Mulch Calculator for Beds and Borders
Works out mulch for one bed or border. Borders are long and narrow, so the figures come out much smaller than a whole-garden estimate — usually in bags rather than loads.
Includes the waste allowance — this is the figure to order.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
volume = length × width × depth × (1 + waste) weight = volume × bulk density
The geometry is trivial; the bulk density is where the answer actually comes from. Mulch runs about 350 kg per cubic metre loose, so a cubic metre weighs roughly 0.35 tonnes. Suppliers quote one or the other, which is why both are given.
What to watch out for
A border is a strip, and strips are easy to over-order for:
- Measure front to back, not across the garden. A 6 m border that is 1.2 m deep is 7.2 m², which is a few bags, not a load.
- Subtract nothing for plants. You mulch around them, and the waste allowance covers the difference.
- Several borders add up. Total the areas first — once you pass about a cubic metre, loose delivery beats bagged on price.
Frequently Asked Questions
How many bags of mulch for a border?
A 6 × 1.2 m border at 75 mm needs about 0.6 m³, which is roughly 9 bags of 70 litres.
Should I remove old mulch before adding new?
Usually not. Bark breaks down into the soil, which is part of the point. Just top up to 75 mm total rather than adding a fresh 75 mm on top of what is already there.
When is loose mulch cheaper than bags?
Above about a cubic metre, which is roughly 15 bags. Below that the delivery charge usually outweighs the saving.