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Concrete Calculator
Enter the dimensions of a slab, footing or round post hole to get the concrete you need in cubic yards and cubic feet, the number of 40, 60 or 80 lb premix bags, and what it costs in bags or as ready-mix. A waste allowance is built in so you do not run short mid-pour.
- Volume in cubic feet
- 36.67 cu ft
- Premix bags
- 62
- Weight of cured concrete
- 5,500 lb
- Cost in bags
- $403.00
- Cost as ready-mix
- $217.28
Saved setups
Save a set of inputs you reuse — your usual rate, your loan, your room sizes — and load it back in one tap.
Your recent calculations
Results you calculate here are kept on this device so you can come back to them.
Formula
How to use it
- Choose slab, footing or round column.
- Enter the dimensions; thickness and diameter are usually easiest in inches.
- Set the quantity if you have several identical pours, such as fence post holes.
- Keep 5–10% waste unless your forms are exact, then read cubic yards and bags.
Worked examples
A 10 × 10 ft slab, 4 inches thick, no waste allowance
- Volume in cubic feet
- 33.33 cu ft
- Concrete needed
- 1.23 yd³
- Premix bags
- 56
- Weight of cured concrete
- 5,000 lb
- Cost in bags
- $364.00
- Cost as ready-mix
- $197.53
A 40 ft footing, 16 in wide and 8 in deep, with 10% extra, in 60 lb bags
- Volume in cubic feet
- 39.11 cu ft
- Concrete needed
- 1.45 yd³
- Premix bags
- 87
Eight post holes 10 in across and 3 ft deep, in 60 lb bags
- Volume in cubic feet
- 13.09 cu ft
- Concrete needed
- 0.48 yd³
- Premix bags
- 30
Bags or a truck?
One cubic yard is 45 bags of 80 lb mix or 60 bags of 60 lb mix. Hand-mixing more than about a yard is hard, slow work, and the first batch may be setting before the last is placed. Above roughly 1 to 1.5 cubic yards most people order ready-mix; many plants have a minimum load or a short-load fee, so ask.
Cured concrete weighs about 150 lb per cubic foot, or roughly 4,050 lb per cubic yard, which matters if you are hauling bags in a pickup.
Typical thicknesses
Walkways and patios are usually 4 in thick. Driveways are 4 in for cars and 5–6 in where heavier vehicles park. A slab formed with 2×4 lumber is 3½ in thick, not 4 in, so enter 3.5 if that is how you are forming it.
For post holes the common rule is a hole three times the post width and one third to one half of the above-ground post height deep, below the frost line where that applies.
Questions people ask
How many bags of concrete do I need for a 10×10 slab?
A 10 × 10 ft slab 4 in thick is 33.3 cubic feet, or 1.23 cubic yards. That is 56 bags of 80 lb mix or 75 bags of 60 lb mix before any waste allowance.
How many 80 lb bags make a cubic yard?
45. An 80 lb bag yields about 0.60 cubic feet and a cubic yard is 27 cubic feet. For 60 lb bags it is 60, and for 40 lb bags it is 90.
How much concrete for a fence post hole?
A 10 in wide hole 3 ft deep holds 1.64 cubic feet with no post in it, which is about three 80 lb bags or four 60 lb bags. The post takes up some of that space, so this slightly overestimates.