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How to pour a concrete slab (DIY basics)

Last reviewed 2026-06-06

A slab is unforgiving — you can’t pause a pour. Here is how to prep, pour and finish one, and how much concrete to order.

A concrete slab for a shed, patio or path is a very doable DIY job — but it is unforgiving, because once you start pouring you cannot stop. The secret is doing all the prep first so the pour itself is quick and calm.

1. Excavate and lay a base

Dig out the area and put down a compacted sub-base of crushed stone (typically 4 inches), levelled and tamped. A solid base stops the slab cracking and settling.

2. Build the forms

Frame the slab with straight timber set to your finished height, braced with stakes. Check it is level (or has a slight fall for drainage on outdoor slabs) and square by measuring the diagonals.

3. Decide the thickness and add reinforcement

Four inches is standard for patios and shed bases; 5–6 inches for anything carrying vehicles. Add wire mesh or rebar for strength on larger or load-bearing slabs.

4. Order the right amount

Run the length, width and thickness through the concrete calculator to get the cubic yards and bag count. Order 5–10% extra — you cannot top up a pour cleanly. Above about a cubic yard, ready-mix delivery beats mixing bags by hand.

5. Pour, screed and float

Pour into the forms, working it into the corners. Screed off the excess by dragging a straight board across the form tops. Once the surface water (bleed water) disappears, float it smooth, then add a broom finish for grip if it is outdoors.

6. Cure it slowly

Concrete gains strength as it cures, not just as it dries. Keep it damp (cover with plastic or mist it) for several days. It will be walkable in a day or two but reaches most of its strength over about a month.

Tip: footings and posts

Setting posts as part of the project? The post hole concrete calculator works out the bags per hole, accounting for the space the post takes up.

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