Circular Bed Calculator — Round Flower Bed Area & Edging

Push a stake into the centre and tie a string to it at exactly half the diameter (the radius) — for a 3 m bed that is 1.5 m. Pull the string taut and walk a full loop, marking the edge with dry sand, line marker or spray paint. For a semi-circle or quarter, peg the straight edges first then swing the string between them.

Flexible steel edging or corrugated plastic edging bends around tight curves without kinking and gives a crisp line. Victorian rope-top tiles suit period gardens, while log roll is the budget choice for gentle curves. Brick or stone setts look best but each unit must be cut or angled to follow the arc, so allow extra for offcuts.

Dig to at least 300 mm for most perennials and annuals so roots have room and drainage is good. If the existing soil is poor, build a raised circular bed 200-400 mm above ground level and fill with a topsoil/compost mix. The RHS recommends improving heavy or sandy soils with bulky organic matter before planting.

Multiply the bed area by the fill depth in metres to get cubic metres. A 3 m full circle is 7.07 m²; filling it 300 mm (0.3 m) deep needs 7.07 × 0.3 = 2.12 m³, which is roughly four 0.6 m³ bulk bags of topsoil. Compost is sold by the litre (1 m³ = 1,000 L), so the same volume is about 2,120 L — around 35 of the 60 L bags.

The radius is always half the diameter, so divide the diameter by two: a 3 m diameter bed has a 1.5 m radius. Measure the diameter across the widest point of the marked circle, through the centre stake. Area uses the radius (πr²) while the full-circle perimeter uses the diameter (πd), so getting this conversion right keeps both figures correct.