Concrete Slab Calculator

How to measure

Measure the finished slab, not the hole. Length and width are the outside dimensions of the forms, taken at the form faces. If the slab is not a rectangle, split it into rectangles, calculate each one and add them together with the Pour List.

Thickness is the number people guess. Measure the depth of the form boards, then check the depth of the base at several points. A base that dips 1 in below the forms makes the slab 25% thicker at that spot on a 4 in design, and all of that extra is concrete you pay for. Use the average depth and let the waste allowance absorb the rest.

The slab formula

The volume of a slab is a box: length × width × thickness. Work in feet so the answer comes out in cubic feet, and that means converting the thickness from inches by dividing by 12. Cubic yards are cubic feet divided by 27, because a yard is 3 ft and 3 × 3 × 3 = 27.

Then add waste. This calculator defaults to 10%, which covers an uneven base, slight form bulge and the concrete that stays in the truck chute and the wheelbarrow. A very flat, well-compacted base with rigid forms can get by on 5%.

Formula: Volume (ft³) = L × W × (T ÷ 12) × quantity. Order (yd³) = Volume ÷ 27 × (1 + waste%).

Worked example: a 12 × 16 ft slab at 4 in

A 12 × 16 ft patio slab at 4 in thick is the classic size for a table, chairs and a grill. Here is the full calculation, using exact fractions:

1. Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Length 12 ft; Width 16 ft; Thickness 4 in = 0.3333 ft
2. Volume = 12 × 16 × 0.3333 × 1 = 64 ft³
3. Net volume = 64 ft³ ÷ 27 = 2.3704 yd³ (× 0.0283168 = 1.8123 m³)
4. Order quantity = 2.3704 yd³ × (1 + 10%) = 2.6074 yd³ = 70.4 ft³
5. 80 lb bags = 70.4 ft³ ÷ 0.6 ft³ per bag = 117.33 → 118 bags (always round up)
6. Weight = 70.4 ft³ × 150 lb/ft³ = 10,560 lb

So you order about 2.61 yd³. As bags that is 118 bags of 80 lb mix, which is a lot of mixing, so it is a short-load truck order or a pump-free wheelbarrow pour with helpers.

How slab thickness changes the order

Thickness is the cheapest variable to get wrong, because every extra inch spreads across the whole footprint. The table shows a 10 × 10 ft slab at four thicknesses with no waste added.

Going from 4 in to 6 in adds half again as much concrete, not just a little. If you are deciding between thicknesses for a particular use, read the slab thickness guide. For a driveway, the driveway calculator starts at 5 in.

Common mistakes when calculating a slab

• Rounding 4 in to 0.33 ft before multiplying. Use 4 ÷ 12 exactly. On a 24 × 24 ft slab the rounded number is about 1% short.
• Measuring to the lot line or the house wall instead of the form edges.
• Forgetting that thickened edges, footings and turn-downs are extra concrete. Add those with the footing calculator.
• Ignoring the base. A low spot of 1 in across a quarter of the slab adds roughly 6% by itself.
• Ordering exactly the net number. Running short mid-pour leaves a cold joint you cannot undo.

Bags or truck for a slab?

Under 1 yd³ is a bagged job. Between 1 and 2 yd³ it is worth comparing prices. Above 2 yd³ ready-mix wins on labor and quality. A 10 × 10 ft, 4 in slab with 10% waste is 1.36 yd³, or 62 bags of 80 lb. That is about 2.5 tons of bags to lift and mix, so the cost per hour of effort matters as much as the price per bag.

Small truck orders often carry a short-load fee, usually quoted on anything below a few yards. The cost calculator finds the break-even between the two routes using your own quotes.

Before you order

• Add reinforcement: see the rebar calculator for grid spacing and weight.
• Price the base too with the gravel base calculator.
• Place control joints at about 24 to 36 times the slab thickness, which is 8 to 12 ft for a 4 in slab (ACI 302.1R). The control joints guide covers layout.
• Use the Project Kit under the result to carry your dimensions to the rebar, gravel and cost tools.

Frequently asked questions