How to use the concrete calculator
- Pick the shape. Use Slab / patio for driveways, sidewalks, shed bases and patios. Use Footing / wall for long narrow pours. Use Round column for fence and deck post holes or sonotube piers.
- Enter the dimensions. Note the units next to each box. Slab thickness, footing width and depth, and column sizes are in inches.
- Set the quantity. Pouring 12 identical post holes? Enter the size of one and a quantity of 12.
- Keep a waste allowance. 5% to 10% covers spillage, an uneven base and form bulging. The default is 10%.
Concrete volume formula
Concrete is sold by the cubic yard. Work out the volume in cubic feet, then divide by 27 (there are 27 cubic feet in one cubic yard).
Column: π × (diameter in ÷ 24)² × height (in) ÷ 12 = cubic feet
Cubic yards = cubic feet ÷ 27
How many bags of concrete do I need?
Premixed concrete bags yield roughly the following volumes once mixed with water:
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 40 lb | 0.30 cu ft | 90 |
Yields vary a little by brand and mix type, so check the bag. For projects over about 1.5 cubic yards, a ready-mix truck is normally cheaper and gives a stronger, more consistent pour than mixing dozens of bags by hand.
Worked examples
10 × 10 ft patio, 4 inches thick
10 × 10 × 4 ÷ 12 = 33.3 cubic feet. Add 10% waste for 36.7 cubic feet, which is 1.36 cubic yards, or about 62 bags of 80 lb mix.
12 fence post holes
Each hole is 10 inches wide and 36 inches deep: 3.1416 × (10 ÷ 24)² × 3 = 1.64 cubic feet. Twelve holes need 19.6 cubic feet, or about 21.6 with waste. That is 36 bags of 80 lb mix. Remember the post itself takes up some space, so this is a safe estimate.
A 24 × 24 ft garage slab, 4 inches
24 × 24 × 4 ÷ 12 = 192 cubic feet, or 7.1 cubic yards. With 10% waste, order about 7.8 cubic yards of ready-mix.
Recommended thickness
| Project | Typical thickness |
|---|---|
| Sidewalk, patio, shed floor | 4 in |
| Driveway for cars | 4 to 5 in |
| Driveway for heavy trucks or RVs | 6 in |
| Garage floor | 4 to 6 in |
| Hot tub pad | 4 to 6 in |
Local building codes and soil conditions may require more. Check with your building department for structural work such as footings and foundations.
Tips before you pour
- Compact a 4 inch gravel base under slabs. Our gravel calculator works out how much base you need.
- Measure the actual depth of the forms in several places. Ground that dips by an inch can add 25% more concrete on a 4 inch slab.
- Order ready-mix a little over your estimate. Running short mid-pour leaves a cold joint that is weaker and visible.
Frequently asked questions
How many bags of concrete are in a yard?
About 45 bags of 80 lb mix, 60 bags of 60 lb mix, or 90 bags of 40 lb mix make one cubic yard of concrete.
How much concrete do I need for a 10x10 slab?
A 10 by 10 ft slab at 4 inches thick needs 33.3 cubic feet, or 1.23 cubic yards. With 10% extra for waste, plan for about 1.36 cubic yards or 62 bags of 80 lb concrete mix.
How do I calculate cubic yards of concrete?
Multiply length by width in feet, multiply by the thickness in inches, divide by 12 to get cubic feet, then divide by 27 to get cubic yards.
Is it cheaper to buy bags or ready-mix concrete?
For small jobs under about 1 to 1.5 cubic yards, bags are usually cheaper. Above that, ready-mix delivery is normally cheaper per yard and saves many hours of mixing, although some suppliers charge a short-load fee for small orders.
How much extra concrete should I order?
Add 5% to 10% to cover spillage, uneven subgrade and forms that bow outward. Use the higher number for uneven ground or hand-dug holes.
How thick should a concrete driveway be?
A residential driveway for cars is usually 4 to 5 inches thick. Use 6 inches if heavy trucks, RVs or equipment will park on it.
More home project calculators
Last updated October 2026.