Short answer: one cubic yard of concrete takes 45 bags of 80 lb or 60 bags of 60 lb premix. One cubic metre takes about 86 bags of 25 kg (or 54 bags of 40 kg). A 10 × 10 ft slab, 4 inches thick, needs 62 bags of 80 lb with 10 % extra.
Step 1: the volume
Length × width × thickness, all in the same unit:
- Feet: 10 ft × 10 ft × 4 in (0.333 ft) = 33.3 ft³ = 1.23 yd³ (divide ft³ by 27).
- Metres: 3 m × 3 m × 0.10 m = 0.9 m³.
Add 10 % for uneven ground, spillage and the bit left in the mixer: 1.23 yd³ becomes 1.36 yd³.
Step 2: bags from the volume
What a bag makes is printed on it (the "yield"). The usual figures for dry premix:
| Bag | Yield per bag | Bags per cubic yard | Bags per m³ |
|---|---|---|---|
| 40 lb | 0.30 ft³ | 90 | 118 |
| 60 lb | 0.45 ft³ | 60 | 79 |
| 80 lb | 0.60 ft³ | 45 | 59 |
| 20 kg | about 9.4 L | 82 | 107 |
| 25 kg | about 11.7 L | 66 | 86 |
The pound sizes are the makers' published yields; the kilogram sizes are worked out from the same density (about 2,136 kg of dry mix per m³ of concrete), so check the figure on your own bag.
Bags of concrete for common slabs
Worked out by the concrete calculator, with the 10 % extra already in:
| 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.
Mixing your own: cement, sand and stone
For bigger jobs, mixing cement, sand and gravel yourself is cheaper than premix bags. The amount of cement depends on the mix:
| 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.
A 1 : 2 : 4 mix (one part cement, two parts sand, four parts stone) is the usual all-round mix for paths, bases and slabs at home; 1 : 1.5 : 3 is stronger, for footings and structural work. Your local building rules or engineer decide what a structural slab needs.
When to order ready-mix instead
Around 1 cubic yard (or 1 m³), mixing 45–90 bags by hand is a long, heavy day. Ready-mix from a truck is often worth it from that size up — ask your supplier about their minimum order and short-load fee. The concrete calculator gives the order in yd³ or m³ as well as in bags.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
45 bags. Each 80 lb bag makes about 0.60 ft³, and a cubic yard is 27 ft³.
How many bags of concrete for a 10 × 10 slab?
At 4 inches thick: 62 bags of 80 lb or 82 bags of 60 lb, including 10 % extra. At 6 inches: 92 bags of 80 lb.
How many 25 kg bags of concrete per m³?
About 86 bags of dry premix (85.4 before rounding). Add your extra on top: with 10 % it is 94 bags.
How many bags of cement for 1 m³ of concrete?
For a 1 : 2 : 4 mix about 317 kg: 6.3 bags of 50 kg or 12.7 bags of 25 kg. For a 1 : 1.5 : 3 mix about 403 kg. The cement calculator also works out the sand and stone.
How much gravel do I need under the slab?
A compacted base of 4 in (100 mm) is usual. Work out the tonnes with the gravel calculator.
Calculators in this guide
Enter your own sizes for an exact answer.
- Concrete calculatorVolume, bags of pre-mix, or cement, sand and crush for a slab, footing or column.
- Cement calculator50 kg cement bags, sand and crush (bajri) for concrete mixed on site — 1:2:4, 1:1.5:3 or your own mix.
- Gravel, sand & soil calculatorVolume, tonnes and bags of gravel, sand, topsoil or mulch.
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