What the mulch calculator does
Bagged mulch is labelled by volume (2 or 3 cubic feet in the US, 50–75 litres in the UK and Australia), while bulk mulch is delivered by the cubic yard or cubic metre. This calculator converts the area of your beds and your chosen depth into both, so you can compare the cost of bags with a bulk delivery.
Add every bed separately; the calculator totals them and shows each bed’s share.
How to use it
- Measure the length and width of each bed. For curved beds, see the measuring tips below.
- Pick a depth: 2, 3 or 4 inches (50, 75 or 100 mm), or choose Custom depth.
- Choose a mulch type for guidance on how it behaves.
- Enter the bag size printed on the bag to get a bag count.
- Optionally enter a price per bag and a bulk price to compare.
Mulch formula
Volume = Length × Width × Depth
Bags = Volume ÷ Bag size, rounded up
Because depth is usually given in inches, divide it by 12 to get feet before multiplying. 1 cubic yard = 27 cubic feet, so one cubic yard covers 108 sq ft at 3 inches deep, 162 sq ft at 2 inches or 81 sq ft at 4 inches. In metric, 1 m³ covers about 13.3 m² at 75 mm deep.
Worked example
Two beds mulched 3 inches deep:
- Front bed 20 ft × 4 ft = 80 sq ft → 80 × 0.25 = 20 ft³
- Side bed 8 ft × 3 ft = 24 sq ft → 24 × 0.25 = 6 ft³
- Total: 26 ft³ = 0.96 cubic yards
- In 2 ft³ bags: 26 ÷ 2 = 13 bags
Metric: a 6 m × 1.2 m border at 75 mm is 6 × 1.2 × 0.075 = 0.54 m³ (540 litres), or 11 bags of 50 litres (10.8 rounded up).
Understanding the results
- Mulch needed – total volume for all beds at your depth.
- Bags – total volume divided by the bag size, rounded up to a whole bag.
- Breakdown – each bed’s area and volume.
- Bagged vs bulk cost – shown side by side when you enter both prices. Bulk is often cheaper for larger areas but may carry a delivery charge.
Measuring and spreading tips
- For a curved bed, measure the longest length and the average width. For a round bed, use the diameter for both length and width and multiply the result by 0.785.
- Pull mulch back a few inches from tree trunks and plant stems to reduce rot and pests.
- Existing mulch counts toward the depth. If 1 inch remains, top up by 2 inches rather than 3.
- Wet mulch is heavier and settles; measure depth after the first watering if you are topping up.
Factors that change how much you need
- Settling – light, fluffy mulches compress after watering and rain.
- Uneven ground – dips and mounds change the effective depth.
- Bag fill – bag volume can be measured loose at packing time; contents may compact in transit.
- Plants – large shrubs and stones take up space, so dense planting needs a little less.
Filling new raised beds? Use the raised bed soil calculator. For stone instead of organic mulch, use the gravel calculator. See also our guide: How much mulch do you need?