The job
Modelled, not recorded. Every figure here is the answer the Concrete Calculator gives for these inputs, written down in both unit systems and re-run whenever the site is built. No job was carried out, so there is no measured overrun on this page.
The job is a plain rectangular pad: 20 ft (6.1 m) long, 30 ft (9.14 m) wide, poured 4 in (10.16 cm) thick. That is a workshop floor, a generous patio or the base for a large shed, and it is a good size to learn the method on because the arithmetic is long enough to need a method and short enough to check on the back of a delivery note.
Thickness is the number to defend. The calculator's own guidance puts a patio, a walkway or a shed floor at the thinner end and a driveway that will carry vehicles at the thicker end, so a pad that stays under foot traffic is entered at 4 in and the thicker figure is kept for the section on changing the job, below. Whether a slab that carries a load needs more than that, and what reinforcement it wants, is a question for the local code and whoever designs it; this page counts the concrete and sizes nothing.
The allowance is left at the calculator's own figure. Concrete is the one material on a job that cannot be topped up an hour later, so the usual error is to order the bare volume and spend the pour hoping the base is flat. A base that has been dug and compacted by hand is never exactly the depth on the drawing.
The inputs, exactly as entered
| Input | Value | Why |
|---|---|---|
| Slab Length | 20 ft (6.1 m) | The longer side, measured between the formwork faces rather than the stakes outside them. |
| Slab Width | 30 ft (9.14 m) | Taken the same way, square to the length; check it at both ends, because a form that has crept out by a hand's width is a few more cubic feet. |
| Slab Thickness | 4 in (10.16 cm) | Entered in inches and held by the calculator as a length, so it can sit beside the feet without a conversion in the arithmetic. |
| Waste Factor (%) | 10 | The calculator's own default: the one allowance on this job that is a percentage of the whole volume rather than a count. |
The working, step by step
1. Volume (no waste)
7.41 yd³ (5.66 m³)
The three dimensions multiply: length, then width, then thickness, each held in the same unit first. The result, 7.41 yd³, is the slab exactly as drawn and nothing more. It is the number a drawing gives you, and the one to write on a sketch before anything is added to it.
2. Volume with waste factor
8.15 yd³ (6.23 m³)
The allowance is a share of that volume, added on top: the 10 per cent turns the slab as drawn into 8.15 yd³. This is the figure to order against, and the reason the headline is larger than the volume of the hole.
3. Cubic feet
220 ft³
The same volume with the allowance, restated as 220 ft³. It is the unit a bag's yield is quoted in, and the easiest of the three units to picture against a pile of bags on a pallet.
4. 80 lb bags needed
367 bags
The last row divides that volume by what one standard bag yields once mixed, as given in the calculator's second source, and rounds up to a whole bag: 367 bags. Nobody buys a fraction of a bag, so the round-up is the only place the answer is not a straight multiplication.
Estimated concrete needed
8.148 yd³
6.23 m³
The headline is the quantity to ask a ready-mix supplier for: 8.148 yd³, or 6.23 m³. Ready-mix is sold in fixed increments with a minimum load, and concrete left in the drum or the chute is not counted, so the figure is an estimate of what the slab takes and not a promise of what a truck will bill. The calculator's confidence for this job is high, which means the inputs are in a range where the arithmetic is reliable and says nothing about the ground underneath.
What to buy
| Quantity | Figure | What it is for |
|---|---|---|
| Estimated concrete needed | 8.148 yd³ (6.23 m³) | The order to place with a ready-mix supplier, in the unit it is quoted in. |
| 80 lb bags needed | 367 bags | What the same pour would take if it were mixed from bags: a number worth reading once, because it shows how far the job is from a hand-mixed one. |
The allowance, and what it does here
The allowance is 10 per cent of the slab's volume, applied once to the whole pour. On this job it turns a slab of 7.41 yd³ into an order of 8.15 yd³. It covers three things the geometry never sees: a subgrade that is a little lower than the string line in places, forms that bow under the weight of wet concrete, and what is left on the wheelbarrow, the chute and the tools. It does not cover a short load charge, a second truck or a pour that stops half way, and a bigger percentage is not a substitute for a flatter base.
The common mistake
Entering the thickness as feet
A slab's footprint is measured in feet and its thickness in inches, so the thickness can be typed as though it were in the same unit as the sides. Twelve inches make a foot, so the answer is then twelve times too large, and the order is for a pour deep enough to fill a cellar. The calculator guards against it by giving thickness its own unit; read the unit beside that field before reading the answer. An order that looks absurdly large for the pad is nearly always this one.
Change the job, and see what moves
The same slab, poured thicker
Slab Thickness: 6 in (15.24 cm) → 12.22 yd³ (9.345 m³)
Pour the same pad 6 in thick, the thickness the calculator's guidance gives to a driveway that carries vehicles, and the order becomes 12.22 yd³, with 550 bags if it were bagged. Half as much again of depth is half as much again of concrete: thickness is the input that moves this order most, and the one worth confirming with whoever specifies the slab before a truck is booked.
The same slab, with a larger allowance
Waste Factor (%): 15 → 8.519 yd³ (6.513 m³)
Raise the allowance to 15 per cent for a base that is rough or forms that are not rigid, and the order becomes 8.519 yd³. The allowance moves the answer far less than the thickness does; it earns its place on a slab dug by hand, and buys little on one laid over a machine-graded base.
What to run next
- Gravel Base Layer Tonnage Calculator
The slab sits on a compacted base, and the stone under it is ordered by weight on the same day as the concrete.
- Rebar Calculator
If the slab is reinforced, the bars are taken off the same plan; the quantity depends on the spacing the designer gives.
- Concrete Formwork Calculator
The forms that hold the pour are measured around its edge, and they are what decides whether the allowance was enough.
Sources
The method and every constant behind these figures belong to the Concrete Calculator, version 1.0.1, last verified 2026-09-02. It rests on:
- Standard formula: cubic yards = length (ft) x width (ft) x thickness (ft) / 27
- An 80 lb bag of pre-mixed concrete yields approximately 0.6 cubic feet once mixed
Frequently asked questions
- How many cubic yards of concrete does a 20 by 30 ft slab need?
- At 4 in thick the slab itself is 7.41 yd³, and with the calculator's 10 per cent allowance the order is 8.148 yd³. A ready-mix supplier will round that to its own increments.
- Why is the answer more than the volume of the slab?
- Because the slab as drawn is 7.41 yd³ and the order has to cover what the drawing leaves out: an uneven base, forms that spread and concrete left behind. The allowance adds 10 per cent, which is how the figure reaches 8.15 yd³.
- How many bags of mix would the same slab take?
- 367 bags, using the calculator's standard bag and rounding up. That is a number to read as a warning: it is a job for a ready-mix delivery, not for mixing by hand.