How Much Concrete Do I Need?
Short answer: multiply length × width × thickness in feet, divide by 27. Enter your dimensions below and you'll have the exact answer — yards, bags and cost — as you type.
Concrete needed
2.96 yd³
80.0 ft³
Estimated cost
$489
at $165 per yd³
Approx. weight
10,640 lb
≈ 5.3 tons
Bagged mix
134
80-lb bags
178
60-lb bags
267
40-lb bags
Before you order: add about 10% for uneven subgrade and spillage — that's 3.26 yd³ or 147 × 80-lb bags.
Formula
The whole question comes down to one line of math:
cubic feet = length (ft) × width (ft) × [thickness (in) ÷ 12] cubic yards = cubic feet ÷ 27
Why 27? A cubic yard is a 3 × 3 × 3 ft cube. Bag counts are the same volume in sack form: an 80-lb bag yields 0.60 ft³, a 60-lb bag 0.45 ft³, a 40-lb bag 0.30 ft³ — round up to whole bags.
Example
- Backyard patio 14 ft × 10 ft, 4 inches thick: 4 ÷ 12 = 0.333 ft.
- Volume: 14 × 10 × 0.333 = 46.7 ft³.
- Yards: 46.7 ÷ 27 = 1.73 yd³ — order 2 yd³ with the 10% allowance.
- Bags instead: 46.7 ÷ 0.60 = 77.8 → 78 × 80-lb bags (86 with waste).
Frequently Asked Questions
How do I calculate how much concrete I need?
Multiply length × width × thickness, all in feet, to get cubic feet — then divide by 27 for cubic yards. If thickness is in inches, divide it by 12 first. The calculator above runs that math as you type, then adds the 10% ordering allowance for you.
How much concrete do I need for a 12×20 slab?
At 4 inches thick: 2.96 cubic yards (80 ft³) — order about 3.3 yards, or in bags, 147 × 80-lb with the allowance. At 6 inches the same footprint needs 4.44 yards, so thickness matters more than anything.
Should I measure in feet or inches?
Measure length and width in feet and thickness in inches — that's how slab work is actually specified (a '4-inch slab'). The calculator handles the conversion; just don't mix units on paper.
How accurate is the estimate?
The math is exact for a rectangular pour; reality adds variance. Uneven excavation, form flex and spillage are why the standard practice is ordering 10% extra — the 'before you order' line in the result already includes it.