Measure the new layer, not the mulch already there.
Geometry = area × depth · Order volume = geometry × loose-volume multiplier × (1 + waste ÷ 100)
Plan the additional depth
If mulch is already present, measure it before deciding how much fresh material to add. Enter only the new layer you intend to apply. Keep bed edges and unmulched spaces out of the footprint. Circular beds can use diameter or radius.
Read the volume on the bag
Use the bag’s labelled cubic feet or litres. Weight alone is a poor substitute for volume when moisture and material vary. If your bag lists only mass, a product-specific loose bulk density is needed. The same garden volume may require different bag counts for different bag sizes.
A worked mulch example
A 6 m × 2 m border with an additional 5 cm layer requires 0.6 m³, or 600 litres. At 50 litres per bag, 12 bags cover the geometric requirement. A user-selected 10% buffer raises the order to 660 litres and 14 whole bags.
Do not count an existing layer twice
Sketch the parts of the border that need fresh material. If only half needs topping up, measure that half rather than using the entire border and assuming a universal reduction. The layer depth entered here is the additional layer you intend to put down.
Compare two bag sizes fairly
For a 600-litre requirement, 40-litre bags need 15 whole bags while 50-litre bags need 12. Enter each bag size with its own price and save a labelled estimate. Compare total purchase cost and leftover volume, not only the price printed on one bag.
Make the shopping list useful at the store
Use labels such as “side border, new layer” instead of an unexplained number. Keep the bag volume visible on the printout. If your supplier gives a depth-specific coverage claim, convert that claim only at the stated depth; doubling the layer thickness halves the ideal coverage area.
Before you place an order
Check dimensions, bag yield and the price basis. Whole bags round up; bulk orders may have minimum quantities or delivery charges that this calculator does not include. All adjustments are your inputs. Read our method, unit definitions and limits.
For material-depth context, see the University of Minnesota Extension mulch guide. Choose a depth appropriate to your plants and existing layer; this calculator does not prescribe one.
Useful questions
Is there a recommended mulch depth built in?
No. The prefilled measurements are examples. Plant type, material and existing mulch matter; check relevant horticultural guidance before choosing a depth.
Should I add a compaction multiplier?
Leave it at 1 unless your supplier or project specification gives a suitable loose-volume adjustment. Do not add the same allowance again as waste.