How to calculate concrete
Volume is length times width times thickness. The rest is detail that matters.
Volume is length x width x thickness for slabs and walls, and π x radius² x height for round columns. A 16 x 10 ft slab at 5 in is 66.7 cubic feet, or 2.59 cubic yards with 5 % waste.
By element type
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| Element | Formula | Example |
|---|---|---|
| Slab | length x width x thickness | 16 x 10 x 0.42 = 66.7 cu ft |
| Strip footing | length x width x depth | 33 x 2 x 1.3 = 86 cu ft |
| Wall | length x height x thickness | 33 x 8 x 0.67 = 177 cu ft |
| Round column | π x (diameter/2)² x height | π x 0.5² x 10 = 7.9 cu ft |
| Post hole | length x width x depth | 1 x 1 x 1.3 = 1.3 cu ft |
Useful conversions
- 1 cubic yard = 27 cubic feet = 0.7646 m³
- 1 m³ = 35.31 cubic feet = 1.308 cubic yards
- A cubic yard of concrete weighs roughly 4,000 lb (about 2 tons)
Common thicknesses
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| Use | Typical thickness |
|---|---|
| Footpath or patio | 4 in |
| Garage slab (light vehicle) | 5-6 in |
| Shed or outbuilding base | 4-5 in |
| Fence post hole | 12-20 in deep |
Bags or ready-mix
Below about half a cubic yard, bags usually win: no minimum order, no waiting. Above that, a truck is clearly cheaper per yard, with two conditions: the truck can get close enough, and you have enough hands to place the concrete before it starts to set.
Bear in mind that many plants have a minimum order (often 1 cubic yard) and charge delivery separately.
Common mistakes
- Measuring thickness from the drawing. On natural ground, the base is never as flat as the plan, and the concrete fills those hollows. Raise the waste to 10 %.
- Ordering exactly what you calculated. Running short mid-pour creates a cold joint that weakens the element. Over-ordering is always better.
- Confusing volume with strength. Knowing how much concrete fits says nothing about whether the element will hold.
Sources and methodology
The formulas are elementary geometry. Conversions use exact factors (1 foot = 0.3048 m by definition). The thicknesses in the table are common values, not code requirements.
Frequently asked questions
How much concrete do I need for a 16x10 slab?
At 5 in thick, 66.7 cubic feet, which with 5 % waste is 2.59 cubic yards. Round up to 3 yards when ordering from a plant.
How many bags of concrete in a cubic yard?
About 45 bags of 80 lb, since each yields roughly 0.6 cubic feet. Check the yield on your bags: it varies.
How much does a cubic yard of concrete weigh?
Around 4,000 lb, close to two short tons. Useful for checking access and load limits.
How much waste should I add for concrete?
5 % on a prepared base with clean formwork, 10 % when pouring onto natural ground or gravel, where the uneven bottom absorbs more.
Can I use this for structural design?
No. Volume tells you how much to order; structural sizing and reinforcement need a qualified engineer.