How much concrete for a foundation?
Foundation quantity depends on the actual formed dimensions: slab area, thickness, footings, stem walls, piers, and any thickened sections. Calculate each part separately when the dimensions are different.
Separate slabs, footings, and walls
A slab, a strip footing, and a wall use different measurements. Slabs use length, width, and thickness. Walls use length, height, and thickness. Round piers use diameter and depth.
Foundation estimates are not structural design
This page estimates concrete quantity only. Foundation design can depend on soil, frost depth, load, reinforcement, drainage, and local code requirements.
Common concrete slab sizes
Cubic yards below are before waste. Add your project waste percentage before ordering.
| Slab size | 4 in thick | 5 in thick | 6 in thick |
|---|---|---|---|
| 10 ft x 10 ft | 1.23 cu yd | 1.54 cu yd | 1.85 cu yd |
| 10 ft x 12 ft | 1.48 cu yd | 1.85 cu yd | 2.22 cu yd |
| 12 ft x 12 ft | 1.78 cu yd | 2.22 cu yd | 2.67 cu yd |
| 20 ft x 20 ft | 4.94 cu yd | 6.17 cu yd | 7.41 cu yd |
| 24 ft x 24 ft | 7.11 cu yd | 8.89 cu yd | 10.67 cu yd |
| 30 ft x 40 ft | 14.81 cu yd | 18.52 cu yd | 22.22 cu yd |
Before you calculate
- Measure the formed dimensions, not just the finished visible surface.
- Break complex foundations into separate rectangular sections.
- Use thickness in inches and keep waste visible for excavation and form variation.
Common mistakes
- Missing thickened edges, grade beams, steps, or separate footing runs.
- Using plan dimensions without checking actual forms.
- Treating a material calculator as structural design.
Formula
cubic yards = (length * width * (thickness / 12) / 27) * (1 + wastePercent / 100)
Assumptions
- Defaults represent a slab foundation.
- Thickness is entered in inches; adjust it to match the actual planned pour.
- Base prep, forms, reinforcement, joints, finishing, delivery, permits, and local requirements are separate planning items.
Example
Estimated concrete needed (cubic yards): 20.74 cubic yards
How to use this calculator
- Enter length in ft.
- Enter width in ft.
- Enter thickness in in.
- Enter waste in %.
- Review the live estimate and compare it with the example result.
- Check the formula, assumptions, product labels, and site conditions before using the Concrete Slab Foundation Calculator result to plan materials.
Before you buy materials
- Confirm dimensions, reinforcement, and inspection requirements before ordering concrete.
- Use the estimate as a quantity planning number for supplier conversations.
FAQ
What is the example concrete slab foundation cubic-yard result?
Use length, width, thickness or depth, and waste, then calculate the planning result. In the default example, the result is 20.74 cubic yards.
How do I calculate concrete for a foundation?
Break the foundation into simple shapes, calculate cubic yards for each shape, add them together, then add waste if needed.
Are footings included in a foundation slab estimate?
Only if you enter them as separate sections. A simple slab calculation does not automatically include footings or stem walls.
Can this calculator design a foundation?
No. It estimates concrete volume only. Foundation design should follow plans, local requirements, and qualified guidance.
Should I add waste for foundation concrete?
Usually yes. Excavation variation, forms, uneven base, and ordering increments can change the final quantity.
Related calculators
This calculator is for planning estimates only. Verify final quantities with product labels, project conditions, and a qualified professional when accuracy matters.