Circular Garden Bed Calculator: Area, Edging and Mulch

This calculator handles a round bed or tree ring. Enter the diameter in feet and the mulch depth in inches; you get the bed area, the length of edging around it, and the cubic yards of mulch to cover it.

Formula

Area = π × (diameter ÷ 2)². Circumference (edging) = π × diameter. Mulch (yd³) = area (ft²) × depth (in) ÷ 324, where 324 = 12 in per ft × 27 ft³ per yd³.

• dia: diameter across the bed in feet
• dep: mulch depth in inches

Worked Example

A bed 8 ft across, mulched 3 in deep.

1. Area = π × 4² = 50.27 ft²
2. Edging = π × 8 = 25.13 ft
3. Mulch = 50.27 × 3 ÷ 324 = 0.4654 yd³ (about 12.6 ft³)

How to use the result

For a ring around a tree, find the area of the outer circle and subtract the inner circle you leave bare around the trunk. Bagged mulch is sold in cubic feet: multiply cubic yards by 27.

Limits and common mistakes

• Measure the diameter through the centre; measuring a chord off-centre gives too small a bed.
• Keep mulch away from tree trunks and plant stems.
• For an oval bed use area = π × a × b with the two half-widths a and b.

Frequently Asked Questions

How much mulch for a 6 ft circle?

At 3 in: π × 3² × 3 ÷ 324 = 0.26 yd³ (about 7.1 ft³).

What if I only know the circumference?

Diameter = circumference ÷ π.

How deep should mulch be?

Often 2-3 in for beds; check advice for your plants.

Does this work in metres?

Enter feet; 1 m = 3.2808 ft.

in dia | Bed diameter | ft | 8 | 0 | 1000
in dep | Mulch depth | in | 3 | 0 | 24
out area | Bed area | ft² | pi * (dia / 2) ^ 2 | 2
out edge | Edging around the bed | ft | pi * dia | 2
out mul | Mulch to cover it | yd³ | area * dep / 324 | 2