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.
Use a current supplier rate for material only. The concrete delivered-cost and short-load fee example shows how to add supplier charges separately before comparing complete project quotes.
Working out an order allowance? Follow the extra-concrete ordering workflow, then keep the base volume and separately measured additions in your free planning workbook.
Bag estimate assumes 0.60 cubic feet per 80-lb bag, as listed for QUIKRETE Concrete Mix No. 1101. Check the exact product yield; bag weight alone does not establish coverage.
Start with geometry, then think about the pour
Planning an outdoor slab with an extension or step? Follow the patio concrete takeoff and bag-count examples to separate the main slab, added shapes and final supplier rounding before using this rectangular calculator.
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