How much waste should I allow?

Ten percent is standard for a flat slab on a well prepared subgrade. Allow 15 percent for footings, trenches, or anything dug by hand, because excavations are always slightly larger than drawn.

How thick should a slab be?

Four inches suits patios, sheds, and footpaths. Five to six inches with reinforcement is normal for driveways and anything carrying vehicles. Local building codes may set a minimum, and a heavy load or poor soil calls for engineering advice.

How long before I can walk or drive on it?

Foot traffic is usually fine after 24 to 48 hours. Vehicles should wait 7 days, and concrete reaches its specified strength at 28 days. Keeping the surface damp for the first week significantly improves the final strength.

Do I need reinforcement?

Steel mesh or rebar controls cracking rather than preventing it. Use it for driveways, any slab over about 10 square metres, and anywhere the ground may move. Control joints cut at intervals of roughly 24 to 30 times the slab thickness give cracks a place to go.

Concrete Estimator

Calculate concrete bags for a slab

Length (ft)

Width (ft)

Depth (in)

Volume

1.23 cubic yards

33.3 cubic feet

60 lb bags

80 lb bags

Add 10% extra for waste and spillage

Quick Facts

•1 cubic yard = 27 cubic feet
•A 60 lb bag yields ~0.45 cu ft
•An 80 lb bag yields ~0.6 cu ft
•4" depth is standard for patios

How to Estimate Concrete

Concrete is measured in cubic yards. For a slab, calculate length × width × depth (in feet), then divide by 27 to convert to cubic yards. Always order 10% extra to account for waste, uneven ground, and spillage. A standard 4-inch thick slab is suitable for patios and walkways.

Understanding the concrete estimator

Concrete is ordered by volume, so the whole job is a volume calculation plus a waste allowance. Getting it wrong in either direction is expensive: too little means a cold joint mid-pour, too much means paying for concrete you have to dispose of.

Worked examples

Concrete for a 10 ft by 12 ft slab, 4 inches thick

1Convert thickness to feet: 4 ÷ 12 = 0.333 ft.
2Volume = 10 × 12 × 0.333 = 40 cubic feet.
3Convert to cubic yards: 40 ÷ 27 = 1.48 cu yd.
4Add 10 percent for spillage and uneven subgrade: 1.63 cu yd.

Order 1.75 cubic yards, since suppliers sell in quarter yard steps.

Concrete for four post holes, 300 mm across and 900 mm deep

1Radius = 0.15 m, so area = π × 0.15² = 0.0707 sq m.
2Volume per hole = 0.0707 × 0.9 = 0.0636 cu m.
3Four holes = 0.255 cu m.
4Subtract the post volume if it is substantial, then add 10 percent: about 0.28 cu m.

About 0.28 cubic metres, which is roughly 14 bags of 20 kg ready mix per hole set.

Slab volume by area and thickness

Cubic yards required before the waste allowance.

Area	4 in thick	5 in thick	6 in thick
50 sq ft	0.62	0.77	0.93
100 sq ft	1.23	1.54	1.85
200 sq ft	2.47	3.09	3.70
400 sq ft	4.94	6.17	7.41
600 sq ft	7.41	9.26	11.11

Bagged concrete yield

Bag size	Yield	Bags per cubic yard
40 lb	0.30 cu ft	90
60 lb	0.45 cu ft	60
80 lb	0.60 cu ft	45
20 kg	0.010 cu m	100 per cu m
25 kg	0.0125 cu m	80 per cu m

Above about one cubic yard, ready mix delivery is usually cheaper and far faster than mixing bags.

Frequently asked questions

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This calculator is for educational and reference purposes only. Always verify critical calculations independently.