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Common questions

How do I calculate how much concrete I need?

Multiply length by width by thickness in the same unit to get the volume, then convert: cubic feet divided by 27 is cubic yards. A 10 by 10 ft slab 4 inches thick is 10 × 10 × 0.333 = 33.3 cu ft, or 1.23 cubic yards, and about 1.36 yards with 10% for waste. The calculator does the unit conversion and the rounding for you.

How many bags of concrete make a yard?

A cubic yard is 27 cubic feet, so it takes 45 bags of 80 lb (0.60 cu ft each), 60 bags of 60 lb or 90 bags of 40 lb. In metric, about 107 bags of 20 kg make a cubic metre. Check the yield on your bag, since mixes differ a little.

How much concrete is in a post hole?

A round hole is a cylinder: π times the radius squared times the depth. A 12 inch hole 4 ft deep holds 3.14 cu ft, about 0.12 yards, which is six 80 lb bags before waste. Choose Column and type the diameter and depth; use How many for several posts.

Why do I need to add extra?

Ground is never flat, forms spread, some mix stays in the barrow and trenches are dug a little wide. 10% covers a prepared base; rough ground can need 15 to 20%. Running short mid-pour leaves a cold joint, which is worse than a few spare bags.

How much does a yard of concrete weigh?

Normal-weight concrete is about 150 lb per cubic foot, so a cubic yard weighs roughly 4,050 lb, a little over two US tons, and a cubic metre about 2,400 kg. The weight shown is for ordering and for checking what a trailer or barrow can carry.

Should I order ready-mix or buy bags?

Below about half a yard, bags are cheaper and simpler. Around a yard it is about 45 bags of 80 lb to mix by hand, which is a long day; a short-load ready-mix service or a towable mixer is usually worth pricing. Type a price per yard and per bag to compare.

Is this calculator free?

Yes, completely free, with no sign-up. The numbers are worked out in your browser and never sent to us.