Mulch & Topsoil Calculator

How mulch quantities are worked out

Multiply bed length × width × depth to get volume in cubic metres, convert to litres (×1000), then divide by your bag size. A 75L bag of mulch or compost is common, but topsoil and bark come in various sizes.

Worked example: a 8 m × 1.2 m border at 50 mm

The bed is 9.6 m². At 50 mm deep that is 0.48 m³, or 480 litres. Bark mulch is sold in 50 or 60 litre bags, so budget 8–10 bags. Expect the layer to settle by roughly 10% in its first season — the small allowance this calculator adds for that is why the bag count rounds up rather than down.

How deep to mulch

50–75 mm suppresses weed seed germination and holds moisture through a dry spell. Around shallow-rooted plants and annuals, 25 mm is plenty and keeps stems dry. Deeper than 100 mm on heavy clay can starve roots of oxygen, so build up in two springs rather than one.

Which mulch suits which job

Composted bark is the best all-rounder for borders and around roses. Woodchip is cheap and lasts well on paths and rough ground, but it robs nitrogen while it breaks down, so keep it off vegetable beds. Composted green waste feeds the soil as it rots — good for hungry beds, less good around acid-lovers like rhododendrons. Leaf mould is low-nutrient and ideal for woodland planting. Gravel or slate suits alpines, pots and modern planting schemes where you want no organic breakdown at all.

Technique that makes it last

Water thoroughly first — mulch seals in moisture that is already there and does little on bone-dry soil. Keep a 50 mm clear collar around trunks and stems so nothing sits wet against the bark. Top up in spring once the soil has warmed; mulching in late winter keeps the ground cold for longer. Five minutes with a hoe before you spread it saves an hour of weeding later.

Where people waste money

Spreading mulch 20 mm thin, which weeds push straight through. Buying 60-litre bags when you need more than about 1 m³ — a bulk bag holds roughly 850 litres and costs a fraction per litre. Piling it against trunks and causing rot. And using fresh woodchip on beds where the plants are already struggling.