🧑‍🔧 Pour a Concrete Patio or Slab

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Every calculator used here is tested against independent reference values · how PhDino checks its numbers

Work out how much concrete and gravel to order, how to batch the concrete if you mix it yourself, and how much rebar to place, in the order you'd actually plan the pour.

A DIY slab or patio pour comes down to a handful of quantities most people guess at: how much concrete to order, how much gravel goes under it, how to batch the concrete if you're mixing your own instead of ordering ready-mix, and how much reinforcing steel to lay down first. Each one depends on the same basic slab dimensions, so it makes sense to work them out in sequence rather than separately.

This is a materials-quantity and batching sanity-check, not a structural design. Slab thickness, rebar size, and spacing for anything load-bearing (a garage slab, a driveway) should still come from your local code or a span table rather than guessed defaults, especially anywhere frost depth or vehicle loading is a factor.

The order of work, and where the numbers fit

Concrete is unforgiving in a way most building materials are not. Once the truck arrives, or the first batch is wet, you have a limited window to place and finish it, an hour or two in mild weather and less in heat, so every quantity has to be settled before the day. The usual sequence is to excavate and grade, compact a gravel base, set the forms, lay the steel on supports, then pour, screed, float, finish and cure.

The calculators cover the four things you must buy or batch: the concrete, the gravel under it, the ingredients if you mix it yourself, and the reinforcing bar. They do not cover forms, finishing, joints or curing, which is where most of the skill in a good slab lies, so the sections below spell those out.

What sets the thickness, strength and steel

These choices come before any calculator, because they are the inputs.

• Thickness. Four inches is a common choice for patios and walks that carry foot traffic; driveways that carry cars are commonly thicker, about five to six inches. Your local code or a supplier will name the minimum for your use.
• Base. A compacted gravel layer of four to six inches keeps the slab off soft or frost-heaving soil and lets water drain away from underneath it.
• Strength and air. For an exposed slab where the ground freezes, ready-mix is typically ordered as air-entrained concrete of roughly 3,500 to 4,500 psi. A volume mix such as 1:2:3 is a rough recipe, not a strength guarantee.
• Steel. Reinforcement limits how wide cracks open; it does not stop concrete cracking. Whether you need it depends on the slab's use, and where it is used it has to sit around mid-depth on supports, because bars lying on the ground do nothing.
• Joints. Slabs shrink as they cure, so they are cut with control joints to tell the cracks where to go. A common guide is to space them at about two to three times the thickness in feet (eight to twelve feet for a four-inch slab) and to cut them a quarter of the thickness deep.

Steps

1. 1📦Concrete Volume
   Start with the slab's footprint: length, width, and thickness, plus a waste margin. This returns the volume in cubic yards, the unit ready-mix is ordered in. You can run it a second time for the compacted gravel base under the slab, which is a volume of stone rather than concrete. Hit "📋 Copy Values" on the slab run: the next two steps reuse its dimensions and net volume.
   🧮 Open the Concrete Volume calculator
2. 2🪣Concrete Mix Design
   If you're mixing your own instead of ordering ready-mix, "📥 Paste Values" fills in the net volume from Step 1. Add your mix ratio and water-cement ratio, and this returns the bags of cement, the sand and gravel, and the mixing water. Paste the net volume, then raise it by your waste margin, or type the with-waste figure yourself (cubic yards × 27 gives cubic feet).
   🧮 Open the Concrete Mix Design calculator
3. 3🔗Rebar Sizing
   "📥 Paste Values" fills in the slab's length and width from Step 1. Add your bar size and spacing to get the bar count and the total steel weight for a two-way grid across the slab.
   🧮 Open the Rebar Sizing calculator

Worked example: a 12 × 10 ft patio, 4 in thick

The patio is 12 ft by 10 ft and 4 in thick, on a 6 in compacted gravel base. The plan is to hand-mix it in a rented mixer at a 1:2:3 volume ratio with a water-cement ratio of 0.5, and to reinforce it with #3 bar on a 12 in grid. The job is to work out what to buy, and to test whether hand-mixing a slab this size is even sensible.

What it adds up to

The shopping list that came out is about 1.6 yd³ of concrete, or 12 bags of cement with the sand, gravel and 64 gal of water if you really do mix by hand, about 2.9 yd³ of gravel for the base, and roughly 288 ft of #3 bar on chairs. The hand-mix numbers are also the argument for a truck.

What the calculators cannot know is the state of the ground and the day itself: a uniform, compacted base, forms that hold their line, joints cut on time, and a week of damp curing. Those decide whether a correctly quantified slab is a good slab, and the sections below cover them.

Notes & limitations

• This treats the slab as one flat rectangular pour. An irregular shape (an L-shaped patio, a slab with a step) needs to be broken into simpler pieces, with each one run through separately.
• Whether a slab needs reinforcing steel at all (and how much) depends on its use. A purely decorative garden path is a very different question from a slab meant to carry a vehicle or support a structure. Check your local code's minimum before skipping the rebar step.

Where hand-batched concrete goes wrong

Most weak or cracked home-made slabs trace back to one of these, not to the arithmetic above.

• Adding water to make the mix easier to place. The calculator's water is for a ratio of 0.5. Raising it to 0.6 adds 13 gal to this pour, and the price is weaker, more porous concrete that shrinks and cracks more and dusts on the surface.
• Forgetting that damp sand already carries water. The figure is the total, so hold back part of what you add from the bucket when the sand is wet.
• Letting the ratio drift. Measure every batch with the same bucket, level-full, rather than by eye.
• Trusting the plan instead of the hole. An excavation is never at one depth, so probe the base in several places before you finalize the order.
• Skipping the cure. Concrete gains strength while there is water in it, so keep the surface damp, or sealed with a curing compound, for about a week.
• Skipping the joints. A slab that is not cut will crack where it likes.

When to order ready-mix, and when to call a professional

Order ready-mix once the pour is more than a yard or so, when it has to be placed in one continuous go, or when the strength and air content matter, as they do for anything outdoors where the ground freezes. Give the supplier the with-waste volume and the use, and let them specify the mix.

Bring in a professional for driveways and garage floors, for any slab that carries a vehicle or a structure, for expansive or poorly draining soils, and for slabs against the house, where the slope, the drainage and the joint between slab and wall all matter. Those are structural and site questions that a quantity calculator cannot answer.

Frequently asked questions