How this calculator works
This calculator runs our concrete, gravel and rebar calculators on one set of measurements. The slab volume is the area times the thickness, with your waste allowance; the gravel base is the area times the base depth with 20% extra because loose stone compacts; the rebar is a grid at 16 inch centers; and the form boards and stakes come from the perimeter.
Worked example: a 12 × 12 ft patio poured 4 inches thick needs 1.96 cubic yards of concrete with 10% extra (88 bags of 80 lb), about 3.0 tons of crushed stone for a 4 inch base, 12 twenty-foot lengths of #3 rebar, and 6 eight-foot form boards with 20 stakes.
Materials for common patio sizes (4 in slab, 4 in base)
10% waste on the concrete, 20% on the gravel.
| Patio | Concrete | 80 lb bags | Gravel | Rebar bars | Form boards |
|---|---|---|---|---|---|
| 10 × 10 ft | 1.36 yd³ | 62 | 2.1 tons | 9 | 5 |
| 12 × 12 ft | 1.96 yd³ | 88 | 3.0 tons | 12 | 6 |
| 12 × 16 ft | 2.61 yd³ | 118 | 4.0 tons | 16 | 7 |
| 16 × 20 ft | 4.35 yd³ | 196 | 6.6 tons | 26 | 9 |
| 20 × 20 ft | 5.43 yd³ | 245 | 8.3 tons | 33 | 10 |
Tips before you buy
- Decide how you will pour. A slab over about a yard of concrete is too much to mix by hand before it starts to set: order ready-mix and have two or three helpers.
- Slope the patio about 1/4 inch per foot away from the house, and keep the finished surface below the door threshold.
- Cut or tool control joints every 8 to 10 feet, so that cracks follow the joints.
- Avoid pouring in heavy rain or near freezing, and keep the slab damp or covered for about a week so that it cures slowly.
- Call 811 before you dig, and ask your building department whether the patio needs a permit or a setback.
Tools & materials for this project
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Frequently asked questions
How much concrete do I need for a 12x12 patio?
A 12 × 12 ft patio 4 inches thick needs 1.78 cubic yards before waste, and 1.96 cubic yards with 10% extra. Order ready-mix to the next quarter yard.
How many bags of concrete for a 12x12 patio?
80 bags of 80 lb mix with no waste, or 88 with 10% extra. That is too much to mix by hand before it sets: most people order ready-mix.
Does a concrete patio need a gravel base?
Yes. A 4 inch compacted gravel base gives the slab even support and lets water drain. For a 12 × 12 ft patio it is about 3.0 tons of crushed stone, and 4.5 tons for a 6 inch base.
Does a patio slab need rebar?
Reinforcement does not stop concrete cracking, but it holds the cracks tight. A #3 grid at 16 inch centers takes 12 bars of 20 ft for a 12 × 12 ft patio, or 3 sheets of wire mesh.
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Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.