How to Pour a Concrete Slab: A Step-by-Step Guide

1. Plan and check before you dig

Before you buy a single bag, pin down the basics. Decide the slab's purpose (patio, shed, walkway), its finished size, and its thickness. Four inches is the typical thickness for patios, walkways, and shed floors. Anything that will carry vehicles or heavy loads usually goes thicker — and that is where a quick call to the building department pays for itself.

Call 811 before you dig. In the US, 811 is the free national “Call Before You Dig” number. Utility locators will come out and mark buried gas, electric, water, and communications lines on your property, usually within a few business days. Digging into a gas line or a primary electric cable is not a mistake you get to make twice — and the locate service is free.

While you are at it, check whether your project needs a permit. Requirements vary by location, but slabs above a certain size or any structural work commonly need one. A short phone call now is cheaper than a stop-work order later.

2. Lay out the slab

Mark the slab's outline with stakes and string, or spray the corners with marking paint. The classic 3-4-5 trick keeps corners square: measure 3 ft along one side from a corner, 4 ft along the adjacent side, and adjust until the diagonal between those points is exactly 5 ft. Check all four corners the same way.

Slope matters. A slab that holds water will spall, grow algae, and make your patio miserable. Set your forms so the finished surface slopes gently away from the house — about 1/8 inch per foot is typical guidance, which works out to about an inch of drop over an 8-foot run.

3. Excavate and build the gravel base

Dig out the area to the full depth you need: slab thickness plus the base. For a 4-inch slab on a 4-inch gravel base, that is 8 inches of excavation below the finished surface level. Strip off the topsoil and any soft organic material first — topsoil compresses over time and a slab poured over it will settle with it.

Spread 4 inches of compactible gravel or crushed stone (not round pea gravel — you want angular material that locks together) and compact it in lifts with a hand tamper or a rented plate compactor. The base should be flat, level with your planned slope, and firm enough that you cannot easily shift it with your boot. This base does three jobs: it spreads the load, drains water away from the slab, and gives you a stable platform to set forms on.

4. Set the forms

Forms are usually 2×4 lumber (which actually measures 3.5 inches tall — close enough to 4 inches for a typical slab, or rip lumber to exact height) or 2×6s for thicker slabs. Set the form boards on edge along your string lines, stake them on the outside every 2–3 feet, and screw through the stakes into the boards so nothing moves when the concrete pushes against them. Wet concrete is heavy — roughly 150 pounds per cubic foot — and it will blow out weak forms without hesitation.

Double-check the slope with a level before you move on. Once the concrete is in the forms, there is no adjusting.

5. Reinforcement: wire mesh and rebar basics

Concrete is strong in compression and weak in tension, so reinforcement is what keeps it from pulling apart as it shrinks and settles. For a typical small slab, welded wire mesh is common practice for crack control. Rebar (usually #4 bar, 1/2-inch diameter, on a grid spacing) is more typical where loads or spans are higher.

Whichever you use, it must sit in the concrete — about the middle of the slab's thickness, not lying on the gravel. Lay the mesh or rebar on chairs or dobies (small concrete or plastic supports) so it stays elevated while you pour. Reinforcement sitting on the ground does nothing; this is the single most common reinforcement mistake DIYers make.

Plan your control joints while you are at it. Control joints are shallow grooves (cut with a groover while the concrete is still wet, or sawn later) that give shrinkage cracks a planned place to form instead of wandering across your slab. Typical guidance: one joint every 8–10 feet in each direction for a 4-inch slab, and never let a single panel get much longer than it is wide.

6. Order the concrete

Run your numbers before pour day. For bagged concrete, use our concrete bags calculator to get exact counts for 40, 60, and 80 lb bags with a waste allowance — remember to add 5–10% extra for uneven subgrade and spillage. For a 10×10 ft slab at 4 inches, that is about 62 eighty-pound bags with waste, which is a lot of mixing by hand.

As a rule of thumb, bagged concrete makes sense up to about 1–2 cubic yards. Beyond that, call a local batch plant for a ready-mix quote — it is usually cheaper per yard and you skip a full day of mixing. If you are not sure which route is cheaper, our concrete calculator gives you the volume in cubic yards, which is exactly what a batch plant will ask for.

On pour day, have everything ready before the truck arrives or before you mix your first bag: wheelbarrows, screed board, floats, trowels, groover, and at least one helper. Concrete waits for no one.

7. Pour, screed, and finish

Start at the far end and work back toward your exit so you never trap yourself. Fill the forms slightly proud of the top, then screed: rest a straight 2×4 across the form tops and saw it back and forth while pulling it along to level the concrete. Work in one pass and resist the urge to keep re-screeding — every pass pushes water and fine material to the surface.

After screeding, float the surface with a magnesium or wood float to push down the aggregate and smooth the surface. Then wait. The critical finishing mistake is working the surface while bleed water is still on top — troweling too early traps water under the surface and weakens the top layer. When the surface sheen disappears and the concrete can hold your weight on kneeboards without sinking more than a quarter inch, cut your control joints and do the final finish: a light broom pass for a patio (traction without roughness) or a smooth trowel finish for a floor that will get paint or epoxy.

8. Cure properly — the step everyone skips

Curing is just keeping the concrete moist while it gains strength. Concrete that dries out in the first few days never reaches its full strength, and no amount of finishing skill compensates for it. Cover the slab with wet burlap, plastic sheeting, or a curing compound, and keep foot traffic off for at least 24–48 hours. Typical guidance is to wait longer — a week or more — before driving on it or loading it heavily. A month gets you to roughly full design strength.

Common mistakes to avoid

• Adding too much water to make the mix flow easier. It weakens the concrete. Use a stiffer mix and a vibrator or rod to settle it instead.
• No control joints. Concrete will crack as it cures; joints decide where.
• Reinforcement on the ground. Mesh or rebar buried under the slab's surface, sitting on the gravel, is decoration.
• Troweling too early. Working the surface while bleed water sits on top traps weak material at the surface.
• Skipping curing. Letting the slab bake dry for the first few days caps its strength permanently.
• Under-ordering. Running out mid-pour creates a cold joint — a visible, weaker seam. Add 5–10% waste and round up.

Ready to plan your pour?

Get your material quantities right first: the concrete bags calculator gives exact bag counts with waste for slabs, walls, footings, and post holes, and the concrete calculator gives cubic-yard volumes for ready-mix quotes. Measure twice, pour once.

Frequently asked questions