.81 days

Final result2.95 in

Confidence note: The base drives this one. BS 8204-1 asks for 2.56 in at every point for a screed spanning compressible insulation under foot traffic and furniture, but a base 0.39 in out needs 2.95 in to reach that minimum over the high spot. Edge insulation at every perimeter and upstand, or the screed bridges to the wall and cracks along it. Price grinding the high spot down against screeding up to it across the whole floor. At this depth the standard's curling guidance applies: reinforce across the joints, or go to 100 mm (about 4 in).

What this calculation does not cover

These are the minimum and specified thicknesses for a sand and cement levelling screed, not a structural design. A screed carrying vehicles, plant or storage racking is designed against its loads, and a figure from a table is the wrong instrument for it.
Thickness cannot rescue a bad bond. A bonded screed depends entirely on preparation — laitance removed by scabbling or shot-blasting, the base sound and free of dust, a bonding coat still wet when the screed goes on — and a debonded screed sounds hollow, curls at the edges and breaks up regardless of how thick it was laid.
BS 8204-1 flags a high risk of CURLING in unbonded and floating screeds and recommends they be reinforced across the joints or laid 100 mm (about 4 in) or more thick. Nothing here places that reinforcement, and a screed at its minimum thickness is not at the depth the standard considers safe from curl on its own.
Heavy commercial traffic, or point loads above about 3.0 kN/m², call for a further 10 to 15 mm over the Category A and B figure. That uplift is not applied here, because whether a floor reaches that threshold is a loading question rather than a build-up one.
Heated screeds are not covered. They carry a second governing rule, a minimum cover over the crown of the pipe, and the pipe displaces screed as well; the heated screed page handles both.
Nothing here places movement joints, and a floating screed needs them. Edge insulation at every perimeter and upstand, a joint at every doorway, and bays sized to the standard are what stop a correctly thick screed cracking anyway.
The drying estimate is a trade rule of thumb in still, heated conditions, not a measurement. Floor coverings fail on residual moisture, and the only honest answer before laying one is a hygrometer reading taken on the floor itself.
The base deviation is taken as a single worst figure across the whole area. A floor with one bad corner is cheaper to grind than to screed to, and this page will not tell you that — compare the extra depth across every square metre against the cost of taking the high spot down.

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This result is a specification — 2.95 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

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Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-10-05 · v1.1.0

Regulatory standards & verification citations8

BS 8204-1 gives a minimum thickness of 25 mm for a bonded levelling screed and 50 mm for an unbonded one, and splits floating screeds by duty category: 65 mm for light duty and domestic work (Category C), and 75 mm for medium and heavy duty (Categories A and B)
The standard distinguishes that MINIMUM from a SPECIFIED thickness for the drawing: a bonded screed is normally specified at 40 mm with a tolerance of ±15 mm, and an unbonded screed at a specified minimum of 70 mm. The ±15 mm on a 40 mm bonded screed is exactly the 25 mm minimum plus an allowance for the base not being flat, which is why this page takes the greater of the standard's nominal and the minimum plus your own measured deviation
BS 8204-1 flags a high risk of curling in unbonded and floating levelling screeds, and recommends they be either reinforced across the joints or made 100 mm or more thick
For heavy commercial traffic, or point loads above about 3.0 kN/m², published guidance on the standard adds a further 10 mm to 15 mm to a floating screed's depth over the Category A and B figure
Drying is the trade rule of thumb — roughly a day per millimetre for the first 50 mm and slower beyond it, in still, heated conditions — rather than a standard, and only a hygrometer reading on the floor settles it
Mineral Products Association, MPA Mortar Data Sheet 22, Screeds (Issue 3, October 2023), reporting BS 8204-1: nominal proportions by weight of cement to fine aggregate of 1:4, with a lower limit of 1:3 and an upper limit of 1:4.5 — the 1:4 by weight this page works the cement and sand at; a specification that names a ratio governs over it
Sika, SikaScreed-40 Binder product data sheet (Version 04.01, February 2026): 400 kg of binder per m³ of fresh mortar at 1:4 with the sand by weight (4 bags of 20 kg to about 320 kg of aggregate in a 200 litre mixer), 2,000 kg of material as batched in each cubic metre — the figure this page and the floor screed page both turn a volume of screed into cement and sand with
QUIKRETE Portland Cement (No. 1124) data sheet (revised August 2022): sold in 94 lb (42.6 kg) bags, among other sizes — the US sack this page counts

Cite this page

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Tools and safety for this job

To lay a floor screed. Generic types, no brands, no prices.

