Show the math
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Diameter 10 ft; Thickness 4 in = 0.3333 ft
- Radius = 10 ÷ 2 = 5 ft
- Volume = π × 5² × 0.3333 × 1 = 26.1799 ft³
- Net volume = 26.1799 ft³ ÷ 27 = 0.9696 yd³ (× 0.0283168 = 0.7413 m³)
- Order quantity = 0.9696 yd³ × (1 + 10%) = 1.0666 yd³ = 28.7979 ft³
- 80 lb bags = 28.7979 ft³ ÷ 0.6 ft³ per bag = 48 → 48 bags (always round up)
- Weight = 28.7979 ft³ × 150 lb/ft³ = 4,320 lb
How to measure
Measure straight across the widest part of the circle, through the center. That is the diameter. If you can only measure from the center to the edge, that is the radius, so double it before you enter it. Thickness is the same as for any slab: the depth of the poured concrete.
For an oval, use the average of the long and short diameter as a rough estimate, then add extra waste.
The circle formula
The area of a circle is π × r², where r is the radius. Multiply that by the thickness in feet to get cubic feet, then divide by 27 for cubic yards. Because π ÷ 4 is 0.785, a round slab uses about 78.5% of the concrete of a square slab with sides equal to its diameter. A 10 ft circle is not 100 ft² like a 10 ft square; it is 78.5 ft².
Formula: V = π × (D ÷ 2)² × (T ÷ 12) × quantity.
Worked example: a 10 ft round patio, 4 in thick
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Diameter 10 ft; Thickness 4 in = 0.3333 ft
- Radius = 10 ÷ 2 = 5 ft
- Volume = π × 5² × 0.3333 × 1 = 26.1799 ft³
- Net volume = 26.1799 ft³ ÷ 27 = 0.9696 yd³ (× 0.0283168 = 0.7413 m³)
- Order quantity = 0.9696 yd³ × (1 + 10%) = 1.0666 yd³ = 28.7979 ft³
- 80 lb bags = 28.7979 ft³ ÷ 0.6 ft³ per bag = 48 → 48 bags (always round up)
- Weight = 28.7979 ft³ × 150 lb/ft³ = 4,320 lb
| Quantity | Result |
|---|---|
| Net volume | 0.970 yd³ (26.18 ft³, 0.741 m³) |
| Order with 10% waste | 1.07 yd³ (0.82 m³) |
| 80 lb bags | 48 |
| 60 lb bags | 64 |
| Weight at 150 lb/ft³ | 4,320 lb |
You order about 1.07 yd³. The same patio as a 10 × 10 ft square needs 1.36 yd³, so the circle saves 0.29 yd³ and 14 bags.
Quick numbers for common circles
| Diameter | Area | Cubic yards | 80 lb bags |
|---|---|---|---|
| 3 ft | 7.1 ft² | 0.087 | 4 |
| 4 ft | 12.6 ft² | 0.155 | 7 |
| 6 ft | 28.3 ft² | 0.349 | 16 |
| 8 ft | 50.3 ft² | 0.621 | 28 |
| 10 ft | 78.5 ft² | 0.970 | 44 |
| 12 ft | 113.1 ft² | 1.396 | 63 |
| 15 ft | 176.7 ft² | 2.182 | 99 |
Forming a circle
Round forms are made from flexible material such as bendable hardboard or thin plywood kerfed on the back, staked every foot or so. The shape is easy to get wrong: the form has a tendency to flatten between stakes. Measure the finished diameter in two directions and use the average. Pour into a round form from the center out, and screed with a pivoting board anchored on a center stake.
Common mistakes
- Entering the radius as the diameter. That cuts the volume to a quarter.
- Using the circumference instead of the diameter.
- Pouring a circle thinner at the edge than the center because of a crowned base.
- Forgetting the control joints: a round slab over 10 ft benefits from joints to a center point.
Forming a round slab
Round slabs are formed with flexible material: thin hardboard, plywood strips or bender board, staked every foot or so. An 8 ft circle at 4 in thick needs 0.68 yd³ with 10% waste, which is 31 bags of 80 lb mix. Mark the center with a stake and swing a string at the radius to set the form, then check the circle at several points before staking it for good.
A circle is easy to estimate but harder to pour than a square. The curved edge cannot be screeded against a straight form, so work from the center outwards with a short screed board, and use a flexible edger for the perimeter. Slope a patio circle slightly from the center towards one side so that water drains.
Frequently asked questions
How do I calculate concrete for a circle?
Halve the diameter to get the radius, square it, multiply by π, then by the thickness in feet. Divide by 27 for cubic yards. A 10 ft circle at 4 in: 3.1416 × 25 × 0.3333 = 26.18 ft³, or 0.97 yd³.
How many bags for a 6 ft round pad?
A 6 ft circle at 4 in thick holds 9.42 ft³, which is 18 bags of 80 lb mix with a 10% allowance.
How much concrete is in a 12 ft circle?
1.396 yd³ at 4 in thick, before waste.
Is a circle cheaper than a square?
For the same width, yes, by about 21%. A round slab has less area than the square that encloses it.
Can I calculate half a circle with this?
Yes. Calculate the full circle and halve the result, or set quantity to 0.5 if you prefer.
What thickness should a round patio be?
Four inches is common for foot traffic. Heavier loads need more, plus reinforcement.
Sources
Estimate only. Results depend on the numbers you enter, ground conditions and your supplier. For structural work consult a licensed engineer and follow local codes. Brand names such as Quikrete and Sakrete are used only to cite published product data. This site is not affiliated with any brand.