Concrete Slab Calculator

How to use the concrete slab calculator

1. Enter the slab size as a rectangle or a circle. Add a row for each separate slab, or use How many for identical ones.
2. Choose the thickness by use (patio, driveway or garage) or pick Custom thickness.
3. Keep the gravel base switched on to estimate the compacted stone under the slab, and set its depth.
4. Check the allowance, the bag size and any prices, then press Calculate.

Concrete slab formula

cubic yards = length (ft) × width (ft) × thickness (in) ÷ 324

A 20 ft × 10 ft driveway pad at 5 inches is 20 × 10 × 5 ÷ 324 = 3.09 cubic yards. Add 5–10% for an uneven subgrade and spillage, then round up to the next quarter yard for ready-mix, or divide the cubic feet by the bag yield for bags.

Worked example: a 12 × 12 ft patio

A 12 ft × 12 ft patio is poured 4 inches thick over 4 inches of compacted road base. Ready-mix costs $175 a yard and gravel $40 a ton:

1. Slab 1: 12 ft × 12 ft = 144 ft²; × 4 in thick = 48 ft³
2. Total: 48 ft³ = 1.778 yd³
3. With allowance: 1.778 yd³ × 1.1 = 1.956 yd³
4. Bags: 52.8 ft³ ÷ 0.6 ft³ per bag = 88 → buy 88 × 80 lb bags
5. Gravel base: 144 ft² × 4 in × 1.15 × 1.5 tons/yd³ = 3.07 tons

Concrete needed: 1.96 yd³; Ready-mix to order: 2 yd³; Or bags: 88 × 80 lb bags; Ready-mix cost: $350.00; Gravel base: 3.07 tons; Gravel base cost: $140.00. Open this example in the calculator.

Concrete per 100 square feet

How much concrete 100 ft² of slab takes at common thicknesses, before any allowance:

How thick should a concrete slab be?

• 4 in (10 cm) is the usual thickness for patios, walkways, shed bases and hot tub pads that sit on well-compacted ground.
• 5 in (12–13 cm) is common for residential driveways carrying cars.
• 6 in (15 cm) or more for garage floors, RV pads and anywhere heavier vehicles or loads will sit.
• Thicken the edges of driveways and patios (often to 8 in around the perimeter) where they carry the most load.

These are common starting points, not a design. Soil, loads, reinforcement and local codes decide the right thickness; check them before you pour.

The gravel base under a slab

Slabs last longer on a compacted base of crushed stone or road base, which spreads the load and lets water drain away. 4 inches is typical under patios and walkways, 6 inches or more under driveways and on soft or clay soil. The calculator adds 15% to the base volume because road base compacts when it is rolled or plate-compacted. Order the base and the concrete together so the base can be laid and compacted before the pour.

Reinforcement and control joints

• Rebar or wire mesh helps hold cracks tight. A common pattern for driveways is #3 or #4 bar on an 18–24 in grid set on chairs mid-depth; follow your plans or local practice. The rebar calculator estimates bars, ties and weight.
• Control joints are cut or tooled about a quarter of the slab’s depth, at spacing of roughly 2–3 times the thickness in feet (8–12 ft for a 4 in slab), so the slab cracks along them instead of at random.

Ordering concrete for a slab

• Order the allowance. Subgrades always have low spots; 5–10% extra avoids a short load.
• Ready-mix for most slabs. Anything over about a cubic yard is far easier from a truck than from bags. Tell the supplier the use (driveway, patio) so they can suggest a mix.
• Plan the pour: forms set and braced, base compacted, tools and helpers ready, and a way for the truck to reach the forms.

Assumptions and limitations

• Each slab has an even thickness. Thickened edges and footings add concrete; add them as extra slabs or with the footing calculator.
• The gravel base is estimated for the slab area only.
• This is a quantity estimate, not a structural design.

Concrete slab questions

Formulas, unit conversions and default values are explained on our methodology page. Results are estimates; check manufacturer data and local codes before you buy or build.