Protection this work requires

Cutting or grinding concrete, masonry, screed, tile or fibre-cement board releases respirable silica: cut wet or extract at the tool, and wear a P2/N95 respirator at minimum — a nuisance dust mask does not filter it.
Wet cement and lime burn skin and eyes painlessly until the damage is done: waterproof gloves to EN 374, safety glasses whenever the mix can splash, and never kneel in wet mix in permeable trousers.
Boards, blocks and bagged material cause most lasting back injuries on small sites: two people or a lifter for full sheets, and never a bag on one shoulder up a ladder.

Show the 6 tools this job needs

Essential

Mixing bucket or tub
Screed bar or rail
Shovel and spade
Spirit level

Recommended

Moisture meter
Paddle mixer

what each flooring tool is for, and the spec that decides which to buy where one does.

The data behind it: Production rates by operation

How to calculate floor screed thickness — bonded, unbonded and floating in 5 steps

Floor areaThe area being screeded.
How the screed sits on the floorBonded to the slab, separated from it by a membrane, or floating on insulation.
How far the base is out of levelThe difference between the highest and lowest point of the base.
Waste allowancePercentage added for overfill, spillage and part batches.
Screed thickness neededThe tool computes the screed thickness needed from those figures and shows the formula, its sources, and a confidence rating alongside it.

Frequently asked questions

Why does the standard give two thicknesses, a minimum and a specified one?: Because they answer different questions. The MINIMUM is a property of the finished screed: below it the screed cannot do its job, and it has to be achieved at every point on the floor including the worst one. The SPECIFIED thickness is what goes on the drawing, and it has to be enough that the minimum is still met after the base turns out not to be flat. BS 8204-1's bonded figures show the arithmetic in the open: the minimum is 25 mm, the normal specification is 40 mm, and the tolerance is ±15 mm — and 40 minus 15 is 25 exactly. The nominal is the minimum plus an allowance for reality. That is why quoting a screed as "25 mm bonded" and then laying it at 25 mm nominal is wrong: half the floor ends up below the minimum, and the half that does is wherever the slab happens to be high.
Why add my base deviation on top instead of trusting the nominal?: Because the nominal's allowance is for a NORMAL base, and yours may not be one. The allowance built into a bonded screed's 40 mm nominal is 15 mm; if your slab is 25 mm out, that allowance is already spent and the screed goes below its minimum over the high spot. So this page takes the greater of the standard's nominal and the minimum plus your own measured deviation, and tells you which one governed. When the deviation governs, the cost is worth pausing on — it applies across the WHOLE floor, not just over the hump, so a slab a centimetre out can be a third more material on every square metre. That is usually the moment to price a morning with a floor grinder against the extra screed, because taking the high spot down is frequently the cheaper of the two and nothing in a quantity calculation will suggest it.
Why does a floating screed have to be so much thicker than a bonded one?: Because of what is underneath it. A bonded screed is glued to the slab and acts with it — the slab carries the load and the screed is a wearing and levelling layer, so 25 mm is enough. A floating screed sits on insulation, which is compressible by design, so it has to span between the points where a load pushes the insulation down. That makes it a slab, and a slab spanning something soft fails under concentrated load rather than under its own weight. A wheel, a rack leg or the foot of a heavy appliance drives force into a small area, and the screed either has the depth to spread it or it does not. That is also why BS 8204-1 splits floating screeds by duty category rather than giving one number: Category C at 65 mm is domestic foot traffic and furniture, while Categories A and B at 75 mm are medium and heavy duty, where the loads are not necessarily bigger overall but are far more concentrated.
What is curling, and why does thickness help?: Curling is the edges and corners of a screed lifting off what they were laid on, and it comes from drying unevenly. An unbonded or floating screed has a membrane or insulation underneath, so it can only dry upward: the top loses water and shrinks while the bottom stays wet and does not, and a layer that shrinks more on one face than the other bends. Depth resists it because a thicker section is stiffer against that bending, and because the moisture gradient through it is gentler. BS 8204-1 treats the risk as high enough that it recommends unbonded and floating levelling screeds be either reinforced across the joints or laid 100 mm or more thick — which is well above every minimum in the table, and worth knowing before a floor at its minimum is signed off as compliant. The symptom on a finished floor is drummy tiles near the edges of bays, cracked grout lines following the bay layout, and a hollow ring where a corner has lifted.
How long before a floor covering can go down?: Longer than most programmes allow, and it is a moisture question rather than a time question. The trade rule of thumb is about a day per millimetre for the first 50 mm and roughly half that rate beyond it, in still, heated conditions — so a 75 mm floating screed is around a hundred days, not a fortnight. Drying is slowed further by anything that stops evaporation: a membrane under an unbonded screed means it can only dry upward, cold weather slows it, and a sealed building with no air movement can effectively stop it. The only honest test before laying vinyl, wood or a resin is a hygrometer reading on the screed itself. Coverings that trap moisture fail by lifting, bubbling or debonding, and the screed is usually blamed for a fault that belongs to the programme.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.