How to Calculate Concrete for a Slab (Formulas & Step-by-Step Guide)
Quick answer: multiply length (ft) × width (ft) × thickness (in) ÷ 12 to get cubic feet, then divide by 27 for cubic yards. Add 10% for waste and round up to what your supplier sells. A 10x10 slab 4 inches thick is 1.23 yd³, or 1.36 yd³ with waste, so you would order 1.5 yards of ready-mix or buy 62 bags of 80-lb mix.
Concrete is sold by volume, but you measure a slab by its length, width and thickness. The math to connect them is simple. Most ordering mistakes come from mixing inches and feet, forgetting thickened edges, or rounding the wrong way. This guide walks through the formulas step by step, covers circles, L-shapes and footings, and finishes with the mistakes that leave people short on pour day.
Step by Step
- Measure length and width in feet. Convert inches to decimal feet by dividing by 12. Ten feet six inches is 10.5 ft.
- Convert thickness to feet. Divide inches by 12. Four inches is 0.333 ft.
- Multiply length × width × thickness to get cubic feet.
- Divide by 27 to get cubic yards (a cubic yard is 3 ft × 3 ft × 3 ft).
- Add waste: multiply by 1.10 for 10% extra.
- Add extras: thickened edges, footings, piers or steps poured at the same time.
- Round up to the supplier's order step, or convert to bags.
The Formulas
If the slab slopes for drainage but keeps the same thickness, the slope does not change the volume. If the thickness varies (for example, because the base is uneven), use the average thickness from several measurements.
Unit Conversions
| Thickness | In feet | Cubic feet per 100 sq ft | Cubic yards per 100 sq ft | Sq ft covered by 1 yd³ |
|---|---|---|---|---|
| 3.5 in | 0.292 | 29.2 | 1.08 | 92.6 |
| 4 in | 0.333 | 33.3 | 1.23 | 81.0 |
| 5 in | 0.417 | 41.7 | 1.54 | 64.8 |
| 6 in | 0.500 | 50.0 | 1.85 | 54.0 |
| 8 in | 0.667 | 66.7 | 2.47 | 40.5 |
The last column is handy for a quick check: at 4 inches, every 81 sq ft of slab needs about a yard before waste.
Waste and Rounding
Add 10% to cover spillage, form bowing, concrete left in the chute and a subgrade that is a little low in spots. Go as low as 5% on a laser-graded base with rigid forms, or up to 15% on rough or irregular ground. The concrete waste calculator shows the difference.
Then round up to the supplier's step. Most ready-mix plants sell in steps of 0.25 or 0.5 yard. For bags, divide cubic feet by the bag yield (0.60 ft³ for an 80-lb bag) and round up to a whole bag. Never round down: a short load means a cold joint and a second delivery fee, while a little leftover concrete can fill a post hole or a few stepping-stone forms.
Worked Examples
A. 12x16 patio, 4 inches thick
B. Round patio, 14 ft across, 4 inches thick
C. 24x24 garage slab, 5 inches thick, 12-inch thickened edge
D. L-shaped patio: 20x10 plus a 10x10 wing, 4 inches thick
Want to price these? The cost per cubic yard guide covers ready-mix pricing and fees, and the slab cost calculator turns a volume into a full project budget.
Common Mistakes
- Multiplying by inches instead of feet. 10 × 10 × 4 = 400 is not cubic feet. Divide the thickness by 12 first.
- Trusting lumber sizes. A 2x4 form board is actually 3.5 inches tall and a 2x6 is 5.5 inches. If you screed to the top of a 2x4, your slab is 3.5 inches, the IRC minimum for a floor slab, not 4.
- Ignoring a low subgrade. If the base is half an inch low across a 4-inch slab, you need about 12.5% more concrete. Check depth with a string line in several places before the truck comes.
- Forgetting extras. Thickened edges, interior footings, piers and steps poured at the same time all add volume. The concrete footing calculator handles them.
- Skipping waste, or rounding down. Both lead to the same place: a short load in the middle of a pour.
- Using the wrong thickness for the job. 4 inches suits patios and walks. Driveways and garage floors usually need 5 to 6. See how thick a concrete slab should be.
Enter your dimensions in the concrete volume calculator to confirm your hand calculation. If you are using bags, our guide to how many bags of concrete you need has tables for every bag size.
Sources and Method
Volumes use standard geometry and the fixed conversions of 12 inches per foot and 27 cubic feet per cubic yard. The waste allowance, bag yields and order steps are this site's working figures (see our methodology). Minimum slab thickness follows the International Residential Code, section R506, and thickened-edge dimensions follow common practice for slab-on-grade foundations under IRC section R403. ACI 302.1R, the American Concrete Institute's guide to floor and slab construction, covers subgrade tolerance and slab thickness.
Frequently Asked Questions
Cubic yards = length (ft) × width (ft) × thickness (in) ÷ 12 ÷ 27. Then add 10% for waste and round up to your supplier's order step or to whole bags.
At 4 inches thick it is 2.37 cubic yards, or 2.61 yd³ with 10% waste. Order 2.75 yd³ if your plant sells in quarter yards.
About 81 square feet at 4 inches thick, 64.8 at 5 inches and 54 at 6 inches, before waste.
Add 10% for a typical slab. Use as little as 5% on a laser-graded base with rigid forms, and up to 15% for rough ground or curved forms. Always round up to the next order step.
Multiply 3.1416 × radius² (in feet) × thickness in inches ÷ 12, then divide by 27. A 14-foot round patio 4 inches thick is 1.90 cubic yards, or 2.09 yd³ with waste.
Calculate Your Concrete Project Budget
Use our comprehensive concrete slab cost calculator to get a complete line-item breakdown of materials, labor, reinforcement, and base preparation.
Launch Concrete Slab Calculator →