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.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Length
Enter this project assumption in ft. The current imperial default is 12 ft.
Width
Enter this project assumption in ft. The current imperial default is 10 ft.
Thickness
Enter this project assumption in in. The current imperial default is 4 in.
Waste allowance
Enter this project assumption in %. The current imperial default is 10 %. Minimum: 0. Maximum: 50.
Concrete yield per bag
Enter this project assumption in ft³. The current imperial default is 0.6 ft³.
Delivery-truck capacity
Enter this project assumption in yd³. The current imperial default is 10 yd³.
Ready-mix price
Enter this project assumption in per yd³. The current imperial default is 150 per yd³. Minimum: 0.
Price per concrete bag
Enter this project assumption in a unitless value. The current imperial default is 7. Minimum: 0.
How to read the outputs
Estimated bags
0.6 ft³ yield each. Default preview: 74.
Truck loads
10 yd³ capacity. Default preview: 1.
Slab volume
before waste. Default preview: 1.48 yd³.
Slab area
rectangular footprint. Default preview: 120 ft².
Ready-mix cost
entered volume price. Default preview: $244.44.
Bagged cost
rounded full bags. Default preview: $518.00.
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
Continue planning the project
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
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.