How much concrete do I need? How to use the calculator
- 1Choose the shape you are pouring and enter its sizes. For a slab, the thickness is the depth of concrete, not the hardcore under it.
- 2Enter how many of the same shape you need (for example 6 post holes or 4 columns).
- 3Choose how you will make the concrete: bags of pre-mix, cement, sand and stone mixed on site, or a ready-mix truck.
- 4For bags, check the yield on the bag. For a site mix, pick the mix and your cement bag size.
How to calculate concrete and bags (formula)
- 01Slab volume = length × width × thickness
- 02Column volume = π × (diameter ÷ 2)² × height
- 03Total = volume × how many × (1 + extra %)
- 04Bags of pre-mix = total ÷ yield per bag, rounded up
- 05Site mix: dry volume = total × 1.54; cement = dry volume × cement parts ÷ all parts
- 06Cement bags = cement volume × 1,440 kg/m³ ÷ bag weight, rounded up
Ground is never perfectly level and some concrete always stays in the mixer or wheelbarrow, so 5–10 % extra is normal.
Mixing by volume: in a 1 : 2 : 4 mix there are 7 parts, so cement is 1/7 of the dry volume, sand 2/7 and stone 4/7. For 1 m³ of 1 : 2 : 4 concrete that is about 6.3 bags of 50 kg cement, 0.44 m³ (15.5 ft³) of sand and 0.88 m³ (31 ft³) of stone.
For large pours (more than about 1 m³ or 1 yd³), ready-mixed concrete delivered by truck is usually cheaper and easier than bags.
Concrete calculation 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.
Concrete bags and mix quantities
| Slab | Thickness | Concrete with 10% extra | 60 lb bags | 80 lb bags | 25 kg bags | 20 kg bags |
|---|---|---|---|---|---|---|
| 10 × 10 ft | 4 in | 1.36 yd³ | 82 | 62 | 89 | 111 |
| 10 × 10 ft | 6 in | 2.04 yd³ | 123 | 92 | 134 | 167 |
| 12 × 12 ft | 4 in | 1.96 yd³ | 118 | 88 | 128 | 160 |
| 12 × 12 ft | 6 in | 2.93 yd³ | 176 | 132 | 192 | 240 |
| 20 × 20 ft | 4 in | 5.43 yd³ | 326 | 245 | 355 | 444 |
| 20 × 20 ft | 6 in | 8.15 yd³ | 489 | 367 | 533 | 666 |
| 3 × 3 m | 100 mm | 0.99 m³ | 78 | 59 | 85 | 106 |
| 3 × 3 m | 150 mm | 1.48 m³ | 117 | 88 | 127 | 159 |
| 4 × 4 m | 100 mm | 1.76 m³ | 139 | 104 | 151 | 188 |
| 4 × 4 m | 150 mm | 2.64 m³ | 208 | 156 | 226 | 282 |
Includes the calculator's 10% extra for spillage and uneven ground. US bag yields are the published ones (60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³); metric bag yields are worked out from the same figure and are approximate, so check your bag.
| Mix (cement : sand : stone) | Concrete | Cement | 50 kg bags | 25 kg bags | Sand | Stone / crush |
|---|---|---|---|---|---|---|
| 1 : 1.5 : 3 (about M20) | 1 m³ | 403 kg | 8.1 | 16.1 | 0.42 m³ | 0.84 m³ |
| 1 : 1.5 : 3 (about M20) | 100 ft³ | 1,142 kg | 22.8 | 45.7 | 42 ft³ | 84 ft³ |
| 1 : 2 : 4 (about M15) | 1 m³ | 317 kg | 6.3 | 12.7 | 0.44 m³ | 0.88 m³ |
| 1 : 2 : 4 (about M15) | 100 ft³ | 897 kg | 17.9 | 35.9 | 44 ft³ | 88 ft³ |
| 1 : 3 : 6 (about M10) | 1 m³ | 222 kg | 4.4 | 8.9 | 0.46 m³ | 0.92 m³ |
| 1 : 3 : 6 (about M10) | 100 ft³ | 628 kg | 12.6 | 25.1 | 46.2 ft³ | 92.4 ft³ |
| 1 : 4 : 8 (very lean) | 1 m³ | 171 kg | 3.4 | 6.8 | 0.47 m³ | 0.95 m³ |
| 1 : 4 : 8 (very lean) | 100 ft³ | 483 kg | 9.7 | 19.3 | 47.4 ft³ | 94.8 ft³ |
Parts by volume, with the standard dry-volume factor of 1.54 and cement at 1,440 kg/m³. Bags are shown to one decimal; for a real job the calculator adds your extra and rounds up to whole bags. Sand and stone are loose volumes as delivered; ft³ is the cft used in Pakistan and India.
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.
- Use clean, sharp sand and washed stone; silt and clay in the sand weaken the concrete.
- Keep new concrete damp for at least 7 days (curing) so it reaches its strength.
- 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 many cement bags for 1 m³ of 1:2:4 concrete?
About 6.3 bags of 50 kg (so buy 7), with 0.44 m³ (15.5 ft³) of sand and 0.88 m³ (31 ft³) of stone. A 1 : 1.5 : 3 mix needs about 8.1 bags.
Why is the dry volume 1.54 times the concrete?
Loose sand and stone have air gaps. When mixed with water, the sand fills the gaps between the stones and the cement paste fills the gaps in the sand, so the mix shrinks by about a third. The 1.54 factor is the standard figure in quantity surveying.
How thick should a concrete slab be?
Garden paths and shed bases are commonly 75–100 mm (3–4 in); driveways, garage floors and roof slabs more. Follow local guidance or an engineer for anything that carries a load.
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Sources
- Quikrete published bag yields (40 lb ≈ 0.30 ft³, 50 lb ≈ 0.375 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³)
- Nominal mixes and the 1.54 dry-volume factor: standard quantity-surveying practice (IS 456 nominal mixes); cement bulk density 1,440 kg/m³.
Formulas last reviewed: September 30, 2026 ·How we check our formulas