Every concrete estimate comes down to one idea: figure out the volume of the space you’re filling, convert it to the unit your supplier uses, and add a small waste allowance. Get those three steps right and you’ll never order two yards short or pay for a yard you didn’t need.
The basic concrete formula
For any rectangular pour — slab, footing, sidewalk, pad — the volume is:
Volume = Length × Width × Thickness
The catch is units. Length and width are usually measured in feet, but thickness is almost always quoted in inches. Convert the thickness to feet first by dividing by 12. A 4-inch slab is 4 ÷ 12 = 0.333 feet thick.
Worked example: a 10 × 10 ft patio at 4 inches
- Convert thickness: 4 in ÷ 12 = 0.333 ft
- Multiply: 10 ft × 10 ft × 0.333 ft = 33.3 cubic feet
- Convert to cubic yards: 33.3 ÷ 27 = 1.23 cubic yards
Ready-mix trucks sell by the cubic yard, so 1.23 yd³ is the number you’d give the dispatcher. With a standard 10% waste allowance, you’d order about 1.35 yd³ — in practice, 1.5 yd³, since most plants batch in quarter-yard increments. Run your own dimensions through our concrete calculator or the dedicated slab calculator to skip the arithmetic.
Unit conversions you’ll actually use
These four conversions cover nearly every concrete job:
| Conversion | Value |
|---|---|
| 1 cubic yard | 27 cubic feet |
| 1 cubic yard | 0.7646 cubic meters |
| 1 cubic meter | 35.31 cubic feet |
| 1 inch | 0.0833 feet |
If you work in metric, measure everything in meters and multiply straight through — a 3 m × 3 m slab at 0.10 m thick is 0.9 m³, no conversion needed. Our cubic yard calculator and cubic meter calculator handle both systems.
The coverage shortcut: 324 ÷ thickness
Here’s a trick estimators use daily. One cubic yard is 27 ft³, which equals 324 “inch-feet” of coverage (27 × 12). So:
Coverage per cubic yard (ft²) = 324 ÷ thickness in inches
- 4-inch slab: 324 ÷ 4 = 81 ft² per yard
- 5-inch slab: 324 ÷ 5 = 64.8 ft² per yard
- 6-inch slab: 324 ÷ 6 = 54 ft² per yard
Flip it around to estimate yards fast: a 400 ft² driveway at 5 inches needs roughly 6.2 yd³ — use our dedicated concrete driveway calculator to factor in #4 rebar and ready-mix yardage.
Converting volume to bags
Small jobs don’t justify a truck. Bagged concrete yields:
- 80 lb bag ≈ 0.60 ft³ (45 bags per cubic yard)
- 60 lb bag ≈ 0.45 ft³ (60 bags per cubic yard)
Our 10 × 10 patio at 33.3 ft³ would take 33.3 ÷ 0.60 = 56 eighty-pound bags. For outdoor slabs, use our concrete patio calculator or compare specific mixes with the Quikrete calculator and general concrete bag calculator.
Why you should always add waste
Sub-bases are never perfectly flat. A grade that’s off by just half an inch across a 10 × 10 slab swallows an extra 4 ft³ of concrete. Spillage, pump line loss, and over-excavated edges add more. The industry standard is 5–10% extra for formed slabs and up to 15% for pours against irregular ground. Running out mid-pour means a cold joint and a second delivery fee — far more expensive than a little surplus.
Related guides & seasonal considerations
Before pouring, check the temperature forecast: in temperatures below 40°F (4°C), follow our guide to pouring concrete in cold weather to prevent early frost damage. Once placed, refer to the concrete curing time chart for the complete 24-hour, 7-day, and 28-day hydration schedule.
Quick checklist
- Measure length and width in feet (or meters).
- Convert thickness to the same unit.
- Multiply all three for cubic feet, then divide by 27 for cubic yards.
- Add 5–10% for waste.
- Round up to your supplier’s minimum increment.
That’s the whole method. For odd shapes — circles, footings, stairs — the same logic applies with different geometry, which we cover in our volume formula guide, or you can jump straight to the volume calculator.