Circular Concrete Slab Calculator: Round Slabs and Pads
A round slab 10 ft across and 4 in thick needs 0.97 cubic yards of concrete, or 1.07 yd³ with 10% extra.
Advanced options: waste, prices
NRMCA advises ordering 4 to 10 percent over the estimate.
Concrete needed (with 10% extra)
1.07 yd³
0.82 m³ · 26.18 ft³ before extra
Round: 10 ft × 4 in. Over about 1 yd³: order ready-mix.
Premix bags
- 80 lb48
- 60 lb64
- 50 lb77
- 40 lb96
- Ready-mix cost
- $171
- Weight
- 3,927 lb
Full breakdown
| Volume before extra | 0.97 yd³ · 0.74 m³ |
|---|---|
| With 10% extra | 1.07 yd³ |
| Weight (150 lb/ft³) | 3,927 lb · 1.96 tons |
| Pallets of 40 lb (80 bags each) | 2 |
| Pallets of 60 lb (56 bags each) | 2 |
| Pallets of 80 lb (42 bags each) | 2 |
| 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
- Round slab volume = 3.1416 × radius² × thickness.
- A 10 ft circle at 4 in = 1.07 yd³ with extra.
- Measure the diameter through the center.
- Hot tub pads: at least 4 in, reinforced (Bullfrog Spas).
How Much Concrete for a Round Slab?
You need 3.1416 × radius² × thickness. A 10 ft circle 4 in thick holds 26.18 ft³, which is 1.07 yd³ with 10% extra, or 48 bags of 80 lb mix.
Circular Slab Formula
ft³ = 3.1416 × (Diameter (ft) ÷ 2)² × (Thickness (in) ÷ 12)
yd³ = ft³ ÷ 27Concrete for Round Slabs by Diameter
| Diameter | 4 in (yd³ + 10%) | 6 in (yd³ + 10%) | 80 lb bags at 4 in |
|---|---|---|---|
| 6 ft | 0.38 | 0.58 | 18 |
| 8 ft | 0.68 | 1.02 | 31 |
| 10 ft | 1.07 | 1.60 | 48 |
| 12 ft | 1.54 | 2.30 | 70 |
| 16 ft | 2.73 | 4.10 | 123 |
| 20 ft | 4.27 | 6.40 | 192 |
Round Fire Pit or Hot Tub Pad
A round pad follows the same formula; Bullfrog Spas advises at least 4 in, reinforced, for a hot tub pad. A 12 ft round patio at 4 in needs 1.54 yd³ with extra.
How to Figure Cubic Yards in a Circle
- Measure the diameter and halve it for the radius.
- Square the radius and multiply by 3.1416 for the area.
- Multiply by the thickness in feet.
- Divide by 27 and add 10%.
Worked Examples: Three Round Pours
8 ft round hot tub pad at 4 in. 3.1416 × 4² × 0.333 = 16.76 ft³; with extra, 31 bags of 80 lb.
14 ft round patio at 4 in. 51.31 ft³; with extra, 2.09 yd³.
Fire pit ring, 6 ft circle minus 3 ft center, 4 in. 9.42 − 2.36 = 7.07 ft³; with extra, 13 bags of 80 lb.
Round Column or Cylinder
A round column uses the same formula with height instead of thickness; for tubes and piers use the concrete tube calculator, which also counts bags per foot.
Half Circle and Quarter Circle Slabs
Divide the full circle by 2 or 4: a half-circle 12 ft across at 4 in holds 18.85 ft³, and a quarter circle with a 6 ft radius holds 9.42 ft³.
Round Slab Cost
Concrete for a 10 ft round slab at 4 in costs $171–$208 at $160–195 per yard; a 12 ft round patio is $246–$299. Installed, a 12 ft circle (113 sq ft) runs about $735–$1,187 at $6.50–10.50 per sq ft (Concrete Network).
How to Form a Round Slab
- Mark the center and swing a string to the radius to draw the circle.
- Use flexible form board or bender board for the curve.
- Stake the form every couple of feet so it holds the circle.
- Check the depth at several points before ordering.
Round Concrete Pad for a Fire Pit or Grill
A 6 ft round pad at 4 in holds 9.42 ft³, or 18 bags of 80 lb mix with extra; an 8 ft pad holds 16.76 ft³, or 31 bags.
Circular Concrete Slab Calculator FAQ
How do you calculate a circular concrete slab?
Multiply 3.1416 by the radius squared and by the thickness in feet, then divide by 27 for cubic yards.
How do you figure cubic yards of concrete in a circle?
Find the circle's area (3.1416 × r²), multiply by thickness in feet and divide by 27.
How do you calculate circle concrete quantity?
Area (3.1416 × radius²) × thickness in feet = cubic feet; divide by 27 for cubic yards.
Does a 4-inch concrete slab need rebar?
Not by code: the 2021 IRC treats slab-on-ground reinforcement as optional (R506.2.4); see the rebar spacing calculator.
Next step
For a round slab next to a square patio, add both as parts in the concrete slab volume calculator.
Official sources
Last reviewed 2026-11-05. Numbers come from our tested engine; see how we calculate and our sources.