Concrete Calculator
Work out how much concrete a slab needs, in cubic yards and cubic metres, and how many bags to buy.
Last updated: June 20, 2026 · Reviewed by TG we-Calculate Editorial TeamUnits
m
m
cm
2 m³ • 70.6 ft³
2 m³
2.62 yd³- 1
Thickness in metres
10 cm ÷ 100 = 0.1 - 2
Slab volume
5 × 4 × 0.1 m = 2 - 3
Cubic yards
2 m³ ÷ 0.7646 = 2.62
How much concrete do I need for a slab?
Multiply the slab length, width and thickness using the same units, then convert to cubic yards or cubic metres. For example, a 16 ft × 13 ft slab 4 inches thick is about 2.6 cubic yards (2.0 m³). Divide by a bag yield (an 80 lb bag ≈ 0.022 yd³) to estimate bags.
Formula
How this is calculated
A slab is treated as a simple rectangular box, so its volume is length × width × thickness. The three inputs are the slab footprint (length and width) and its depth (thickness). To multiply them safely all three must be in the same unit. In metric mode length and width are entered in metres and thickness in centimetres; the thickness is divided by 100 to convert it to metres, then the three are multiplied to give cubic metres directly.
In imperial mode length and width are in feet and thickness is in inches; the thickness is divided by 12 to get feet, the product gives cubic feet, and that is divided by 27 (since 27 ft³ = 1 yd³) to get cubic yards. Cubic yards convert to cubic metres using the exact factor 1 yd³ = 0.7646 m³.
Bag counts divide the slab volume by each bag's published yield (about 0.60, 0.45 and 0.30 ft³ for 80, 60 and 40 lb bags) and round up. The estimate assumes a flat, uniform slab and ignores reinforcement, waste and over-excavation, so add roughly 5–10% extra when ordering.
Examples
| Input | Result |
|---|---|
| 5 m × 4 m slab, 10 cm thick | 2.00 m³ ≈ 2.62 yd³ (~118 × 80 lb bags) |
| 16 ft × 13 ft slab, 4 in thick | ~2.57 yd³ ≈ 1.96 m³ |
| 10 ft × 10 ft slab, 4 in thick | 1.23 yd³ ≈ 0.94 m³ (~56 × 80 lb bags) |
About this calculator
A concrete slab is a rectangular box, so its volume is simply length × width × thickness. The only trick is keeping units consistent: in metric the thickness is entered in centimetres and converted to metres before multiplying, giving a result in cubic metres. In imperial the thickness is entered in inches and converted to feet, giving cubic feet, which is divided by 27 to get cubic yards (since 27 ft³ = 1 yd³).
Concrete is usually ordered by the cubic yard from a ready-mix truck or, for small jobs, mixed from dry bags. Published yields are roughly 0.6 ft³ for an 80 lb bag, 0.45 ft³ for a 60 lb bag and 0.30 ft³ for a 40 lb bag, so this tool divides the slab volume by those yields and rounds up. Add about 5–10% extra for spillage, uneven sub-base and over-excavation.
Frequently asked questions
Roughly 45 × 80 lb bags, 60 × 60 lb bags or 90 × 40 lb bags, based on yields of about 0.6, 0.45 and 0.30 ft³ per bag respectively. Bagged mixing only makes sense for small slabs.
Yes. Add about 5–10% to cover spillage, an uneven sub-base and slight over-excavation, since running short on a pour is far worse than a small surplus.
A cubic metre is larger: 1 cubic yard equals 0.7646 cubic metres, and 1 cubic metre equals about 1.308 cubic yards. Ready-mix is sold by the yard in the US and by the metre elsewhere.
Also known as
Sources & references
TG we-Calculate Editorial Team. (2026). Concrete Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/concrete-calculator
TG we-Calculate Editorial Team. "Concrete Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/concrete-calculator.
TG we-Calculate Editorial Team, "Concrete Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/concrete-calculator
@misc{wecalculate_concrete_calculator, title = {Concrete Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/concrete-calculator}}, year = {2026}, note = {TG we-Calculate} }
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