Why would tins of the same colour not match?

Because a colour is manufactured to a tolerance, not to an identity. Tinted paint is a base with measured amounts of colourant dispensed into it, and every step of that has a permitted variation: the base itself varies slightly between its own production batches, the dispensing equipment has a tolerance, and the machine in a shop is calibrated periodically rather than continuously. The result is matched closely enough that nobody can tell two tins apart by looking at them, in the tin or on a stirrer or on a card — which is exactly the problem, because the one place the difference does become visible is a large flat area in raking light, as a soft vertical step where one tin ran out and the next began. The step is not a defect anybody will accept responsibility for, because each tin is within tolerance. Boxing sidesteps the argument entirely by making sure no wall is ever painted from two different tins.

What do I do when it will not all fit in one container?

Two things, and the first is the one people get wrong. Distribute the tins between the containers rather than filling them in sequence: a tin from each batch code into container one, a tin from each into container two, and so on, like dealing cards. Filling the first container from the first tins to hand and the second from what is left leaves the two containers as different from each other as the tins were, which does not solve the problem — it just relocates the step from the middle of the job to a place you have not chosen. Dealing them out makes every container a near-identical average of the whole supply. The second thing is to decide in advance where the changeover falls, because there will be one. An external corner, a door casing, a ceiling line or a change of plane all hide a small colour shift completely, since the light striking the two surfaces differs far more than the paint does. Halfway up a continuous wall hides nothing. Work out which wall each container will finish on before you open anything.

Is the leftover in the container better touch-up paint than a spare tin?

Considerably better, and it is the quiet reason boxing pays for itself long after the job. What is left in the container is, by construction, the exact material that went on the wall — the same average of the same batches, mixed the same way. An unopened spare tin is not: it is one batch that never entered the blend, and it will differ from the boxed average by at least as much as the tins differed from each other. Touching up a scuff with it puts a small patch of a slightly different colour on the wall, which is more noticeable than the scuff was, because the eye finds an isolated patch far more readily than a gradual change. So decant the leftover into a clean, well-sealing container sized close to what is actually left — air in a part-full tin skins the surface and eventually ruins the rest — and label it with the colour, the product, the sheen, the room and the date. Then keep the spare tin sealed for the next full repaint, where it can go into a new blend.

How long should I stir it, and does anything else change?

Longer than looks necessary, and with the right motion. Paint reaches uniform appearance well before it reaches uniform composition — the colourant disperses through the base faster than the base itself circulates — so a blend that already looks even can still be layered, particularly in a tall container where the bottom third barely moves. The motion matters more than the duration: lift material from the bottom and fold it over rather than spinning the top, which is why a flat paddle beats a spiral one and why a drill paddle should be run slowly and moved around the vessel rather than held in the centre at speed. A high-speed vortex also drags air into the paint, and entrained air shows up as pinholes in the finished film. Two other things change once paint is boxed: it is now in a vessel that seals less well than the tins did, so it wants covering between sessions and using within days rather than months, and its viscosity may differ very slightly from any single tin, so if the first wall is being sprayed, set the gun on the blend rather than on a tin.

Renovation & Construction

Paint Boxing Calculator — Blending Tins Before You Start

How many containers it takes to blend your tins into one consistent colour, and where the changeover has to fall when they will not all fit in one.

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Every formula citedNamed published standards
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SettingsSettings for this calculationUS

Market

Imperial · sales tax

Number of tins

How many tins of this one colour the job will use.

One colour at a time. A job with walls in one shade and a ceiling in another is two separate boxing exercises with two separate containers, and the two must never meet. Count only the tins that will actually be opened for this colour — a spare kept back unopened is not part of the blend, and that is precisely why it will not match it later.

Size of each tin

The volume printed on the tin.

The common trade sizes are 5 and 10 litres in metric markets and 1 US gallon (3.8 L) or 5 US gallons (18.9 L) elsewhere. This assumes every tin is the same size and full; for a mix of sizes, or for tins already part-used, work out the real total and divide it by the number of tins to get an average to enter here.

Container capacity

How much the container holds filled to the brim.

The nominal size of the vessel you will blend in — a 20 litre pail, or the five-gallon bucket that is the same thing in US sizes. Larger tubs of 25 litres and up are common and hold a bigger job in one blend, which is always better than two. It must be clean, and it must be a material paint is happy in — a drum that held something else is the fastest way to ruin every tin at once.

Stirring headroom (%)

The share of the container left empty so it can be stirred.

A container filled to its rim cannot be mixed. A paddle on a drill sets up a vortex and throws paint out well before the rim, which is why 15 per cent is a reasonable default and 20 is safer for a powered stir; a careful hand stir with a flat paddle needs less. This is the input that most often decides the answer, because it is what the total is really divided by.

Number of different batch codes

How many distinct batch numbers are printed across the tins.

Read them off the labels before you start — it takes two minutes and it is the only thing that tells you whether boxing matters on this job. The code is usually a printed or stamped string near the barcode, distinct from the colour reference. Tins bought together are often, but by no means always, from one batch, and a top-up bought a fortnight later almost never is.

