How to use this calculator
- Choose the shape you are pouring.
- Enter the sizes. For a slab, the thickness is the depth of concrete, not including any hardcore under it.
- Enter how many of the same shape you need (for example 6 post holes).
- Pick your bag size and check the yield against the bag label.
The formula
Slab volume = length × width × thickness
Column volume = π × (diameter ÷ 2)² × height
Total = volume × how many × (1 + extra %)
Bags = total ÷ yield per bag, rounded up
Ground is never perfectly level and some concrete always stays in the mixer or wheelbarrow, so 5–10 % extra is normal.
For large pours (more than about 1 m³ or 1 yd³), ready-mixed concrete delivered by truck is usually cheaper and easier than bags.
Worked example
A 10 ft × 10 ft slab, 4 in thick, with 10% extra needs 1.36 yd³ (36.67 ft³) of concrete.
Using 80 lb bags that make 0.6 ft³ each, buy 62 bags.
Tips before you buy
- Order a little more rather than less — a cold joint between two pours is a weak spot.
- For slabs over about 1 m³ / 1.3 yd³, get a price for ready-mix delivery.
- Check whether your job needs reinforcement mesh or rebar and a sub-base; this calculator only covers the concrete.
Frequently asked questions
›How many 80 lb bags of concrete make a cubic yard?
About 45 — each 80 lb bag makes about 0.6 ft³, and a cubic yard is 27 ft³.
›How many 25 kg bags make a cubic metre?
Roughly 85 bags, depending on the product. Check the yield printed on your bag.
›How thick should a concrete slab be?
Garden paths and shed bases are commonly 75–100 mm (3–4 in); driveways and garage floors more. Follow local guidance or an engineer for anything that carries a load.
Sources
Formulas last reviewed: September 29, 2026. How we check our formulas