How to calculate concrete
Concrete is sold by volume, so every concrete estimate comes down to one number: cubic feet (for bags) or cubic yards (for ready-mix trucks). The calculator converts your measurements to feet, multiplies them, adds a waste allowance, then divides by the yield of the bag size you choose.
Cubic yards = cubic feet ÷ 27
Round column (cu ft) = π × (diameter in ÷ 24)² × height (ft)
Worked example: a 10 × 10 ft patio, 4 in. thick
- Convert thickness to feet: 4 ÷ 12 = 0.333 ft.
- Volume: 10 × 10 × 0.333 = 33.3 cu ft.
- Add 10% waste: 33.3 × 1.10 = 36.7 cu ft.
- Cubic yards: 36.7 ÷ 27 = 1.36 cu yd.
- 80 lb bags: 36.7 ÷ 0.60 = 61.1, round up to 62 bags.
At that size, a ready-mix truck is worth pricing out. Sixty-plus bags is roughly 5,000 lb of material to haul, mix and place before it starts to set.
How many bags of concrete per cubic yard?
Bag yields below are the typical figures printed on standard premixed concrete (such as Quikrete and Sakrete) sold at US home centers. High-strength and fast-setting mixes can yield slightly differently, so check the bag.
| Bag size | Yield per bag | Bags per cubic yard | Bags per 10 sq ft at 4 in. |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 11.1 |
| 50 lb | 0.375 cu ft | 72 | 8.9 |
| 60 lb | 0.45 cu ft | 60 | 7.4 |
| 80 lb | 0.60 cu ft | 45 | 5.6 |
Per pound, the bag sizes cost roughly the same, so the choice usually comes down to what you can lift. An 80 lb bag means fewer trips to the mixer; 40 and 60 lb bags are easier on your back and easier to fit in a car.
Recommended slab thickness
| Project | Typical thickness |
|---|---|
| Walkway, patio, shed floor | 4 in. |
| Hot tub pad, light garage floor | 4–5 in. |
| Residential driveway | 5–6 in. |
| Driveway with heavy vehicles (RV, truck) | 6 in. or more |
These are common rules of thumb. Footings, structural slabs and anything attached to a house are governed by local building codes, which often specify depth below the frost line, rebar and minimum strength. Check with your local building department before you dig.
Bags vs. ready-mix: which should you buy?
- Under about 1/2 cubic yard (fence posts, small pads, steps): bags are the clear choice.
- About 1/2 to 1 cubic yard: bags still work, but rent a portable mixer and plan on help.
- Over 1 cubic yard: get a ready-mix quote. A truck delivers consistent concrete in one continuous pour, which means fewer cold joints and a stronger slab. Ask about short-load fees and whether you need a pump or wheelbarrow access.
Tips for measuring
- Measure the inside of your forms, not the outside.
- Check thickness in several spots. A subgrade that dips 1 in. over a 10 × 10 slab adds about 8 cu ft, or 14 more 80 lb bags.
- For irregular shapes, split the area into rectangles, calculate each one and add them together, or use our square footage calculator first.
- For the gravel base under a slab, use the gravel calculator. A 4 in. compacted base is common.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
About 45. Each 80 lb bag yields roughly 0.60 cubic feet, and a cubic yard is 27 cubic feet (27 ÷ 0.60 = 45).
How many bags of concrete do I need for a 10×10 slab?
At 4 inches thick, a 10×10 slab is 33.3 cubic feet. That works out to 56 bags of 80 lb concrete with no waste, or about 62 bags with a 10% allowance.
How much concrete do I need for a fence post?
A 10-inch-diameter hole that is 3 feet deep holds about 1.6 cubic feet. Subtract the post (a 4×4 takes up about 0.26 cu ft over 3 feet), and you need around 1.4 cubic feet, or about four 50 lb bags of fast-setting concrete. Our fence calculator does this for a whole fence line.
Should I add extra concrete for waste?
Yes. Plan on 5–10% extra. Running short during a pour leaves a cold joint, which is a weak spot. Leftover bags can be returned or saved for small repairs.
How much does a cubic yard of concrete weigh?
Normal-weight concrete weighs about 4,000 lb (2 tons) per cubic yard once cured, or roughly 145–150 lb per cubic foot.
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