How to Calculate Concrete Volume
By the TapeMeasureCalc team · Updated · 7 min read
Ordering concrete starts with one number: the volume of the space you are filling. This guide shows how to work it out for the shapes you meet on most home projects, how to convert between cubic feet, cubic yards and cubic metres, and how much extra to allow.
Step 1: Put every measurement in the same unit
Most volume mistakes come from mixing units. Slabs are usually measured in feet but their thickness in inches; in metric, lengths are in metres and thickness in millimetres. Convert everything before multiplying:
- Inches to feet: divide by 12 (4 in = 0.333 ft, 6 in = 0.5 ft).
- Millimetres to metres: divide by 1,000 (100 mm = 0.1 m).
- Feet and inches: 12 ft 6 in = 12.5 ft.
Step 2: Calculate rectangular volumes
Slabs, footings, pads and walls are rectangular boxes:
Volume = Length × Width × Thickness
Example: a 16 ft × 12 ft patio at 4 in thick is 16 × 12 × 0.333 = 64 cubic feet. A strip footing 30 ft long, 1.5 ft wide and 1 ft deep is 30 × 1.5 × 1 = 45 cubic feet.
For L-shaped or irregular slabs, split the area into rectangles, work out each volume and add them. A thickened edge around a slab is another rectangle: its length is the perimeter, and its cross-section is the extra width × extra depth.
Step 3: Calculate round columns and holes
Piers, cardboard tube forms and post holes are cylinders:
Volume = π × (Diameter ÷ 2)² × Height
Example: a 12 in (1 ft) tube 4 ft deep holds π × 0.5² × 4 = 3.14 cubic feet. Ten of them hold 31.4 cubic feet. For a post hole, the post displaces some concrete, so the result is slightly more than you will use.
Step 4: Convert to the unit your supplier uses
| Convert | Calculation |
|---|---|
| Cubic feet → cubic yards | ÷ 27 |
| Cubic yards → cubic metres | × 0.7646 |
| Cubic metres → cubic yards | × 1.308 |
| Cubic feet → litres | × 28.317 |
Our 64 ft³ patio is 64 ÷ 27 = 2.37 cubic yards, or 1.81 m³. A metric slab of 5 m × 4 m × 100 mm is 2.0 m³, about 2.62 cubic yards.
Step 5: Add a waste allowance
The geometric volume is the minimum. Real pours lose concrete to uneven sub-base, forms that bow slightly, spillage and what is left in the chute or pump. Many people add 5–10%, using the higher end when the ground is uneven or running short would be costly.
Our patio: 2.37 yd³ × 1.10 = 2.61 yd³. If the supplier sells in quarter-yard steps, that rounds to 2.75 yd³.
Step 6: Decide between ready-mix and bags
For small pours, pre-mixed bags may be simpler. Divide the volume by the yield printed on the bag. An 80 lb bag of many general-purpose mixes yields about 0.6 ft³, so 10 post holes needing 15 ft³ would take 25 bags. Check your product, as yields vary.
For anything more than a cubic yard or so, compare against a ready-mix delivery, which also saves hours of mixing.
Use the calculators
The concrete calculator handles rectangles and round columns with quantities and waste. The concrete slab calculator adds slab area and form length, and the concrete bag calculator converts any volume into bags. Structural details such as thickness, reinforcement and footing size should come from your plans or a qualified professional.