Concrete Calculator
Use our free concrete calculator to find how much concrete you need in cubic yards, add a waste allowance and estimate material cost using a price per cubic yard.
Your project details
Change the assumptions below to match your project.
Start with geometry, then think about the pour
The calculator converts length × width × thickness into cubic feet, divides by 27 to get cubic yards, then applies the allowance you select. That gives a quantity target. It does not yet tell you the final delivered order or installed project cost.
Worked example: 20 × 12 ft slab at 4 inches
Four inches is 0.333 ft. The theoretical volume is 20 × 12 × 0.333 = about 79.9 cubic feet. Divide by 27 and the slab is about 2.96 yd³ before an allowance. With 5% extra, the planning quantity is about 3.11 yd³.
Do not use “waste” to hide missing geometry
If the slab has a thickened edge, footing, step or deeper section, calculate that volume separately and add it. A percentage allowance is for uncertainty and handling loss, not for parts of the project you already know exist.
What usually sits outside the per-yard material price
| Item | Why it matters |
|---|---|
| Delivery / short load | Small orders can carry fixed charges or minimums. |
| Pumping or buggy placement | Needed when the truck cannot discharge close to the forms. |
| Base preparation | Excavation, aggregate and compaction are separate from concrete volume. |
| Reinforcement | Rebar, mesh or fibers depend on the project design. |
| Forms and finishing | Labor can be a large part of installed cost. |
| Washout / cleanup | Site logistics should be planned before delivery. |
Three checks before ordering
- Re-measure the forms after they are set.
- Confirm the designed thickness and any thickened sections.
- Call the ready-mix supplier for minimum order, increment, delivery and waiting-time rules.
Allowance scenarios
Use the selector to test 5%, 10% and 15% rather than assuming one percentage is always correct. An accurately formed slab over a well-prepared base can have less uncertainty than an irregular excavation with changing depth.
Related guides
See the 12×12 slab example, how much extra concrete to order and what concrete cost per yard actually includes.
Three checks before you trust the concrete number
- Measure the forms. Length, width and approved slab thickness drive the base volume.
- Add separate geometry. Thickened edges, footings, steps and ramps should be calculated separately rather than buried in a waste percentage.
- Confirm supplier rules. Minimum loads, delivery increments, waiting time and access can change the practical order.
Concrete calculator reference
Formula
Cubic yards = length × width × thickness in feet ÷ 27. Add separate volumes for footings, steps or thickened edges.
Best input
Field dimensions from the forms and the approved slab thickness. Do not combine aggregate-base depth with concrete depth.
Before ordering
Confirm supplier minimums, delivery increments, access, waiting time and the final order allowance.
On this page
Calculator · Cubic yards · Ordering allowance · Ready-mix vs bags