Concrete Math

Concrete calculator

Calculate how much concrete you need, how many bags to buy, and which supplier quote costs less.

Pour dimensions

Use finished dimensions and set an allowance for uneven ground, spills, or measurement variation.

Concrete to order

1.358 yd³

1.038 m³ · 36.667 ft³ including 10% extra

Volume before allowance
1.235 yd³
80 lb bags
62
Compare supplier quotes

Compare concrete quotes

Enter each supplier’s rates in the same currency. Leave optional charges at zero if they don’t apply. Use the full quoted delivery charge for your order.

Supplier A

Enter the supplier’s rate to estimate its total.

Supplier B

Enter the supplier’s rate to estimate its total.

Add a price to compare material costs.

Enter two supplier rates to compare their totals.

Minimum order sets the floor. Order increment rounds up to a supplier’s selling multiple. A short-load fee applies below the stated threshold after rounding. Tax here applies to the full subtotal. Bag cost covers bags only.

Concrete calculators by project

Concrete bag calculator converts a known volume into whole bags. The fence post concrete calculator estimates concrete around the post for one hole or an entire fence.

How to calculate concrete volume

For a rectangular slab or footing, multiply length × width × thickness in the same unit. With feet and inches, divide thickness by 12 before multiplying. For a round pier, multiply π × radius² × depth. Multiply by the number of identical pours, then add your allowance. Divide cubic feet by 27 to get cubic yards.

Worked example · 10 × 10 ft slab at 4 inches

The slab needs 10 × 10 × (4 ÷ 12) = 33⅓ cubic feet. With 10% extra, that becomes 36⅔ cubic feet, or about 1.358 cubic yards. Divide by 0.60 cubic feet per 80 lb bag and round up to 62 bags. The calculator keeps full precision before rounding bag counts or order increments.

Working in millimetres? See the cubic metre formula and metric bag examples.

How much does a bag of concrete make?

These approximate yields come from the QUIKRETE Concrete Mix No. 1101 technical data sheet. For a different mix, select a custom yield and enter the value from its label.

Concrete mix yield by bag size
Bag weightApproximate yieldBags for 1 yd³, before allowance
40 lb0.30 ft³90
50 lb0.375 ft³72
60 lb0.45 ft³60
80 lb0.60 ft³45
90 lb0.675 ft³40

Why a lower price per cubic yard can cost more

For the slab above, a supplier charging $150/yd³ with a 2 yd³ minimum and $80 delivery would charge $380 before tax. A supplier charging $165/yd³ and selling in 0.25 yd³ increments would bill 1.5 yd³. With $100 delivery and a $25 short-load fee below 2 yd³, its total would be $372.50 before tax.

How do I estimate an irregular pour?

Split the footprint into non-overlapping rectangles or circles. Calculate each section with the allowance set to zero, add the volumes, then apply the allowance once to the total. Treat thickened edges as additional volume below the slab and count any overlapping corners only once.

Before ordering, use the ready-mix quote checklist and cost example to check what each supplier includes.

This is a material estimator. Use the dimensions and concrete specifications required by your project plans.