How it works
A slab is a rectangular prism, so volume is length × width × thickness. The only complication is that thickness is quoted in different units from the plan dimensions — inches against feet, or millimetres against metres — so it has to be converted before multiplying.
Bag counts come from the yield printed on the bag, not from its weight. Dry mix yields less volume than you would expect, because the aggregate packs down once water is added. Typical yields are 0.45 ft³ for a 60 lb bag, 0.60 ft³ for an 80 lb bag, and about 9 litres for a 20 kg bag — check your supplier's figure, as it varies by mix.
Bag counts are always rounded up. Two-thirds of a bag is not something you can buy, and running short mid-pour means a cold joint.
Worked example
A 12 ft × 10 ft shed slab, 4 inches thick, with 10% wastage.
- Convert thickness to feet: 4 ÷ 12 = 0.333 ft.
- Multiply out: 12 × 10 × 0.333 = 40 ft³.
- Add 10% wastage: 40 × 1.10 = 44 ft³.
- Convert to cubic yards: 44 ÷ 27 = 1.63 yd³.
Order 1.63 yd³, or about 74 eighty-pound bags if mixing on site.
Common questions
Should I order ready-mix or use bags?
Bags stop being practical somewhere around half a cubic yard — beyond that you are mixing dozens of them against the clock, and the pour will not be monolithic. Most ready-mix suppliers have a minimum load, so ask before assuming it is available for a small job.
How thick should a slab be?
Four inches (100 mm) is typical for a shed base or footpath; six inches (150 mm) for a driveway carrying vehicles. Anything structural should follow an engineer's specification rather than a rule of thumb — this calculator works out quantity, not adequacy.
Why does the bag count seem so high?
Because yield is volume, not weight. An 80 lb bag produces only about 0.6 ft³ of wet concrete, so even a modest slab needs a surprising number. That arithmetic is exactly why people underestimate this job.