Blending containers needed

3 containers

High confidence

12 tins make 12.0 gal, which will not fit in one container, so it becomes 3 blends and the job carries 2 colour changes. Deal the tins out between the containers like cards, a tin from each of the 3 batch codes in turn, rather than filling the first container from the first tins to hand — pouring them in sequence leaves the containers as different from each other as the tins were and simply moves the step. Then put every changeover on a corner, a casing or a ceiling line.

Paint to blend in total: 12 gal
Usable capacity per container: 4.25 gal
Paint in each container: 4 gal
Tins into each container: 4 tins
Colour changes on the finished job: 2 changes

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

Paint is tinted in BATCHES and matched to a tolerance rather than to an identity, so tins of one colour can differ slightly. The difference is invisible in the tin and shows up once, as a soft step on the wall where one tin ran out — which is what boxing (combining every tin before starting) exists to prevent
A container's stated size is not its stirrable capacity. Mixing needs headroom above the paint, and more of it for a paddle on a drill than for a hand stir, so the usable volume is the nominal volume less that allowance
Where the paint will not fit in one container, the tins are distributed between containers rather than poured in sequence, and each changeover is placed at a corner, a door casing or a ceiling line — a step that is invisible across an external corner is obvious on a continuous wall

Inputs used

Number of tins: 12
Size of each tin: 1 gal
Container capacity: 5 gal
Stirring headroom (%): 15
Number of different batch codes: 3

Intermediate steps

Paint to blend in total: 12 gal
Usable capacity per container: 4.25 gal
Paint in each container: 4 gal
Tins into each container: 4 tins
Colour changes on the finished job: 2 changes

Final result3 containers

Confidence note: 12 tins make 12.0 gal, which will not fit in one container, so it becomes 3 blends and the job carries 2 colour changes. Deal the tins out between the containers like cards, a tin from each of the 3 batch codes in turn, rather than filling the first container from the first tins to hand — pouring them in sequence leaves the containers as different from each other as the tins were and simply moves the step. Then put every changeover on a corner, a casing or a ceiling line.

What this calculation does not cover

Boxing evens out the difference between tins; it cannot tell you what colour those tins are. This site works in quantities and ratios rather than colour, so if the shade itself is the question the answer is a fan deck and a sample pot held against the wall, not a calculator.
This is for single-pack paint only. A two-pack must NEVER be boxed: its two parts begin curing the moment they meet, so a blended container of it has a working life measured in minutes and then becomes a solid. Two-pack coatings are mixed in batches sized to the pot life instead, which is a different calculation with a different page.
Never blend different products, different sheens or different bases together, even where the colour reference is identical. A matt and a mid-sheen of the same shade are two products, and the result of combining them is neither — it is a film with unpredictable gloss, unpredictable durability and no manufacturer's position on how it should behave.
Boxing concentrates the job's risk in one vessel. It is genuinely the point of the technique that every tin ends up in the same container, and the corollary is that one dirty bucket, one piece of grit or one splash of the wrong thing now spoils all of it rather than a fifth of it. Strain the blend if the container's history is at all uncertain.
Boxing also concentrates the job's solvent release into a few minutes. A dozen tins opened together and stirred in an open container put far more vapour into a room than brushing the same paint out over a day would, and it happens before anyone has thought about ventilation. With solvent-borne paint, box in a through-draught or outdoors rather than in the room you are about to paint, and keep ignition sources away from an open container while it is being stirred. Water-based paint is a much smaller problem, though not a zero one.
Blending averages the variation between tins; it cannot rescue a tin that is actually wrong. A mis-tint is a different failure from batch drift — it is outside the tolerance rather than inside it — and stirring it into the rest pulls the whole job off colour instead of hiding one tin. Anything visibly different from its neighbours goes back to the merchant unopened.
The arithmetic assumes every tin is the same size and full. Part-used tins, a mix of sizes, and the film left clinging to the inside of each emptied tin all move the real total, and the last of those is not negligible across a dozen tins — allow for it rather than working to the nominal figure to the last decimal.
Once the paint is in the container, the labelling on the tins no longer describes what you are painting with. Write the colour, the product, the sheen and the batch codes on the container before you start, and transfer the same details to whatever the leftover is decanted into.

Estimated cost — your price

Price per container ($)

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

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 2026-09-15 · v1.0.0

Regulatory standards & verification citations3

Paint is tinted in BATCHES and matched to a tolerance rather than to an identity, so tins of one colour can differ slightly. The difference is invisible in the tin and shows up once, as a soft step on the wall where one tin ran out — which is what boxing (combining every tin before starting) exists to prevent
A container's stated size is not its stirrable capacity. Mixing needs headroom above the paint, and more of it for a paddle on a drill than for a hand stir, so the usable volume is the nominal volume less that allowance
Where the paint will not fit in one container, the tins are distributed between containers rather than poured in sequence, and each changeover is placed at a corner, a door casing or a ceiling line — a step that is invisible across an external corner is obvious on a continuous wall

Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Protection this work requires

Stop and check first. Paint under later coats in older housing can be lead-based, and sanding or heat-stripping it produces lead dust that is especially harmful to children and to anyone pregnant. Test before you disturb it — this is a job for a tested, controlled method, not a better mask.

