RHACOVA

Free contractor tool

Concrete Slab Calculator

Length, width and slab thickness in, cubic yards out — plus the waste allowance you choose and the quantity you would actually order.

Slab dimensions

Units

Switching converts what you typed, so the size stays the same. Units only — this never changes a currency.

Slab length
Slab width

Take it from the drawings or your own spec — this tool never chooses a thickness for you.

Your number, not ours. RHACOVA does not publish a standard allowance because the honest answer depends on your subgrade, forms and crew.

The step your supplier actually sells in. 0.25 is only a starting value.

Your result

Quantity decision

Order volume

Order quantityWaste-adjusted volume rounded up to your increment2.75 cu yd
Waste-adjusted volume2.716 cu yd
Base volume (geometry only)2.469 cu yd

Same volume in cubic feet

Base volume66.67 cu ft
Waste-adjusted volume73.33 cu ft
Waste allowance10.00% · 0.247 cu yd
Why this number?

cubicFeet = (lengthIn * widthIn * thicknessIn) / 1728; cubicYards = cubicFeet / 27; adjusted = cubicYards * (1 + waste); order = ceilToIncrement(adjusted)

Inputs

Length
240.00 in
Width
120.00 in
Thickness
4.00 in
Waste allowance
10.00%
Order increment
0.2500 cu yd

Results

Theoretical volume (cu ft)
66.67 cu ft
Theoretical volume (cu yd)
2.4691 cu yd
Waste allowance volume
0.2469 cu yd
Waste-adjusted volume
2.7160 cu yd
Order quantity
2.7500 cu yd

Formula concrete.rectangularSlab · engine shared.concrete@1.1.0 · rectangular-volume · rounding half-even to 6 decimals.

A quantity is not a bid

Cubic yards tell you what to order. What you charge still depends on material cost, loaded labor, equipment, overhead and the margin you need. RHACOVA carries this quantity into the job bid so you price it instead of guessing.

Price this pour: build the job bid

Still no account, and nothing you type leaves your device. Signing in only comes up if you choose to save the bid as an estimate.

What a slab quantity actually includes

A slab quantity is the volume inside the forms: plan area × slab thickness. It does not include a thickened edge, a turndown, a footing under a bearing wall or a haunch at a door opening. Those are extra rectangles and they add up faster than people expect — a 12 in × 12 in turndown around a 20 ft × 10 ft slab is roughly another 0.7 cu yd on its own.

If the pour has more than one rectangle, use the main concrete calculator and add a section for each one, or the footing calculator for the turndown.

Formula, units and precision

  • Cubic inches = length (in) × width (in) × thickness (in)
  • Cubic feet = cubic inches ÷ 1,728
  • Cubic yards = cubic feet ÷ 27
  • Waste-adjusted volume = base volume × (1 + your allowance)
  • Order quantity = waste-adjusted volume rounded up to your increment

Feet and inches are converted in the same shared unit system every RHACOVA calculator uses, with exact decimal arithmetic rather than floating-point. Rounding happens once, for display; the underlying value is never quietly changed to make a number look tidy.

Common slab mistakes

  • Forgetting the turndown, the thickened edge and the door haunches.
  • Using nominal thickness when the subgrade is 1/2 in low across half the slab.
  • Mixing feet and inches in the same multiplication.
  • Ordering the base volume with no allowance at all.

What this page is not

RHACOVA calculates from your inputs. It does not determine slab thickness, mix design, reinforcement, joint spacing or code compliance — those are design decisions that belong with the drawings and your engineer.

Worked example

A 20 ft × 10 ft slab at 4 in thick, with a 10% allowance and a 0.25 cu yd order increment:

  • Base volume: 66.67 cu ft
  • Base volume: 2.4691 cu yd
  • Waste allowance: 0.2469 cu yd
  • Order quantity: 2.75 cu yd

Other concrete quantities

After the quantity: cost, price, profit

  • True Job Cost Calculator

    A quantity is not a cost. Add material, loaded labor, equipment, disposal and the rest to see what the pour actually costs you.

  • Markup & Margin Calculator

    Turn that cost into a price, and see the margin the price actually leaves.

Learn the method