Concrete Slab Calculator

Estimate cubic yards, cubic metres and bags of concrete for a slab, patio or pad — with waste factor, the formula and a shopping list.

✓ Method shownUpdated Oct 18, 2026By DIYCalcLab Editorial TeamSources (4)How we calculate

Quick answer: A 10 × 10 ft slab 4 in thick needs about 1.23 cu yd of concrete before waste — 1.36 cu yd with 10% extra, or about 62 bags of 80 lb premix. In metric, a 3 × 3 m slab 10 cm thick needs 0.99 m³ including 10% waste. Volume = length × width × thickness; always round bags up.

Concrete needed (incl. 10% waste)

1.36 cu yd

Before waste: 1.23 cu yd (33.3 cu ft)
Cubic feet: 36.7 cu ft
80 lb bags: 62
60 lb bags: 82
40 lb bags: 123

Over ~1 cu yd (0.76 m³), ready-mix delivery is often cheaper and easier than bags.

Show the math

Area = length × width = 100 sq ft
Volume = area × thickness × slabs = 1.23 cu yd (33.3 cu ft)
With 10% waste = 1.36 cu yd (36.7 cu ft)
Bags = volume ÷ bag yield (0.6 cu ft) = 61.1 → round up to 62

Shopping list & cost

Shopping list
Item	Spec	Qty	Cost
Premix concrete	80 lb bag	62 bags	—

Prices are what you enter under Advanced options. Estimates only — check stock and pricing with your supplier.

Diagram

Labels update as you type. Focus an input to highlight its dimension.

How to use this calculator

Measure the inside of your forms (not the outside of the boards) for length and width. For a circle, measure the diameter.
Choose a thickness: 4 in (10 cm) for patios, paths and shed bases; 5–6 in (12–15 cm) for driveways or anything a car will drive on.
Leave the waste factor at 10% unless your sub-base is perfectly level and compacted. Uneven ground eats concrete quickly.
Open Advanced options to pick your bag size and enter the price per bag for a cost estimate.
Above roughly 1 cubic yard (0.76 m³), compare the bag total with a ready-mix quote — the result card will remind you.

Method & assumptions

Formula

Volume (cu ft) = Length (ft) × Width (ft) × Thickness (in) ÷ 12
Volume (cu yd) = Volume (cu ft) ÷ 27
Volume (m³) = Length (m) × Width (m) × Thickness (cm) ÷ 100
Bags = Volume × (1 + waste %) ÷ yield per bag, rounded up

Length, Width: inside form dimensions
Thickness: finished slab depth
Yield per bag: from the bag label: ~0.60 cu ft for 80 lb, 0.45 for 60 lb, 0.30 for 40 lb (typical)
Waste %: allowance for spillage, sub-base variation and form flex

Assumptions & limits

The sub-base is flat; dips and soft spots need extra concrete.
Bag yields are typical label values for standard premix — high-strength and fast-setting mixes can differ.
Rebar, mesh and gravel base are not included in the volume.
Not a structural design tool: footings, suspended slabs and load-bearing pads need local code or an engineer.

Rounding: purchase quantities are always rounded up to whole bags, boxes, sheets or bundles. See our methodology.

Worked examples

Imperial: 12 × 16 ft shed base, 4 in thick

12 × 16 = 192 sq ft
192 × 4 ÷ 12 = 64 cu ft
64 ÷ 27 = 2.37 cu yd; with 10% waste = 2.61 cu yd
That is about 118 bags of 80 lb premix — a ready-mix delivery is the sensible choice here.

Metric: 2.4 × 3.0 m patio, 10 cm thick

2.4 × 3.0 = 7.2 m²
7.2 × 0.10 = 0.72 m³; with 10% waste = 0.79 m³
At ~11.7 L per 25 kg bag: 0.79 ÷ 0.0117 = 67.7 → 68 bags

In depth: concrete slab planning notes

Thickness, base and reinforcement

The thickness you choose drives the order more than any other input. Going from 4 in to 5 in adds 25% to the volume, and going from 10 cm to 15 cm adds 50%. Patios, paths and garden shed bases are usually 4 in (100 mm) on a compacted granular base. Driveways and slabs that carry vehicles are typically 5–6 in (125–150 mm), often with mesh or rebar. If you are unsure, check what your local authority or code requires before you set the forms — changing thickness after the truck arrives is not an option.

