Concrete foundation calculator

How It Is Calculated

This calculator estimates the volume, weight, and material cost of concrete needed for a rectangular foundation slab. The calculations follow these steps:

1. Compute the slab volume in cubic feet: V_ft³ = L × W × T / 12, where L is length (ft), W is width (ft), and T is thickness (in).
2. Convert to cubic yards: V_yd³ = V_ft³ / 27.
3. Apply the waste factor: V_total = V_yd³ × (1 + w / 100), where w is the waste percentage.
4. The final volume in cubic yards is V_total rounded to 2 decimals.
5. Volume in cubic feet: V_ft³,total = V_total × 27, rounded to 1 decimal.
6. Weight: W_lb = V_total × 4050, rounded to the nearest pound.
7. Material cost: C = V_total × P, where P is the price per cubic yard, rounded to 2 decimals.

Assumptions

• Concrete density is 150 lb/ft³ (4050 lb/yd³).
• The foundation is a rectangular slab with uniform thickness.
• Waste factor includes spillage, over-excavation, and typical ordering rounding.
• Concrete price is per cubic yard delivered, excluding taxes, labor, and other materials.

Worked Example

For a slab with Length = 20 ft, Width = 20 ft, Thickness = 4 in, Waste factor = 10%, and Concrete price = $150/yd³:

1. V_ft³ = 20 × 20 × 4 / 12 = 133.33 ft³
2. V_yd³ = 133.33 / 27 = 4.938 yd³
3. V_total = 4.938 × (1 + 10/100) = 5.432 yd³ → 5.43 yd³
4. V_ft³,total = 5.432 × 27 = 146.6 ft³ → 146.6 ft³
5. W_lb = 5.432 × 4050 = 21992 lb → 21992 lb
6. C = 5.432 × 150 = 814.8 → $814.50 (rounded to 2 decimals)

These results match the example outputs.

Practical Tips

• Always order a little extra concrete (waste factor) to account for spillage, uneven ground, or miscalculations. A 10% waste factor is typical for slabs.
• For larger projects, consider using the Concrete slab calculator for more detailed estimates.
• If you are also planning a driveway, the Concrete driveway cost calculator can help with cost breakdowns.
• Check local concrete delivery minimums—often 1 to 2 cubic yards—to avoid paying for short loads.
• Ensure the slab thickness is uniform; measure in multiple spots to confirm.
• Concrete weight is important for transport and formwork strength; use the Weight concrete calculator for other shapes.

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