How to use this concrete figuring calculator
This concrete figuring calculator helps contractors, builders, and DIY homeowners figure the exact volume of concrete required for rectangular slabs, footings, patio pads, driveways, and walkways. Enter your project dimensions (length and width in feet, slab thickness in inches), specify your safety waste allowance, and select your bagged yield to compute cubic yards (yd³), cubic feet (ft³), cubic meters (m³), and required bag quantities in real time.
Concrete figuring formula: Yards, feet & bags
To figure concrete volume accurately by hand, use the following standard equations:
Because one cubic yard contains exactly 27 cubic feet, dividing your total cubic feet by 27 gives the ready-mix truck order volume. If mixing on site using dry bags, figure 0.45 cubic feet per 60 lb bag and 0.60 cubic feet per 80 lb bag.
Concrete figuring reference chart (Standard Slabs at 4" Depth)
| Slab Dimensions (ft) | Surface Area (ft²) | Volume (yd³ with 10% waste) | 60 lb Bags (0.45 ft³ yield) | 80 lb Bags (0.60 ft³ yield) |
|---|---|---|---|---|
| 8 × 10 ft | 80 ft² | 1.09 yd³ | 66 bags | 49 bags |
| 10 × 10 ft | 100 ft² | 1.36 yd³ | 82 bags | 62 bags |
| 12 × 12 ft | 144 ft² | 1.96 yd³ | 118 bags | 88 bags |
| 12 × 16 ft | 192 ft² | 2.61 yd³ | 157 bags | 118 bags |
| 16 × 20 ft | 320 ft² | 4.35 yd³ | 261 bags | 196 bags |
Step-by-step worked concrete figuring example
Here is how to figure concrete for a 12 ft by 10 ft backyard patio slab poured at a 4-inch depth:
- Convert thickness to feet: 4 inches ÷ 12 = 0.333 feet.
- Calculate geometric volume: 12 ft × 10 ft × 0.333 ft = 40.0 cubic feet.
- Add 10% waste allowance: 40.0 ft³ × 1.10 = 44.0 cubic feet.
- Convert to cubic yards: 44.0 ft³ ÷ 27 = 1.63 cubic yards.
- Figure total bags: At 0.60 cubic feet yield per 80-lb bag, 44.0 ÷ 0.60 = 73.3, which rounds up to 74 bags.
Frequently asked questions about figuring concrete
What is a concrete figuring calculator?
A concrete figuring calculator is a construction calculation tool that determines the geometric volume of concrete needed for slabs, footings, post holes, and walls, converting the measurements into cubic yards for ready-mix trucks or individual dry bags (60 lb and 80 lb) for on-site mixing.
How many 80 lb bags of concrete do I figure per cubic yard?
It takes approximately 45 bags of 60 lb concrete mix or 34 bags of 80 lb concrete mix to figure one cubic yard (27 cubic feet) of mixed concrete.
Why should I add 10% waste when figuring concrete?
Ground subgrades are rarely perfectly level, forms flex under hydraulic pressure, and some concrete sticks to mixing equipment. Adding a 10% safety margin prevents running short before finishing the pour.
Looking for post hole concrete?
If you are setting fence posts or deck footings in cylindrical holes, use our dedicated Post-hole concrete calculator to account for post displacement.
Created and calculation-tested by Velour Trends · Updated 8 November 2026. Formulas verified against standard ASTM concrete yield guidelines. Report a calculation issue.