🧱 Concrete Calculator
Work out how much concrete you need for a slab, footing or post hole – in cubic yards, cubic meters and bags of premix.
Quick answer: Volume = length × width × thickness. A 10 × 10 ft slab 4 inches thick holds 33.33 cubic feet – about 1.23 cubic yards (0.94 m³) – which is 56 bags of 80 lb premix.
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Concrete Calculator inputs
Result
Concrete needed
1.23 yd³
33.33 cubic feet · 0.944 m³
| Cubic yards | 1.23 |
| Cubic meters | 0.944 |
| Cubic feet | 33.33 |
| 40 lb bags of premix | 112 |
| 60 lb bags of premix | 75 |
| 80 lb bags of premix | 56 |
| 25 kg bags of premix | 79 |
Premixed bags vs ready-mix
| Bag size | Approx. yield | Bags needed |
|---|---|---|
| 40 lb bag | 0.3 ft³ | 112 |
| 60 lb bag | 0.45 ft³ | 75 |
| 80 lb bag | 0.6 ft³ | 56 |
| 25 kg bag | 0.012 m³ | 79 |
Above roughly 1 cubic yard (0.76 m³), ordering ready-mix concrete by truck is usually cheaper and easier than mixing bags.
How to calculate concrete volume
Concrete is ordered by volume. Convert every measurement to the same unit, multiply to get the volume, then convert to cubic yards (US ready-mix) or cubic meters.
Slab = length × width × thickness · Column = π × (diameter ÷ 2)² × height · 1 yd³ = 27 ft³
Worked example
A 10 × 10 ft patio slab 4 inches thick: 4 in = 0.333 ft, so the volume is 10 × 10 × 0.333 = 33.33 ft³. Divide by 27 for 1.23 cubic yards (0.94 m³). At 0.6 ft³ per 80 lb bag that is 55.6 – round up to 56 bags.
Bag yields
| Bag | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
| 25 kg | 0.012 m³ | about 64 |
Shapes supported
- Slab: length and width in feet or meters, thickness in inches or centimeters.
- Wall footing: a continuous strip – length in feet/meters, width and depth in inches/centimeters.
- Round column or post hole: diameter in inches/centimeters and height in feet/meters.
Tips for pouring
- Compact the sub-base and use gravel under slabs to prevent cracking.
- For more than about a cubic yard, ready-mix delivery saves hours of hand mixing.
- Add control joints every 8–12 ft in slabs to direct shrinkage cracks.
- Keep fresh concrete damp for several days so it cures to full strength.
Cubic yards versus cubic meters
US ready-mix suppliers sell concrete by the cubic yard, while most other countries use cubic meters. One cubic yard is about 0.765 m³, and one cubic meter is about 1.31 cubic yards, so always confirm which unit your supplier quotes.
Frequently asked questions
How many bags of concrete make a yard?
One cubic yard is 27 cubic feet. An 80 lb bag yields about 0.6 cu ft, so you need 45 bags; with 60 lb bags (0.45 cu ft) you need 60, and with 40 lb bags (0.3 cu ft) 90.
How thick should a concrete slab be?
Four inches (10 cm) is standard for patios, walkways and shed bases. Driveways and garage floors are usually 5–6 inches (12–15 cm), thicker where heavy vehicles park.
Should I order extra concrete?
Yes. Uneven ground, form bulges and spillage use more than the theoretical volume. Add 5–10% under advanced options – running short mid-pour leaves a weak cold joint.
How do I calculate concrete for a post hole?
Choose the round column shape and enter the hole diameter and depth. Subtract the post itself if it is large: a 4 × 4 post displaces about 0.11 cu ft per foot of depth.