How much concrete for a slab?
Concrete slabs are ordered by volume, not just square footage. Multiply slab length by width, convert thickness from inches to feet, then divide by 27 to get cubic yards. Add a practical waste buffer before ordering.
Common slab sizes
A 10 x 10 ft slab at 4 inches thick is about 1.23 cubic yards before waste. A 12 x 12 ft slab at 4 inches thick is about 1.78 cubic yards. A 20 x 20 ft slab at 4 inches thick is about 4.94 cubic yards.
Slab thickness and waste
Thickness changes concrete volume quickly. Moving from 4 inches to 6 inches increases the volume by 50%. Uneven base, forms, and ordering variance are common reasons to add waste.
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 actual formed length and width in feet.
- Enter slab thickness in inches, then handle thickened edges or footings as separate estimates.
- Break irregular slabs into rectangles so the volume calculation stays transparent.
Common mistakes
- Using square feet without converting thickness into volume.
- Missing thickened edges, turndowns, steps, or separate pads.
- Ordering exactly the calculated amount with no allowance for site variation.
Formula
cubic yards = (length * width * (thickness / 12) / 27) * (1 + wastePercent / 100)
Assumptions
- Defaults represent a concrete door landing.
- 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): 0.34 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 Door Landing Calculator result to plan materials.
Before you buy materials
- Compare cubic yards, cubic feet, and bag counts before deciding between ready-mix and bags.
- Confirm final slab details with plans, product labels, or a qualified professional when accuracy matters.
FAQ
What is the example concrete door landing cubic-yard result?
Use length, width, thickness or depth, and waste, then calculate the planning result. In the default example, the result is 0.34 cubic yards.
How do I calculate concrete for a slab?
Use cubic yards = (length x width x (thickness / 12)) / 27, then add waste if needed. Measure length and width in feet and thickness in inches.
How much concrete is needed for a 10 x 10 slab?
At 4 inches thick, a 10 x 10 ft slab needs about 1.23 cubic yards before waste. With 10% waste, plan around 1.36 cubic yards.
Does slab thickness matter?
Yes. A 4 inch slab and a 6 inch slab with the same footprint require different volumes. This calculator estimates quantity, not the required structural thickness.
Should I order ready-mix or bags for a slab?
Small slabs may be possible with bags, but larger slabs often make more sense as ready-mix. Compare cubic yards, bag yield, labor, delivery, and supplier minimums.
Related calculators
This calculator is for planning estimates only. Verify final quantities with product labels, project conditions, and a qualified professional when accuracy matters.