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Concrete Slab Calculator

Computes the concrete volume of a rectangular slab, footing or pad from length, width and thickness, adds a waste allowance and converts it to cubic yards, cubic feet and the number of 80 lb, 60 lb and 25 kg bags of premixed concrete.

When to use

You need the amount of ready-mix concrete to order or the number of premix bags for a slab, patio, shed base, footing or pad of uniform thickness.

Do not use when: The pour is a cylinder such as a post hole or pier (use cylinder for the volume), or you need mortar for brickwork (use brick).

Formula

volume = length × width × thickness (thickness/100 in m or thickness/12 in ft); with_waste = volume × (1 + waste_percent/100); yd³ = ft³ / 27; m³ = ft³ × 0.028316847; bags_80lb = ceil(ft³ / 0.60); bags_60lb = ceil(ft³ / 0.45); bags_25kg = ceil(m³ × 96).

Bag yields follow the QUIKRETE data sheet (80 lb → 0.60 ft³, 60 lb → 0.45 ft³). The 25 kg count assumes ≈ 2400 kg of dry mix per m³ (96 bags), a conservative figure: manufacturer yields of 0.010–0.0125 m³ per 25 kg bag correspond to 80–100 bags/m³, so check the bag.

Inputs

Parameter	Type	Unit	Required	Description
`unit_system`	enum: metric \| imperial		default metric	Unit system of the length inputs (metric: m / cm / mm; imperial: ft / in). Outputs are reported in both systems.
`length`	number	m or ft	yes	Slab length in metres or feet. Range: > 0, ≤ 1000
`width`	number	m or ft	yes	Slab width in metres or feet. Range: > 0, ≤ 1000
`thickness`	number	cm or in	yes	Slab thickness in centimetres (metric) or inches (imperial). Typical: 10 cm / 4 in for paths and patios, 15 cm / 6 in for driveways. Range: > 0, ≤ 500
`waste_percent`	number	%	default 10	Extra material for cuts, breakage and offcuts, applied as a multiplier (1 + waste_percent/100). Covers uneven sub-base, spillage and over-excavation; 5–10 % is usual. Range: ≥ 0, ≤ 50

Outputs

Output	Type	Unit	Description
`volume_m3`	number	m³	length × width × thickness, before waste.
`volume_cubic_feet`	number	ft³	Same volume in cubic feet.
`volume_cubic_yards`	number	yd³	Same volume in cubic yards (1 yd³ = 27 ft³ = 0.7646 m³).
`volume_with_waste_m3`	number	m³	Volume × (1 + waste_percent/100).
`volume_with_waste_cubic_yards`	number	yd³	Volume to order in cubic yards.
`bags_80lb`	integer		ceil(volume_with_waste_ft³ / 0.60 ft³ per bag).
`bags_60lb`	integer		ceil(volume_with_waste_ft³ / 0.45 ft³ per bag).
`bags_25kg`	integer		ceil(volume_with_waste_m³ × 96 bags per m³), assuming about 2400 kg of dry premix per m³ of placed concrete.

Example

10 ft × 20 ft slab, 4 in thick, 10 % waste: {"unit_system":"imperial","length":10,"width":20,"thickness":4,"waste_percent":10} → {"volume_cubic_feet":66.67,"volume_cubic_yards":2.469,"volume_m3":1.888,"volume_with_waste_cubic_yards":2.716,"volume_with_waste_m3":2.077,"bags_80lb":123,"bags_60lb":163,"bags_25kg":200}

4 m × 3 m slab, 10 cm thick, 10 % waste: {"unit_system":"metric","length":4,"width":3,"thickness":10,"waste_percent":10} → {"volume_m3":1.2,"volume_cubic_yards":1.57,"volume_with_waste_m3":1.32,"volume_with_waste_cubic_yards":1.727,"bags_25kg":127,"bags_80lb":78,"bags_60lb":104}

GET https://tttkmbb.com/api/v1/calculate/concrete-slab?unit_system=imperial&length=10&width=20&thickness=4&waste_percent=10

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/concrete-slab (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/concrete-slab · Markdown: https://tttkmbb.com/construction/concrete-slab.md · JSON definition: https://tttkmbb.com/construction/concrete-slab.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="concrete-slab"
OpenAPI operationId: estimate_concrete_slab
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

FAQ

Should I order ready-mix or use bags?

Bags are practical up to roughly 0.5 m³ (about 30 × 80 lb bags); above that, ready-mix delivered by truck is cheaper and gives a consistent mix. Ready-mix is ordered by the m³ or yd³ using the with-waste figure.

Why do bag counts differ between brands?

Yield per bag depends on the mix design and how much water is added; the figures here are QUIKRETE's for US bags and a conservative 2400 kg/m³ assumption for 25 kg bags. Use the yield printed on the bag when it differs.

Related calculators

Gravel Calculator — Sub-base aggregate under the slab.
Brick Calculator — Bricks and mortar for walls on the slab.
Cylinder Calculator — Volume of round post holes or piers.