How the concrete calculator works
Concrete is measured by volume. The calculator turns your measurements into cubic feet, adds extra for waste, then converts the total into bags or cubic yards.
Slab: length (ft) × width (ft) × thickness (in) ÷ 12
Footing: length (ft) × width (in) ÷ 12 × depth (in) ÷ 12
Round hole: 3.1416 × (diameter ÷ 2 ÷ 12)² × depth (in) ÷ 12 × number of holes
Cubic yards = cubic feet ÷ 27
Bag yields
Bagged concrete mix makes less finished concrete than its weight suggests. These are the standard yields printed on most bags; check yours, as specialty mixes can differ.
| Bag size | Yield per bag | 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 |
Worked example
A 10 by 10 foot patio, 4 inches thick: 10 × 10 × 4 ÷ 12 = 33.3 cubic feet. Adding 10% gives 36.7 cubic feet, which is 1.36 cubic yards or 62 bags of 80 lb mix. That is a lot of bags to mix by hand, so the calculator suggests ready-mix delivery for jobs over 1 cubic yard.
Measuring for a pour
- Measure inside the forms, not the outside of the boards.
- Check the depth in several places. If the ground dips, use the average depth, or the deepest point if you want to be safe.
- For an L-shaped patio, split it into two rectangles, calculate each, and add the bags together.
- For fence posts, a common rule is a hole about three times the post width and deep enough to reach below your area's frost line. Ask your local building department for the frost depth.
Bags or ready-mix?
| Job size | Usually best |
|---|---|
| Under 1/3 cubic yard (up to about 15 bags) | Bags, mixed in a wheelbarrow or tub |
| 1/3 to 1 cubic yard (15 to 45 bags) | Bags with a rented drum mixer and a helper |
| Over 1 cubic yard | Ready-mix delivery; ask about short-load fees |
Before you pour
- Most patios and walkways sit on a compacted gravel base of about 4 inches. Use the gravel calculator for it.
- Have everything ready before the first bag is mixed: forms set, base compacted, tools and water at hand.
- Do not pour on frozen ground or when frost is expected; follow the temperature range on the bag.
- Keep the surface damp, or cover it, for the curing time the manufacturer recommends.
Common questions
How many 80 lb bags of concrete make a yard?
One cubic yard is 27 cubic feet, and an 80 lb bag yields about 0.60 cubic feet, so it takes 45 bags. With 60 lb bags (0.45 cu ft) it takes 60, and with 40 lb bags (0.30 cu ft) it takes 90.
How many bags for a 10x10 slab 4 inches thick?
A 10 by 10 foot slab at 4 inches is 33.3 cubic feet. With 10% extra that is 36.7 cubic feet, or 62 bags of 80 lb mix. At about 1.4 cubic yards, ready-mix delivery is usually the easier choice.
When should I order ready-mix instead of bags?
Around 1 cubic yard (45 bags of 80 lb mix) is where most people switch. Mixing that many bags by hand is slow, and the first part of a slab can start setting before you finish. Some suppliers charge a short-load fee for small orders, so ask.
How thick should a concrete slab be?
4 inches is typical for patios, walkways and shed floors. Driveways and areas that carry vehicles are usually 5 to 6 inches. Local codes and soil conditions can require more, so check with your building department.
How much extra concrete should I buy?
Plan for 5 to 10% extra. Ground is rarely perfectly flat, forms bow a little, and running short mid-pour leaves a weak joint. The calculator adds 10% unless you change it.