DIY Home CalcsConcrete Calculator
Work out how much concrete to buy for a slab, patio, footing or round post hole. You get the volume in cubic yards and cubic meters, plus the number of premixed bags in every common size.
How to calculate concrete
Concrete is ordered by volume. For a rectangle, multiply length × width × thickness, keeping every measurement in feet. Thickness is usually measured in inches, so divide it by 12 first. Divide cubic feet by 27 to get cubic yards, the unit ready-mix suppliers use.
cu yd = cu ft ÷ 27 | bags = cu ft × (1 + waste) ÷ bag yield
For a round hole the diameter is in inches, so dividing by 24 turns it into a radius in feet. The calculator then multiplies by the number of slabs or holes and adds your waste allowance.
Worked example: a 10 × 10 patio
- Volume: 10 × 10 × (4 ÷ 12) = 33.33 cu ft.
- In cubic yards: 33.33 ÷ 27 = 1.23 cu yd.
- With 10% waste: 36.67 cu ft, or 1.36 cu yd.
- In 80 lb bags: 36.67 ÷ 0.60 = 61.1, so buy 62 bags.
Sixty-two 80 lb bags weigh almost 2.5 tons. That is a lot of hand mixing, which is why a slab this size is often a good candidate for a short-load ready-mix delivery.
Bag yields
| Bag size | Yield per bag | Bags per cubic yard | Bags for one 10 in × 3 ft post hole |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 6 |
| 60 lb | 0.45 cu ft | 60 | 4 |
| 80 lb | 0.60 cu ft | 45 | 3 |
Yields are the typical figures printed on standard premixed concrete bags. Fast-setting and high-strength mixes can differ slightly, so check the bag. Post-hole figures include 10% waste and ignore the volume of the post itself, which gives you a small safety margin.
Common slab thicknesses
- 4 in: patios, sidewalks, garden paths, shed and hot-tub pads (check the tub's weight).
- 5–6 in: driveways and garage floors that carry cars and light trucks.
- Footings: sized by local building code, often at least 12 in wide and below the frost line.
Tips before you pour
- Measure the depth of your forms in several places. A slab that averages 4.5 in instead of 4 in needs 12.5% more concrete.
- Compacted gravel under the slab is not part of the concrete volume. Use the gravel calculator for the base.
- Have one or two extra bags on hand even when you order ready-mix, for filling low spots.
- Read bagged concrete vs ready-mix to decide which is right for your job.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
A 10 ft × 10 ft slab at 4 inches thick is 33.3 cu ft, or about 1.23 cu yd. That is 56 bags of 80 lb mix before waste, or about 62 bags with a 10% allowance.
How much does an 80 lb bag of concrete make?
A standard 80 lb bag of premixed concrete yields about 0.60 cu ft. A 60 lb bag yields about 0.45 cu ft and a 40 lb bag about 0.30 cu ft. It takes 45 of the 80 lb bags to make one cubic yard.
How thick should a concrete slab be?
Four inches is standard for patios, walkways and shed floors. Driveways and slabs that carry vehicles are usually 5 to 6 inches. Check local code and your project's load before you pour.
How much extra concrete should I order?
Add 5% to 10%. Ground that is not perfectly level, forms that bow, and spillage all eat concrete, and running short in the middle of a pour is far worse than having a little left over.
When should I order ready-mix instead of bags?
Bags are practical for small jobs up to roughly one cubic yard. Beyond that, mixing 45+ bags by hand is hard work and slow, so a ready-mix truck is usually cheaper and gives a stronger, more uniform pour. See our guide to bagged concrete vs ready-mix.