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Concrete Slab Calculator
Estimate concrete volume for a rectangular slab.
Concrete Slab Calculator: Find the volume of concrete for a rectangular slab from its footprint and thickness, then see an order allowance rounded upwards to the next tenth of a cubic yard. The calculated volume describes the shape; the order estimate also covers the waste percentage you enter. Measure thickness separately from the length and width. Runs 100% locally in your browser with zero server file uploads.
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Concrete to order (estimate)
2.8 cu yd
- Slab area (estimate)
- 200 sq ft
- Calculated concrete (estimate)
- 2.47 cu yd
Order yd³ = length × width × (thickness ÷ 12) ÷ 27 × (1 + waste)- Every result is an estimate; verify surface preparation, measurements, and local practices before ordering.
- Allow for the stated waste and product-coverage assumptions; openings are deducted only where requested.
- Compaction, settling, substrate condition, cuts, and installation method can change actual quantities.
Formula and worked example
Order yd³ = length × width × (thickness ÷ 12) ÷ 27 × (1 + waste)
A worked example with sample values:
| Inputs | |
|---|---|
| Length | 20 ft |
| Width | 10 ft |
| Thickness | 4 in |
| Waste allowance | 10% |
| Results | |
|---|---|
| Slab area | 200 sq ft |
| Calculated concrete | 2.47 cu yd |
| Concrete to order (estimate) | 2.8 cu yd |
Slab formula and a worked pour
Let L and W be length and width in feet, T thickness in inches and w the waste percentage. Area = L × W. Volume in yd³ = L × W × (T ÷ 12) ÷ 27. Order = volume × (1 + w ÷ 100), rounded upwards in 0.1 yd³ steps.
For L = 20 ft, W = 10 ft, T = 4 in and w = 10%, area = 200 ft² and volume = 200 × (4 ÷ 12) ÷ 27 = 2.469136 yd³, displayed as 2.47 yd³. Adding the illustrative allowance gives 2.716049 yd³, so the order estimate is 2.8 yd³. Rounding is applied after the allowance.
Volume units before you order
NIST's conversion factors (https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8) give 1 yd³ = 0.7645549 m³ approximately and 1 ft² = 0.09290304 m² exactly. Thus the 2.8 yd³ order is about 2.141 m³. Convert metric dimensions to the field units first, rather than entering metres into feet fields.
For an irregular footprint, divide the slab into non-overlapping rectangles and add their volumes before discussing delivery increments. The calculator's tenth-yard rounding is a purchasing convention for this estimate; confirm the actual increment and any minimum order with the concrete producer.
How to use it
- Enter measurements using the units shown beside each field.
- Choose the product coverage and allowances for your measured job.
- Calculate, then compare the rounded quantity with your layout and product label.
Privacy & limitations
Inputs and calculations stay in your browser. No values are uploaded.
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Frequently asked questions
Why add concrete overage if my slab dimensions are exact?
The rectangle assumes an even base and constant thickness. Depressions, spillage and material left in equipment can increase consumption. Choose an allowance for the actual pour; the example's 10% is an arithmetic assumption, not a recommended value for every slab.
Can I use bagged mix instead of ordering ready-mixed concrete?
Yes, but this result is volume, not a bag count. Multiply the order in yd³ by 27 to obtain ft³, divide by the mixed yield per bag and round up. Bag weight alone does not tell you the yield; check the product label.
Does the slab estimate include thickened edges or footings?
No. Calculate each extra rectangular section separately and avoid counting the overlapping slab volume twice. Steps, beams, tapered edges and reinforcement design need their own measurements; changing the waste percentage does not accurately describe those shapes.
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