How the concrete calculator works
A slab is a simple box, so its volume is length times width times thickness. The only traps are units: thickness is usually quoted in centimetres or inches while the plan is measured in metres or feet, so everything is converted to one unit before multiplying. A waste allowance is then added for the sub-base that is never perfectly flat, the formwork bowing slightly and the concrete left in the mixer and wheelbarrow.
Volume = length × width × thickness × (1 + waste ÷ 100) · Bags = volume ÷ yield per bagThe bag count relies on the yield of dry premix. Bagged concrete makes roughly one cubic metre of placed concrete for every 2,130 kg of dry mix, which is the same as 133 lb per cubic foot. That figure gives the yield of metric bags (11.7 L for 25 kg, 18.8 L for 40 kg). For US bags the calculator uses the yield printed on the bag itself: 0.60 ft³ for 80 lb, 0.45 ft³ for 60 lb and 0.30 ft³ for 40 lb. The result is always rounded up, because a part bag cannot be bought.
Worked example
A garden shed base of 4 m × 3 m poured 10 cm thick holds 4 × 3 × 0.10 = 1.2 m³. With a 10% allowance you plan for 1.32 m³. At 2,130 kg per cubic metre that is about 2,812 kg of dry mix, so 2,812 ÷ 25 = 112.5, which rounds up to 113 bags of 25 kg, or 71 bags of 40 kg. In imperial units the same approach applied to a 12 × 10 ft pad 4 inches deep gives 40 ft³, or 44 ft³ (1.63 yd³) once the 10% is added: 44 ÷ 0.60 = 73.3, so 74 bags of 80 lb.
Yield of standard premixed bags
| Bag size | Approximate yield | Bags per m³ | Bags per yd³ |
|---|---|---|---|
| 25 kg | 11.7 L (0.41 ft³) | 85 | 65 |
| 40 kg | 18.8 L (0.66 ft³) | 53 | 41 |
| 40 lb | 8.5 L (0.30 ft³) | 117 | 90 |
| 60 lb | 12.8 L (0.45 ft³) | 78 | 60 |
| 80 lb | 17.0 L (0.60 ft³) | 59 | 45 |
Check the yield printed on your bag: mixes with larger aggregate or added fibres can differ by a few percent. The figures above are for bags only and exclude any water you add.
Bags or ready-mix?
Hand-mixing 45 bags of 80 lb for a single cubic yard is a long, heavy day, and a large slab should be placed in one continuous pour so it does not set in layers. As a rough guide, bags suit jobs up to about half a cubic metre (two-thirds of a yard); from about one cubic metre upward, a ready-mix delivery or a mixer rental is usually the sensible choice. Ask the supplier about minimum loads, since small orders often carry a surcharge.
Tips before you pour
- Measure depth in several places. A sub-base that dips by 2 cm across a 4 m slab adds about 20% to the volume of a 10 cm pour.
- Do not include the hardcore. The compacted gravel under the slab is a separate material; size it with the gravel calculator.
- Plan control joints. A common guide is to space joints two to three times the slab thickness in inches, expressed in feet: a 4 in slab gets a joint every 8 to 12 ft.
- Load-bearing work needs a design. Foundations for walls, garages or anything carrying a structure must follow local building codes and, where required, an engineer's specification.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
A 10 × 10 ft slab 4 inches thick holds 33.3 ft³ (1.23 yd³). With a 10% margin that is 36.7 ft³: 36.7 ÷ 0.60 = 61.1, so 62 bags of 80 lb, or 82 bags of 60 lb at 0.45 ft³ each.
How many 80 lb bags of concrete make a cubic yard?
Each 80 lb bag yields about 0.60 ft³, so a cubic yard (27 ft³) takes 45 bags, or roughly 50 once you add a 10% allowance.
How many 25 kg bags of concrete are in a cubic metre?
About 85 bags of 25 kg make one cubic metre, or 53 bags of 40 kg. Add 5–10% for waste.
How thick should a concrete patio or driveway be?
Patios, paths and shed bases are usually 100 mm (4 in) thick on a compacted gravel base. Driveways for cars are usually 125–150 mm (5–6 in); heavier vehicles need a thicker, reinforced slab designed to local rules.
How much does a cubic yard of concrete weigh?
Cured normal concrete weighs about 2,400 kg per cubic metre, which is roughly 4,000 lb per cubic yard. That is why a single yard is a full load for a small trailer.
Last reviewed: 2026-09-27 · How we calculate