What a slab takeoff actually measures
A slab takeoff converts three dimensions into one number you can hand to a batch plant: the volume of plastic concrete the forms will hold. Everything else on the delivery ticket — mix design, strength class, slump, air entrainment, fibre dose — is a specification. The quantity is pure geometry, and it is the only part of the order you can get wrong by arithmetic alone.
Concrete is sold by volume rather than by weight, and in the United States that volume is the cubic yard: a cube three feet on each side, so 27 cubic feet. Outside North America the same trade uses the cubic metre, which is 1.308 cubic yards. This calculator reports both, plus cubic feet, because forms are dimensioned in feet and inches and the intermediate figure in cubic feet is the one you can check in your head.
The number that matters on the day is not the neat drawn volume but the order quantity. Ready-mix is delivered in one pass; if the last eight feet of a driveway comes up short you cannot buy a partial truck, and a cold joint at a random location is a defect rather than a detail. Every experienced concrete crew therefore orders a deliberate surplus and dumps the tail end into a spare form. That surplus is what the waste allowance in this calculator represents.
Because volume scales linearly with thickness, thickness is where the money hides. Going from 4 inches to 5 inches on a 480 square foot garage floor adds 40 cubic feet — 1.48 cubic yards, a quarter more concrete for a slab that still looks identical from above. Measure the thickness you are actually going to build, not the one on the sketch.
The formula, and why the two divisors are what they are
Multiply length by width to get the plan area, multiply that by the thickness expressed in feet, and you have cubic feet. The two divisors in the formula are unit conversions, not fudge factors:
- ÷ 12 turns a thickness in inches into feet, so that all three dimensions share a unit before you multiply them.
- ÷ 27 turns cubic feet into cubic yards, because 3 × 3 × 3 = 27.
Combine them and a useful shortcut falls out: one cubic yard spread at T inches covers 324 ÷ T square feet. At 4 inches that is 81 square feet — the single most-quoted number in residential concrete. At 6 inches it is 54 square feet, at 3 inches it is 108. Memorise 81 and you can sanity-check almost any residential order in your head: divide the square footage by 81 for a 4 inch slab, and if the calculator disagrees with that by more than a rounding step, one of your inputs is wrong.
Measure length and width to the inside face of the forms. Form boards are 1½ inches thick, so which face of the board you set on your layout line decides whether the pour comes out larger or smaller than nominal. Put the board's inside face on the line and a 20 × 24 slab is exactly 480 ft². Put its outside face on the line and the concrete gains 1½ inches on each side: 20.25 × 24.25 = 491.1 ft², which at 4 inches is 0.14 cubic yards more than you took off. Set it the other way, 19.75 × 23.75 = 469.1 ft², and you are 0.14 cubic yards short. Either error is small on its own; what is not small is measuring to a string line offset from the forms, or to the outside face of a foundation wall the slab does not actually reach. Those errors run to whole feet.
The order quantity applies your waste percentage and then rounds up to the increment your supplier batches in — usually a quarter of a cubic yard. Rounding up is deliberate. Rounding a concrete order down is how a crew ends up screeding with a wheelbarrow of hand-mixed bag concrete that cures a different colour.
Worked example: a 20 ft × 24 ft garage slab at 4 inches
You are pouring a detached garage floor. The forms are set 20 ft by 24 ft inside face to inside face, the slab is 4 inches thick over a compacted base, and your supplier quotes $175 a cubic yard delivered and batches in quarter yards. Work it through:
- Plan area. 20 ft × 24 ft = 480 ft².
- Thickness in feet. 4 ÷ 12 = 0.3333 ft.
- Volume in cubic feet. 480 × 0.3333 = 160 ft³.
- Volume in cubic yards. 160 ÷ 27 = 5.926 yd³. Check it against the shortcut: 480 ÷ 81 = 5.93. They agree.
- Volume in cubic metres. 160 × 0.0283168 = 4.531 m³.
- Add 10% waste. 5.926 × 1.10 = 6.519 yd³.
- Round up to the next quarter yard. 6.519 ÷ 0.25 = 26.07, so 27 quarter-yards = 6.75 yd³.
- Cost of concrete. 6.75 × $175 = $1,181.25, before delivery, pump or short-load charges.
Two sanity checks are worth doing before you pick up the phone. First, 6.75 yd³ is comfortably inside one truck, so this is a single-load pour with no joint planning needed. Second, the surplus you have bought is 6.75 − 5.926 = 0.82 yd³, which is 22 cubic feet. Have a form ready for it: a splash pad, a shed pier, or a pair of parking blocks.
Now change one thing. Thicken the same slab to 5 inches for a truck: 480 × (5 ÷ 12) = 200 ft³ = 7.41 yd³, and the order becomes 7.41 × 1.10 = 8.15, rounded to 8.25 yd³. That single inch adds 1.48 cubic yards of neat volume and raises the order by 1.50 cubic yards, or $262.50 at the same unit price.
How much to over-order, and what thickness to build
Order between 5% and 10% more than the neat volume for a formed slab on a properly graded base, and more than that when the base is rough. The allowance is not padding; it covers three real losses. The subgrade is never perfectly flat, so the slab is thicker than nominal almost everywhere. Wood forms bow outward under the head of wet concrete. And some concrete stays in the chute, the wheelbarrow and the pump line.
Put a number on the first of those, because it dominates. If the base of the 480 ft² garage slab averages just half an inch low, the slab is really 4.5 inches thick: 480 × (0.5 ÷ 12) = 20 ft³ = 0.74 yd³ of extra concrete, which is 12.5% over the drawn volume all by itself. That is why a 10% allowance over a good base is normal and why a rough, un-rolled base can swallow far more.