The base matters because concrete fills every low spot. A sub-base that is 1 in low across a 10 × 10 ft slab swallows another 8 cu ft — almost a third of a cubic yard. Compact the gravel in layers, screed it to a string line and check the depth in several places before you pour. That is the cheapest way to keep your waste allowance at 5–10% rather than 15%.

Ordering ready-mix versus mixing bags

Bagged premix is ideal for anything up to about one cubic yard (0.76 m³). Above that the bag count climbs quickly: a 12 × 16 ft shed base at 4 in is around 118 bags of 80 lb mix, which is more than five tonnes to lift, mix and place before the first batch starts to set. Ready-mix arrives consistent, in one continuous pour, and at a predictable strength.

When you call a plant, give them the volume from this calculator including waste, the slab thickness and whether the truck can reach the forms. Many plants deliver in quarter-yard or 0.2 m³ steps and charge a short-load fee below a minimum; ask about it so you can compare honestly with bags. If the truck cannot get close, budget for a pump or a team with wheelbarrows.

Whichever route you take, have a place ready for leftover concrete — a set of paver moulds or an extra post hole — so it does not end up in a drain or on the lawn.

Reference table

Typical concrete waste allowances (vary by project)
Situation	Extra to order
Formed slab on compacted, level base	5%
Typical DIY patio or shed base	8–10%
Uneven ground or hand-dug edges	10–15%
Post holes (irregular sides)	10–20%

Key facts

1 cubic yard: 27 cu ft = 0.7646 m³
1 m³: 35.31 cu ft = 1.308 cu yd
Concrete per 100 sq ft at 4 in: 1.23 cu yd (before waste)
80 lb bags per cubic yard: 45 (at 0.6 cu ft each)

Tips & common mistakes

Thickness is entered in inches but area is in feet — the most common mistake in hand calculations is forgetting to divide by 12.
Order ready-mix to the next ¼ yard and tell the plant your waste allowance — short loads often carry a surcharge, so ask.
Have a plan for leftover concrete (a few paver molds or a post hole) rather than washing it into the drain.
In hot weather, small batches from bags can start to set before the last one is placed; get help or use ready-mix.

Frequently asked questions

How many bags of concrete do I need for a 10×10 slab?

At 4 in thick, about 56 bags of 80 lb premix before waste, or 62 with a 10% allowance. With 60 lb bags it is about 75 before waste.

How thick should a concrete slab be?

Most patios, walkways and shed bases are 4 in (10 cm). Driveways and garage floors are typically 5–6 in (12–15 cm). Check local code for anything structural.

Is it cheaper to buy bags or ready-mix?

For small jobs (under about 1 cubic yard) bags are usually cheaper and simpler. Above that, ready-mix tends to win on cost and finish quality — get a local quote including any short-load fee.

How much waste should I add?

5–10% for a well-prepared base. Use 10–15% if the ground is uneven or the edges are hand-dug.

Does this include the gravel base?

No. The gravel calculator handles the sub-base; enter the same length and width with your base depth.

Can I calculate a round slab?

Yes — choose Circle and enter the diameter. Area = π × (diameter ÷ 2)².

Sources

Quikrete – product yield information for bagged concrete mixes — checked 2026-10; verify before launch
Portland Cement Association – concrete basics — checked 2026-10; verify before launch
American Concrete Institute – residential concrete resources — checked 2026-10; verify before launch
NIST – unit conversion factors (SP 811) — checked 2026-10; verify before launch

Default values (bag yields, coverage rates, densities) are typical figures from manufacturer literature. Always check the label on the product you buy.

Related calculators

Learn more in our guides

Going deeper: Need slab or footing design checks? See StructuralEngineerLab.

Cite or embed this calculator

DIYCalcLab. “Concrete Slab Calculator.” Oct 18, 2026. https://diycalclab.com/concrete/slab-calculator/

An embeddable version for blogs and contractor sites is planned.

Estimates only. Verify quantities with your supplier and local building codes. For structural, electrical or safety-critical work, consult a licensed professional. Full disclaimer.

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