How this is calculated
Volume = length × width × thickness, with all three converted to the same unit. Thickness in inches is divided by 12 to become feet, then the result in cubic feet is divided by 27 to give cubic yards.
Worked example on a slab 10 feet by 10 feet at 4 inches thick: thickness is 0.333 feet, so the volume is 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards. Adding 5% for waste gives about 1.3 cubic yards.
Concrete weighs roughly 4,000 lb per cubic yard, so that slab is close to 5,200 lb — about 2.4 tonnes. Worth knowing before assuming a wheelbarrow and an afternoon will cover it.
Bags or ready-mix?
An 80 lb bag of premix yields about 0.6 cubic feet, so roughly 45 bags make a cubic yard. A 60 lb bag yields about 0.45 cubic feet, needing around 60 bags per cubic yard.
The 1.23 cubic yard example above is therefore about 58 bags of 80 lb premix — well over two tonnes of material to carry, open and mix, and more than most people can place before the first batches begin to set.
Ready-mix delivery generally becomes competitive on cost somewhere around one cubic yard and is unambiguously better above two, both on price and on the quality of the result. Suppliers usually have a minimum load and a short-load surcharge, so it is worth asking for the total delivered price rather than the per-yard rate.
Why slabs fail, and it is rarely the concrete
The usual causes are inadequate base preparation and placing too much material in one pour to finish properly. A compacted sub-base of gravel matters more to a slab's life than a small increase in thickness.
Concrete gains most of its strength in the first week but continues curing for far longer, and keeping it damp during that period substantially improves the outcome. Rapid drying causes surface cracking, which is why slabs poured in hot sun without protection so often craze.
Control joints are the other omission. Concrete will crack as it shrinks; joints decide where. Cutting them at intervals of roughly 24 to 30 times the slab thickness gives the cracking somewhere to go that is not across the middle of your slab.
How to use the concrete calculator
- Enter length and width in feet. Measure the actual formed area, not the nominal size — forms are rarely exactly where you planned.
- Enter thickness in inches. Four inches is standard for a footpath or patio, five to six for a driveway. Note the unit differs from length and width.
- Compare bag count against ready-mix. Above about one cubic yard, check a delivered price. The bag count at that volume is usually more work than it first appears.