How to use
- Choose what you are pouring: slab, footing, square or round column, or stairs.
- Pick ft · in or m · cm. Lengths, widths and heights use the larger unit (ft or m); thickness, column sections and step sizes use the smaller one (in or cm).
- Enter a quantity for repeated items and an extra for waste percentage (5% by default).
- You get the volume to order in cubic yards (or m³) and the number of pre-mixed bags if you plan to mix it yourself.
Formulas
| Item | Volume |
|---|---|
| Slab / footing | length × width × thickness |
| Square column | width × depth × height |
| Round column | π × (diameter ÷ 2)² × height |
| Stairs (solid, on grade) | width × run × rise × n(n + 1) ÷ 2 |
| Amount to order | net volume × quantity × (1 + waste%) |
Stairs are treated as a solid block: the first step is one rise tall, the second two rises, and so on. Stairs poured as an inclined slab with open space underneath need less concrete.
Unit facts: 1 cubic yard = 27 cubic feet = 0.7646 m³, and 1 m³ = 1,000 liters.
Example
A 10 × 10 ft patio, 4 inches thick:
- Net volume: 10 × 10 × (4 ÷ 12) = 33.3 ft³ = 1.23 yd³
- With 5% extra: 35.0 ft³ = 1.30 yd³
- Mixing it yourself: 35.0 ÷ 0.60 = 59 bags of 80 lb, or 78 bags of 60 lb — at this size, a short-load ready-mix delivery is often easier.
Things to watch
- Mind the thickness unit. Thickness is entered in inches (or cm); 4 ft instead of 4 in would be 12 times too much.
- The waste allowance covers spillage, form bulging, uneven subgrade and concrete left in the pump. Use up to 10% on rough ground.
- Bag yields follow the common label ratio of about 0.60 ft³ per 80 lb bag (0.45 ft³ per 60 lb, 0.30 ft³ per 40 lb). Check your product’s label.
- Rebar volume is not subtracted; it is usually small enough that the waste allowance covers it.
FAQ
How much does a cubic yard of concrete weigh?
Normal-weight fresh concrete is roughly 2,300–2,400 kg per m³, which is about 3,850–4,050 lb per cubic yard, depending on the mix.
I only know the area of my slab. What now?
Area × thickness = volume. Find odd-shaped areas with the area calculator, and box or cargo volume with the CBM calculator.