How to Calculate Concrete

The basic concrete formula

Concrete is sold and ordered by the cubic yard, so every estimate comes down to finding a volume and expressing it in cubic yards. For any rectangular pour the formula is:

Cubic yards = (Length ft × Width ft × Thickness ft) ÷ 27

The only trick is that the three measurements must all be in the same unit — feet — before you multiply them. The number 27 appears because a cubic yard is a cube three feet on each side, and 3 × 3 × 3 = 27 cubic feet. Divide cubic feet by 27 and you have cubic yards.

Step by step

1. Measure the pour. Record the length, the width, and the thickness (or depth). For a slab, length and width are usually in feet and the thickness in inches.
2. Convert everything to feet. Divide any measurement in inches by 12. A 4 inch slab is 4 ÷ 12 = 0.333 feet thick.
3. Multiply for cubic feet. Length × width × thickness gives the volume in cubic feet.
4. Divide by 27. That converts the cubic feet into cubic yards, the unit your supplier uses.
5. Add a waste allowance. Increase the total by 5–10% to cover spillage and uneven ground.

If you would rather skip the arithmetic, the concrete slab calculator and the yardage calculator run these exact steps for you and add the waste automatically.

Converting units

Most mistakes come from mixing units, so it pays to convert before you multiply. This table shows the common conversions into feet:

Worked example: a patio slab

Say you are pouring a patio that is 12 feet long, 10 feet wide, and 4 inches thick.

1. Convert the thickness: 4 ÷ 12 = 0.333 ft.
2. Multiply: 12 × 10 × 0.333 = 40 cubic feet.
3. Divide by 27: 40 ÷ 27 = 1.48 cubic yards.
4. Add 10% waste: 1.48 × 1.10 = 1.63 cubic yards to order.

If you were bagging it instead of ordering ready-mix, that 40 cubic feet is about 67 × 80 lb bags (each 80 lb bag yields roughly 0.60 ft³). The bag calculator works that out for every bag size at once.

Calculating other shapes

Not every pour is a simple rectangle. The principle is the same — find the volume, then divide by 27 — but the area formula changes:

• Footings and walls are long boxes: length × width × depth. Use the footing calculator or the wall calculator.
• Round columns and tube forms use π × radius² × height. The Sonotube calculator handles cylinders.
• Stairs are stacked prisms; the stairs calculator adds up each step for you.
• Odd shapes can be split into rectangles and circles, calculated separately, and added together.

Why add a waste allowance

A calculated volume is the theoretical minimum. In the real world the subgrade is never perfectly level, some concrete sticks to the wheelbarrow, and a slab settles slightly into soft spots. Ordering 5–10% extra means you finish the pour in one go. Running short is far worse: a second batch arrives after the first has begun to set, creating a cold joint that is weaker and more visible. When in doubt, round up to the next quarter yard.

Ready to put it into practice? Use the slab calculator for flat pours, or browse all concrete calculators to find the right tool for your project.