How much concrete do I need?
Multiply length x width x thickness in feet to get cubic feet, then divide by 27 for cubic yards. Thickness is the step people get wrong: 4 inches is 0.33 ft, not 0.4. Add about 10% for uneven ground, form bowing and spills.
Example: a 12' x 12' patio at 4" is 12 x 12 x 0.333 = 48 cu ft, or 1.78 yd³. With 10% extra that's 1.96 yd³ — about 88 bags of 80 lb mix, or a 2-yard ready-mix order.
How many bags make a yard?
| Bag | Yield per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | about 0.60 cu ft | 45 |
| 60 lb | about 0.45 cu ft | 60 |
| 40 lb | about 0.30 cu ft | 90 |
These are the yields printed on standard concrete mix bags. High-strength and fast-setting mixes are close but check the bag.
Bags or a ready-mix truck?
Under about a cubic yard, bags are simplest. Between one and three yards it depends on your price and patience — that's 45 to 135 bags to mix by hand, while a small truck load often carries a short-load fee. Above three yards, order ready-mix: you can't mix and place that many bags before the first ones start to set. The calculator rounds a truck order up to the nearest quarter yard.
How thick should a slab be?
Walks and patios are usually 4 inches. Driveways for cars are 4 to 5 inches, and 6 inches for pickups, RVs or heavy use. Slabs go on a compacted gravel base — typically 4 inches — which drains water and spreads the load.
Rebar, mesh and control joints
A grid of rebar (often #3 or #4 at 12 to 24 inches) or welded wire mesh helps hold a cracked slab together; it doesn't stop cracks. Control joints decide where cracks happen. A common rule is joint spacing in feet of about 2.5 times the slab thickness in inches, so every 10 feet for a 4" slab. Cut them a quarter of the slab's depth.
Post holes and footings
For a fence post, dig a hole about three times the post's width and about a third of its length deep, and subtract the post itself from the volume — the calculator does both. Deck and structural footings must go below your local frost line and may need a permit.
Building a patio? Pair this with the tile calculator or plan the room next to it with the flooring and paint calculators.