Concrete calculator
Pick what you are pouring and enter its size. You get cubic feet, cubic yards, the ready-mix order and the number of bags in every common size, with waste included.
Inputs
Result
| Bag | Bags | Weight to haul |
|---|
How far a cubic yard goes
Area one cubic yard covers at each thickness, and the 80 lb bags it takes to pour 100 sq ft.
| Thickness | Sq ft per cubic yard | 80 lb bags per 100 sq ft |
|---|
How the volume is worked out
A 10 by 10 ft slab 4 in thick is 10 x 10 x 4 / 12 = 33.3 cu ft, or 1.23 cubic yards. With 10% waste that is 36.7 cu ft: 1.36 yards, 62 bags of 80 lb, or 82 bags of 60 lb.
Steps are worked out from the side, as a staircase of blocks. Each step is as tall as all the risers below it plus its own, so a three-step stoop with 7 in risers is one block 7 in tall, one 14 in and one 21 in, the top one as deep as the landing.
Bags or ready-mix?
An 80 lb bag of Quikrete concrete mix makes about 0.6 cu ft, so a cubic yard takes 45 of them: 3,600 lb to lift, mix and place. Most people switch to ready-mix somewhere around a yard. Plants sell in quarter or half yards and often add a short-load charge to small orders, so ask before you book. The calculator rounds the ready-mix figure up to the next quarter yard.
Why add waste
Forms bow, the subgrade is never perfectly flat, and some concrete stays in the wheelbarrow and the truck chute. Running short in the middle of a pour leaves a cold joint, which is far worse than a few leftover bags. Use 5% for small, well-formed pours and 10% for slabs on rough ground.
Questions
How many bags of concrete are in a yard?
A cubic yard is 27 cu ft. At the Quikrete yields of 0.6 cu ft for an 80 lb bag and 0.45 cu ft for a 60 lb bag, that is 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb.
How much concrete do I need for a 10x10 slab?
At 4 in thick, 33.3 cu ft or 1.23 cubic yards. With 10% waste, order 1.5 yards of ready-mix or 62 bags of 80 lb. At 6 in thick, 1.85 yards before waste.
How many bags of concrete per fence post?
A 10 in hole 36 in deep around a 4x4 holds about 1.38 cu ft of concrete, which is 2.3 bags of 80 lb, so three bags with a little waste. A 12 in hole the same depth takes about 2.1 cu ft, or four bags.
How thick should a concrete slab be?
4 in for patios, walks and shed floors; 5 to 6 in for driveways that carry cars, and more with reinforcement for heavy trucks. A thicker slab on a poorly compacted base still cracks, so the base matters as much.
How much does a yard of concrete weigh?
About 3,900 lb, or nearly two tons, at the 145 lb per cubic foot of normal-weight concrete. That is why a full ready-mix truck carries no more than about 10 yards.
How many square feet does a yard of concrete cover?
81 sq ft at 4 in thick, 65 at 5 in and 54 at 6 in. The table on this page has the full range.