On thickness, the practical rules in residential work are these. Four inches is the customary minimum for foot traffic — patios, walkways, shed floors, basement slabs. Five to six inches is customary where passenger cars and light trucks park or drive. Anything carrying repeated heavy vehicles, forklifts or point loads from equipment is a designed slab, not a rule-of-thumb slab, and the thickness, reinforcement, joint layout and subgrade modulus should come from an engineer rather than from a table. ACI 302.1R, the industry guide to concrete floor and slab construction, is the reference the specifying engineer will be working from.
Finally, read the order quantity as a decision, not just a number. Under about one cubic yard, the short-load fee a plant charges for a partial delivery often exceeds the cost of buying bags, so run the volume through the concrete bag calculator before booking a truck. Above roughly 10 cubic yards you are into multiple loads, and you need a placement sequence and a planned joint location so that the truck change does not land in the middle of an open panel.
Concrete coverage by slab thickness
| Thickness | Area one yd³ covers | yd³ per 100 ft² | ft³ per 100 ft² | 80 lb bags per 100 ft² |
|---|---|---|---|---|
| 2 in | 162 ft² | 0.617 | 16.7 | 28 |
| 3 in | 108 ft² | 0.926 | 25.0 | 42 |
| 3½ in | 92.6 ft² | 1.080 | 29.2 | 49 |
| 4 in | 81 ft² | 1.235 | 33.3 | 56 |
| 5 in | 64.8 ft² | 1.543 | 41.7 | 70 |
| 6 in | 54 ft² | 1.852 | 50.0 | 84 |
| 8 in | 40.5 ft² | 2.469 | 66.7 | 112 |
| 10 in | 32.4 ft² | 3.086 | 83.3 | 139 |
| 12 in | 27 ft² | 3.704 | 100.0 | 167 |
Bag counts use the published 0.60 ft³ yield of an 80 lb bag and are rounded up; they carry no waste allowance.
Mistakes that leave a slab short
- Measuring to the string line instead of the forms. The layout line is often offset from the form face. Pull your tape between the boards that will actually hold the concrete.
- Ignoring the thickened edge. Many slabs on grade turn down at the perimeter into an integral footing. That turndown is extra concrete this calculator does not include — take it off separately with the concrete footing calculator and add the two volumes.
- Forgetting the base is not flat. Half an inch of average dip over a 480 ft² slab is three quarters of a cubic yard. Screed the base, roll it, and check it with the same string you will screed the concrete to.
- Ordering the neat volume. Zero waste means the first low spot is paid for out of a slab you have not finished yet.
- Rounding down to hit a round number. Suppliers batch in quarter yards; 6.5 is not cheaper than 6.75 by enough to justify running out.
- Confusing an L-shaped pour for a rectangle. Break irregular footprints into rectangles, run each through the calculator, and add the cubic feet before converting to yards.
- Using the same allowance for pumped work. A pump line holds concrete you will not place. Ask the pump operator how much their line holds and add it on top.
What the standards say about the quantity you are buying
ASTM C94/C94M, the specification for ready-mixed concrete, defines the cubic yard you are billed for as a volume of freshly mixed, unhardened concrete, and it is the document your delivery ticket is written against. When there is a dispute about whether a truck actually delivered the volume on the ticket, the test used is ASTM C138/C138M, which measures the density of a sample in a container of known volume and back-calculates the yield of the batch. That is worth knowing for one practical reason: yield is verified against measured density, and a batch with more entrained air than specified will produce more apparent volume from the same materials. If a load repeatedly finishes long or short against a carefully measured form, ask for the batch ticket and the air content.
When to use a different takeoff instead
This calculator assumes a rectangular slab of constant thickness. Several common pours break at least one of those assumptions, and each has its own geometry:
Footings and turndowns. Continuous strip footings and isolated pads are a different cross-section entirely; run them through the footing calculator. If your slab has a monolithic thickened edge, take the slab off here at its constant thickness and add the turndown as a strip footing.
Round pours. Sonotube piers, deck footings and column bases are cylinders, so their volume is πr²h rather than L × W × T. The column and Sonotube calculator handles the tube diameters that are actually sold, and the deck footing size calculator sizes the pier before you take off its volume.
Steps and stairs. Cast-in-place stairs are a stack of prisms whose volume depends on the tread and riser layout, not on a single thickness; use the concrete stairs calculator.
Small hand-mixed pours. Below roughly a cubic yard, bagged mix usually wins on total cost once short-load fees are counted. The bag calculator converts any of these volumes into 40, 50, 60 or 80 lb bags.
The base under the slab. The compacted granular base is a takeoff of its own, priced by weight rather than volume. Size it with the gravel tonnage calculator, and remember that every inch you fail to build in base is an inch of concrete you will pay for instead.
Once the volume is settled, the remaining quantities on a slab job are reinforcement and joints. Bar spacing and quantity come from the rebar spacing calculator; the mix proportions behind a site-batched or specified mix come from the concrete mix ratio calculator.
Key terms
- Neat volume
- The geometric volume of the slab as drawn, with no allowance for over-excavation, form spread or waste. It is the first output of this calculator.
- Short load
- A ready-mix delivery below the plant's minimum quantity, usually carrying a surcharge that can rival the cost of the concrete itself on very small pours.
- Yield
- The volume of fresh concrete produced by a batch. Measured in the field by ASTM C138 from the density of a sample of known mass and volume.
- Subgrade and base
- The compacted soil beneath the slab and the graded granular layer on top of it. Their flatness controls how much concrete the slab actually consumes.