How to pour a concrete slab: a step-by-step plan with the numbers

1. Plan the size and thickness

Decide the footprint and how thick the slab will be. Quikrete’s standard concrete mix is for pours 2 in thick or more; patios and walks are commonly around 4 in. Remember that forms built from 2×4s on edge are 3½ in deep, not 4. Then work out the volume — a 10 × 10 ft slab at 4 in is 1.23 cubic yards before waste. Worked examples: how much concrete for a 10×10 slab, how many bags of concrete for a 4×4 slab and how much concrete for a 10×12 shed.

2. Call before you dig

Call 811 in the US (or your local locate service) a few business days before digging, even for a shallow slab. Check with your building department whether the slab needs a permit.

3. Prepare the base

Strip sod and topsoil, dig to the depth of the slab plus the base, and compact the soil. Most slabs sit on a few inches of compacted gravel — crusher run packs hard, #57 stone drains well. Compact it in lifts with a plate compactor, and set its top exactly one slab-thickness below the planned top of the concrete. For quantities, see how much gravel under a concrete slab.

4. Build the forms

Set form boards on edge around the slab, staked every few feet and screwed together at the corners. Check the top edges with a level and string: for a patio next to a house, slope them away from the wall so water drains. Brace the forms well — wet concrete pushes hard — and check that the slab will be square by measuring the diagonals.

5. Add reinforcement if your plans call for it

Steel and fibers don’t stop a slab cracking — every slab shrinks a little — but they hold cracks tight so the pieces stay level, while control joints decide where cracks form. Welded wire mesh is common in patios, walks and shed floors; rebar where plans call for more strength, such as driveways and thickened edges; and fiber-reinforced mixes reduce small shrinkage cracks. Many small 4 in pads use mesh or a fiber mix, and some use neither. For a structure, a hot tub or vehicles, ask your building department (and the shed or spa maker) what they require.

Rebar or welded wire mesh goes on chairs so it ends up inside the slab, not on the gravel. The counts are worked through on how much rebar for a 20×20 slab and how much wire mesh for a slab. Whether a slab needs steel, and how much, is a design decision.

6. Order the concrete

Add 5–10% to the volume and decide between bags and a truck. Up to about a yard, bags and a rented mixer are reasonable for two people; beyond that a ready-mix truck is usually far less work. See ready-mix vs bagged concrete and, for big pours, how many yards of concrete for a 24×24 slab.

7. Pour, screed and float

1. Dampen the base (no puddles) so it doesn’t suck water from the concrete.
2. Place the concrete to full depth, starting at the far end, working continuously. Don’t spread it thin and top it up later — that leaves a weak layer.
3. Strike it off level with the forms using a straight screed, moving it back and forth like a saw.
4. Float the surface right away with a bull float (big slabs) or a magnesium hand float to push stones down and close the surface.
5. Run an edger along the forms to round the edges, and cut control joints with a groover — Quikrete suggests about 1 in deep every 6–8 ft.

8. Finish

Let the concrete stiffen slightly. Quikrete says to wait until all the water has evaporated from the surface before troweling or applying a broom finish, and not to overwork the material. Then trowel for a smooth finish or drag a broom across for grip. Resist sprinkling water on a stiffening surface to make it easier to finish.

9. Cure it

Curing is what turns a hard slab into a strong one. Keep it moist for 5 days at 70 °F or warmer, 7 days between 50 and 70 °F (Quikrete), with a curing compound, plastic sheeting or a sprinkler. Keep it from freezing for at least 48 hours. Light foot traffic is fine once it has hardened; heavier use waits — see how long before you can walk on concrete. In a cold snap, see can you pour concrete in cold weather before you book the pour.

10. Seal (optional)

Once cured, a sealer helps resist stains and de-icing salt. Coverage runs roughly 150–225 sq ft per gallon depending on the product and texture — see how much sealer for a concrete driveway.

Tools checklist

• Tape, string line, level, stakes and form boards
• Plate compactor (rental) or hand tamper
• Mixer and wheelbarrow, or a clear path for the truck chute
• Screed, bull float or magnesium float, edger, groover, finishing trowel or broom
• Waterproof gloves, boots and eye protection