Tools and safety for this job

To roll and cut in two coats and prepare a surface for paint. Generic types, no brands, no prices.

Joint-compound dust is very fine and travels: a P1/FFP1 mask is the minimum, and a damp-sponge finish avoids most of it.
A dust mask does nothing about solvent vapour: ventilate through, and use an organic-vapour (A1) cartridge respirator in an enclosed room.
Most fatal falls on small jobs are from under three metres: use a tower or a ladder tied at the top, and never work off the top two rungs.

Show the 11 tools this job needs

Essential

Brushes
Dust sheets and masking
Filling knife
Roller frame, sleeves and tray
Scraper

Recommended

Caulking gun
Extension pole
Pole sander
Random orbital sander
Step ladder

Optional

Dust extractor or vacuum with a fine filter

Also needed as materials: abrasive discs, caulk and sealant cartridges, masking tape, plastic sheeting, roller sleeves, sanding screens or paper.

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

Worked example: Bedroom repaint — step 7 of 7

The data behind it: Production rates by operation

How to calculate paint boxing — blending tins before you start in 6 steps

Number of tinsHow many tins of this one colour the job will use.
Size of each tinThe volume printed on the tin.
Container capacityHow much the container holds filled to the brim.
Stirring headroom (%)The share of the container left empty so it can be stirred.
Number of different batch codesHow many distinct batch numbers are printed across the tins.
Blending containers neededThe tool computes the blending containers needed from those figures and shows the formula, its sources, and a confidence rating alongside it.

Blending containers needed by size of each tin

Page defaults, not your figures above.

Size of each tin	Blending containers needed (containers)
1 gal	3
1.5 gal	5
2 gal	6
2.5 gal	7

Frequently asked questions

Why would tins of the same colour not match?: Because a colour is manufactured to a tolerance, not to an identity. Tinted paint is a base with measured amounts of colourant dispensed into it, and every step of that has a permitted variation: the base itself varies slightly between its own production batches, the dispensing equipment has a tolerance, and the machine in a shop is calibrated periodically rather than continuously. The result is matched closely enough that nobody can tell two tins apart by looking at them, in the tin or on a stirrer or on a card — which is exactly the problem, because the one place the difference does become visible is a large flat area in raking light, as a soft vertical step where one tin ran out and the next began. The step is not a defect anybody will accept responsibility for, because each tin is within tolerance. Boxing sidesteps the argument entirely by making sure no wall is ever painted from two different tins.
What do I do when it will not all fit in one container?: Two things, and the first is the one people get wrong. Distribute the tins between the containers rather than filling them in sequence: a tin from each batch code into container one, a tin from each into container two, and so on, like dealing cards. Filling the first container from the first tins to hand and the second from what is left leaves the two containers as different from each other as the tins were, which does not solve the problem — it just relocates the step from the middle of the job to a place you have not chosen. Dealing them out makes every container a near-identical average of the whole supply. The second thing is to decide in advance where the changeover falls, because there will be one. An external corner, a door casing, a ceiling line or a change of plane all hide a small colour shift completely, since the light striking the two surfaces differs far more than the paint does. Halfway up a continuous wall hides nothing. Work out which wall each container will finish on before you open anything.
Is the leftover in the container better touch-up paint than a spare tin?: Considerably better, and it is the quiet reason boxing pays for itself long after the job. What is left in the container is, by construction, the exact material that went on the wall — the same average of the same batches, mixed the same way. An unopened spare tin is not: it is one batch that never entered the blend, and it will differ from the boxed average by at least as much as the tins differed from each other. Touching up a scuff with it puts a small patch of a slightly different colour on the wall, which is more noticeable than the scuff was, because the eye finds an isolated patch far more readily than a gradual change. So decant the leftover into a clean, well-sealing container sized close to what is actually left — air in a part-full tin skins the surface and eventually ruins the rest — and label it with the colour, the product, the sheen, the room and the date. Then keep the spare tin sealed for the next full repaint, where it can go into a new blend.
How long should I stir it, and does anything else change?: Longer than looks necessary, and with the right motion. Paint reaches uniform appearance well before it reaches uniform composition — the colourant disperses through the base faster than the base itself circulates — so a blend that already looks even can still be layered, particularly in a tall container where the bottom third barely moves. The motion matters more than the duration: lift material from the bottom and fold it over rather than spinning the top, which is why a flat paddle beats a spiral one and why a drill paddle should be run slowly and moved around the vessel rather than held in the centre at speed. A high-speed vortex also drags air into the paint, and entrained air shows up as pinholes in the finished film. Two other things change once paint is boxed: it is now in a vessel that seals less well than the tins did, so it wants covering between sessions and using within days rather than months, and its viscosity may differ very slightly from any single tin, so if the first wall is being sprayed, set the gun on the blend rather than on a tin.

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.