How this calculator works for circles
A circle's volume is π × (diameter/2)² × depth. The calculator handles π and the squaring for you — you just enter the diameter (the widest distance across the circle, edge to edge) and the depth.
Don't enter the radius by mistake. If you measure radius (center to edge), double it before typing. A 6 ft radius circle has a 12 ft diameter — a 4× difference in area if you mix them up.
The pi shortcut
For a quick mental check: a circle's area is roughly 0.785 × diameter². A 10 ft circle is about 78 sq ft; a 12 ft circle about 113 sq ft. Useful when comparing fire-pit sizes at the supply yard.
Recommended depth & material
| Use | Depth | Material | Notes |
|---|---|---|---|
| Around a fire pit (decorative ring) | 3" | Pea gravel or river rock | Ring shape: use annulus mode on home calc. |
| Tree well (mulch ring) | 2" – 3" | Pea gravel or river rock | Keep 2" ring away from trunk. |
| Circular patio (loose gravel) | 4" | #57 base or pea gravel top | Edge to keep stones contained. |
| Round paver patio | 4" base + 1" sand | CA6 base, concrete sand bedding | See paver-base for two-layer math. |
| Round French drain (catch basin) | 12" | #57 washed | Around a perforated catch basin. |
How to measure a circle
Measuring circles is straightforward — just be precise about diameter vs. radius.
- Stake the center. Drive a stake at the dead center of the planned circle. Tie a string to it.
- Set the radius with the string. Mark the desired radius on the string (half the diameter you want). Pull the string taut and walk a circle, marking with chalk or paint.
- Measure the diameter to confirm. Use a tape across the widest part of the marked circle. Verify it matches twice your intended radius.
- Enter the diameter in the calculator. Diameter, not radius. The calculator squares the radius internally — you'd be inflating your estimate by 4× if you typed the radius.
- Set depth and material. 3 inches of pea gravel is the default for a decorative circle. Adjust per use case.
- Add 10% buffer. Circles waste a tiny bit more material than rectangles because the edge isn't a clean line.
Worked example: 12-ft circular gravel area, 3" deep
A small circular patio or fire-pit area: 12 ft diameter, 3 inches deep, pea gravel.
- Area: π × (12/2)² = π × 36 = 113 sq ft
- Volume: 113 × 3 ÷ 324 = 1.05 yd³
- Weight: 1.05 × 2,450 ÷ 2,000 = 1.29 tons
- With 10% buffer: ~1.4 tons of pea gravel.
- Cost: at $55/ton, ~$77 in stone, plus delivery. Or 65 retail bags (0.5 cuft) at ~$5 each = $325 if you go bagged.
Bigger circle? At 16 ft diameter, area jumps to 201 sq ft, and the order roughly doubles to ~2.5 tons.