Concrete Slab Cost Calculator

How to measure a concrete slab

Mark out the slab with stakes and string, then measure the length and width inside the planned forms. Decide on the thickness before you order: 4 inches (about 10 cm) is common for patios, walkways and shed bases on well-compacted ground, while slabs that carry vehicles are usually thicker. Your local building code, your contractor or an engineer has the final say, especially for garages and driveways.

Check how level the ground is. If the subgrade dips, the slab will be thicker in those spots and you will use more concrete than the simple volume suggests. That is the main reason to add extra concrete. If the slab has a thickened edge or integrated footings, calculate those separately with the concrete calculator and add them to the total.

The concrete slab formula

In metric, multiply length × width × thickness, all in metres, for cubic metres. For bagged premix, divide the volume by the yield of one bag. The calculator uses the yields commonly printed on US bags of concrete mix: 0.30 cu ft for a 40 lb bag, 0.45 cu ft for 60 lb and 0.60 cu ft for 80 lb. For metric bags (20, 25 and 30 kg) it applies the same yield per kilogram, so a 25 kg bag gives roughly 0.0117 m³ (11.7 litres). Yields vary slightly between products, so check the bag.

Worked example

A 12 × 12 ft patio slab, 4 inches thick, with 10% extra:

1. Area: 12 × 12 = 144 sq ft.
2. Volume: 144 × (4 ÷ 12) = 48 cu ft, or 48 ÷ 27 = 1.78 yd³.
3. With 10% extra: 1.78 × 1.10 = 1.96 yd³ (52.8 cu ft).
4. Ready-mix order rounded up to the nearest quarter yard: 2 yd³.
5. Bagged alternative: 52.8 ÷ 0.6 = 88 bags of 80 lb (or 118 bags of 60 lb, or 176 of 40 lb).
6. With the example prices of $175 per yard and $6.50 per 80 lb bag, ready-mix comes to $350 and bags to $572, before labor and other costs.

In metric, a 3.6 × 3.6 m slab, 10 cm thick, holds 1.30 m³. With 10% extra that is 1.43 m³, ordered as 1.5 m³ of ready-mix, or about 122 bags of 25 kg.

How much extra concrete to add

Most people add 5% to 10%. Use the higher end when the subgrade is uneven, the forms are long or flexible, or you are pumping concrete. The cost of a small leftover is minor compared with running short in the middle of a pour, which can leave a weak cold joint. Ready-mix suppliers often deliver in increments such as a quarter or half yard (or cubic metre), so the calculator rounds the order up to the nearest quarter.

Estimating the cost of a concrete slab

A slab's cost is more than the concrete. This calculator separates the pieces so you can enter your own numbers rather than rely on a national average:

• Concrete: ready-mix is priced per cubic yard or cubic metre and depends on your region, the mix strength and any additives such as fibres or accelerators. Many suppliers add a short-load fee for small orders, so ask for the delivered price for your quantity.
• Bagged mix: priced per bag. It avoids delivery and short-load fees but takes far more labor to mix and place, and it is hard to keep a large pour consistent.
• Labor and finishing: contractors often quote per square foot or square metre for forming, placing, finishing and curing. Enter their rate in the labor field.
• Other costs: excavation, a compacted gravel base, form boards and stakes, rebar or wire mesh, a vapor barrier, a pump, sealer and permits. Enter them as a lump sum.

The default prices are examples only. They are there so the calculator shows working numbers, not as a claim about what concrete costs where you live. Get at least two quotes for anything beyond a small DIY pad.

Bags or ready-mix?

As a rough guide, bags make sense for small, scattered pours such as post holes, steps and small pads. For a single slab above about one cubic yard (0.75 m³), the number of bags climbs quickly: 45 bags of 80 lb for every cubic yard. At that point ready-mix delivery, a mini-mix trailer or a rented mixer usually saves a great deal of effort, and gives a more consistent slab. Whatever you choose, have enough helpers and tools on hand to place and finish the concrete before it begins to set.

Do not forget the base

Most exterior slabs sit on a compacted layer of crushed stone or road base, often around 4 inches (10 cm) deep, which provides drainage and a stable surface. Use the gravel calculator with the crusher-run preset to work out how much base material to order, and add its cost to the "other costs" field.