How it works
The calculator multiplies length, width and thickness for one slab, multiplies by the number of slabs, then adds your extra allowance.
Volume = Length × Width × Thickness × Number of slabs
Concrete to order = Volume × (1 + Extra ÷ 100)
Cubic yards = Cubic feet ÷ 27
All dimensions are converted to one internal unit first, so you can enter the slab in feet and the thickness in inches. The result is not rounded; round up to the increment your supplier sells.
This is an estimate of material quantity, not a structural design. Thickness, reinforcement and base preparation are decisions for a qualified professional.
Worked example
Inputs
- Slab length
- 10 ft
- Slab width
- 10 ft
- Thickness
- 6 in
- Number of slabs
- 1
- Extra concrete
- 0 %
Result
- Concrete to order
- 1.85 cu yd (50 cu ft, 1.42 m³)
- Volume before extra
- 1.85 cu yd (50 cu ft)
Common mistakes
- Entering the thickness in feet instead of inches. A 4 in slab is a third of a foot; typing “4 ft” makes the order twelve times too large. Check the unit next to the field.
- Measuring the outside of the forms, or the planned surface, instead of the space the concrete will fill.
- Ordering exactly the calculated volume. A low spot in the base or a bowed form uses concrete you didn’t plan for, and a short load is much harder to fix than a little left over.
- Forgetting thickened edges, footings under the slab or steps that are poured at the same time. Calculate them separately and add them.
Assumptions and limits
- Each slab is a rectangle of uniform thickness. Thickened edges, sloped tops, haunches and holes are not modeled.
- The result is a material quantity only. It says nothing about thickness, reinforcement, base preparation, joints or whether the slab is suitable for its load – those questions need a qualified professional.
- The volume of rebar, mesh or embedded items is not subtracted.
- The extra allowance you enter is applied to the whole volume. Nothing is rounded – ready-mix suppliers have their own ordering increments.
Frequently asked questions
How do I convert cubic feet of concrete to cubic yards?
Divide by 27. One cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet (NIST Handbook 44). The calculator shows both, plus cubic meters.
How thick should my concrete slab be?
That depends on what the slab carries, the soil and local requirements, so this calculator doesn’t suggest a figure. Use the thickness from your plans, or ask a qualified contractor or engineer.
Should I use bags or ready-mix?
It mostly comes down to volume and access. Enter your slab here, then put the volume into the concrete bag calculator to see how many bags it would take; compare that with a ready-mix quote, including any delivery or short-load charges.
Can I calculate a circular or L-shaped slab?
Split an L shape into rectangles and add the results. For a circle, work out the area (π × radius²), then multiply by the thickness yourself – this calculator handles rectangles only.
Sources
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 2, 2026