Concrete Slab Calculator
Calculate concrete slab volume, waste allowance, full bag quantity, delivery loads and material cost in imperial or metric units.
Default project preview
1.63 yd³
Estimated bags
74
Truck loads
1
Slab volume
1.48 yd³
Slab area
120 ft²
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Imperial volume = length ft × width ft × thickness in ÷ 12 ÷ 27. Metric volume = length m × width m × thickness cm ÷ 100. Waste is applied before bags and delivery loads are rounded up.
Related tools
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FAQ
Concrete Slab Calculator questions
How much extra concrete should I order?+
A 5% to 10% allowance is common for spillage, uneven base, form variation and measurement differences. Complex pours may need more.
How many concrete bags do I need?+
The calculator divides the waste-adjusted volume by the entered bag yield and rounds up to a whole bag.
Is a four-inch slab thick enough?+
Four inches is common for some residential patios and walkways, but loads, soil, climate, reinforcement and local code can require a different design.
Project guide
Use this calculator with confidence
Last reviewed: July 19, 2026
Use this slab calculator when the project can be represented by a rectangular length, width and thickness. It converts every dimension into a consistent volume, adds the selected ordering allowance, and reports both bulk volume and planning cost. The result is intentionally separated from structural decisions: the tool estimates how much concrete to order, while slab thickness, reinforcement, base preparation, drainage, joints and concrete strength remain project-specific choices.
How to use it
- 1Measure the finished slab length and width at several points.
- 2Enter the planned thickness rather than the excavation depth.
- 3Choose a waste allowance that reflects uneven forms and subgrade.
- 4Enter a local ready-mix price only when a cost comparison is useful.
Worked example
A 12 ft × 10 ft slab at 4 in. thick has a base volume of 40 ft³, or about 1.48 yd³. With 10% extra, the planning order becomes about 1.63 yd³ before supplier minimums or short-load charges.
What the defaults mean
4 in. thickness
A familiar planning example, not a structural recommendation.
10% waste
Allows for small form and subgrade variation; edit it for the actual site.
Price per volume
A placeholder for comparison. Replace it with a current local quote.
| Quantity | Imperial | Metric |
|---|---|---|
| Area | length × width in ft² | length × width in m² |
| Thickness | inches ÷ 12 = feet | millimetres ÷ 1000 = metres |
| Order volume | cubic yards | cubic metres |
Common measurement mistakes
- Using excavation depth instead of concrete thickness.
- Mixing feet and inches without conversion.
- Measuring only one side of an irregular form.
- Forgetting supplier minimum-order and delivery rules.
Limits and safety
- Not a structural-design or code-compliance tool.
- Does not select mix strength, reinforcement, base depth or joint layout.
- Confirm the final order and truck access with the concrete supplier.
Continue the project
calculate concrete bags for a small slab — Compare bagged mix with ready-mix volume.
estimate reinforcing bar quantity — Plan reinforcement separately from concrete volume.
estimate slab formwork — Turn perimeter dimensions into form materials.
plan saw-cut length — Estimate joint-cutting quantities after the slab layout is set.