Concrete Triangle Calculator: Triangular and Trapezoid Slabs
A triangular slab with a 10 ft base and 12 ft height at 4 in needs 0.74 cubic yards, or 0.81 yd³ with 10% extra.
Advanced options: waste, prices
NRMCA advises ordering 4 to 10 percent over the estimate.
Concrete needed (with 10% extra)
0.81 yd³
0.62 m³ · 20 ft³ before extra
Triangle / trapezoid: 10 ft × 0 ft × 12 ft × 4 in. Small pour: premix bags make sense.
Premix bags
- 80 lb37
- 60 lb49
- 50 lb59
- 40 lb74
- Ready-mix cost
- $130
- Weight
- 3,000 lb
Full breakdown
| Volume before extra | 0.74 yd³ · 0.57 m³ |
|---|---|
| With 10% extra | 0.81 yd³ |
| Weight (150 lb/ft³) | 3,000 lb · 1.50 tons |
| Pallets of 40 lb (80 bags each) | 1 |
| Pallets of 60 lb (56 bags each) | 1 |
| Pallets of 80 lb (42 bags each) | 1 |
| Truckloads at 12 yd³ | 1 |
| Truckloads at 8 yd³ | 1 |
Bag counts round up, as Quikrete's own calculator does. Cost uses your prices; delivery and short-load fees are extra.
Key takeaways
- Triangle volume = ½ × base × height × thickness.
- Trapezoid = ½ × (a + b) × height × thickness.
- Measure height at a right angle to the base.
- Split odd shapes into triangles and rectangles.
How Much Concrete for a Triangle?
You need ½ × base × height × thickness. A 10 ft base and 12 ft height at 4 in hold 20 ft³, which is 37 bags of 80 lb mix with 10% extra.
Triangle Concrete Formula
ft³ = ½ × Base (ft) × Height (ft) × (Thickness (in) ÷ 12)Trapezoid Slabs
For a trapezoid, average the two parallel sides: ½ × (a + b) × height × thickness. Sides of 10 and 6 ft with a 12 ft height at 4 in hold 32 ft³.
How to Measure a Triangle for Concrete
- Measure one side as the base.
- Measure the height at a right angle from the base to the opposite corner.
- For odd shapes, split the area into triangles and rectangles and add them as parts.
Bags of Concrete for a Triangular Slab
| Triangle (base × height, 4 in) | yd³ with 10% | 80 lb bags |
|---|---|---|
| 6×6 ft | 0.24 | 11 |
| 10×12 ft | 0.81 | 37 |
| 12×16 ft | 1.30 | 59 |
| 20×20 ft | 2.72 | 123 |
Worked Examples: Triangular Areas
Corner patio, 12 ft base × 12 ft height at 4 in. ½ × 12 × 12 × 0.333 = 24 ft³; with extra, 44 bags of 80 lb or 0.98 yd³.
Driveway flare, 6 ft base × 10 ft at 4 in (two flares). 2 × 10 ft³ = 20 ft³; with extra, 0.81 yd³.
Trapezoid walkway, sides 4 and 8 ft, 20 ft long at 4 in. ½ × 12 × 20 × 0.333 = 40 ft³; with extra, 1.63 yd³.
Triangle Concrete in Cubic Meters
In metric, use ½ × base (m) × height (m) × thickness (m): a 3 m base, 4 m height and 10 cm thick triangle holds 0.6 m³. Switch the calculator to Metric to enter meters and centimeters.
Right, Isosceles and Irregular Triangles
The formula works for any triangle as long as the height is measured at a right angle to the base; for a triangle without a clear height, split it into two right triangles or use Heron's formula for the area and multiply by thickness.
Triangle Slab Cost
Concrete for a 12×12 ft triangular slab at 4 in (24 ft³) costs $156–$191 at $160–195 per yard, or 44 bags of 80 lb mix at your bag price; installed slabs run $6.50–10.50 per sq ft (Concrete Network), so 72 sq ft is about $468–$756.
Triangle and Rectangle Together
A slab with a triangular end, such as a 10×12 ft rectangle plus a triangle with a 10 ft base and 6 ft height, at 4 in: 40 ft³ + 10 ft³ = 50 ft³, or 2.04 yd³ with extra. Add both shapes as parts in the calculator.
Concrete for a Triangle in Bags
Autocomplete asks how many bags a triangle takes: divide the cubic feet (with 10% extra) by 0.6 for 80 lb bags or 0.45 for 60 lb bags.
| Triangle (4 in) | ft³ | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 8×8 ft | 10.67 | 20 | 27 |
| 10×10 ft | 16.67 | 31 | 41 |
| 12×16 ft | 32 | 59 | 79 |
Concrete Triangle Calculator FAQ
How do you work out concrete for a triangle?
Multiply half the base by the height and by the thickness in feet, then divide by 27 for cubic yards.
Next step
Add the triangle as a part next to rectangles in the slab yardage calculator to total an irregular slab.
Official sources
Last reviewed 2026-11-05. Numbers come from our tested engine; see how we calculate and our sources.