Mulch Calculator
Works out mulch for beds, borders and around trees. Gives cubic yards and cubic metres along with bag counts, since mulch is sold both loose and bagged.
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
Depth is the whole decision, and the window is narrower than it looks:
- Under 50 mm and light reaches the soil, so weeds come through and you have mulched for decoration only.
- 75 mm is the target. Enough to block light and hold moisture.
- Over 100 mm and water struggles to get through to the roots, while the soil underneath can go anaerobic.
Keep it off the stems. Mulch piled against a trunk holds moisture against the bark and invites rot — leave a clear ring of 50–75 mm around anything woody.
Bark mulch is light: about 350 kg per cubic metre, a fifth of the weight of gravel. A bulk bag goes much further than the same bag of stone would.
Frequently Asked Questions
How deep should mulch be?
75 mm. Below 50 mm weeds push through; above 100 mm water has trouble reaching the roots and the soil underneath can turn sour.
How much area does a cubic yard of mulch cover?
About 10 m² at 75 mm deep, or roughly 110 square feet. A cubic metre covers about 13 m² at the same depth.
How many bags of mulch in a cubic yard?
At 70 litres a bag, about 11. Bagged mulch typically costs two to three times as much per cubic metre as loose, so loose is worth it above roughly a cubic yard.