Tinker 
How to use the mulch calculator
Choose imperial or metric units. Then enter the length and width of the bed and the depth of mulch you want. For an irregular bed, split it into rectangles, run each one, and add the results.
Add a percentage for settling if you like, then pick the bag size sold at your garden centre. Bulk mulch is usually sold by the cubic yard or cubic metre, so those figures are shown as well.
The formula
Volume = length × width × depth. In imperial units the depth is in inches, so divide it by 12 to get cubic feet. Divide cubic feet by 27 to get cubic yards. In metric units the depth is in centimetres, so divide it by 100 to get cubic metres. One cubic metre is 1,000 litres, and one cubic foot is 28.317 litres.
Bags needed = volume × (1 + extra %) ÷ bag volume, rounded up to a whole bag.
Worked example
Take a bed 12 ft by 9 ft, mulched 3 in deep. 12 × 9 × 3 ÷ 12 = 27 cubic feet, which is exactly 1 cubic yard or about 0.76 m³. With 2 cu ft bags, 27 ÷ 2 = 13.5, so you buy 14 bags. If you add 10% extra and use 3 cu ft bags, 29.7 ÷ 3 = 9.9, so you buy 10 bags.
Metric example: a bed 5 m by 2 m at 10 cm deep needs 1 m³, or 1,000 litres. That is 20 bags of 50 litres.
Practical tips
A depth of 5–8 cm (2–3 in) suppresses weeds without smothering roots. Keep mulch a few centimetres away from plant stems and tree trunks. Fresh, fluffy mulch settles over time, so 5–10% extra is sensible. The results are estimates. Bag fill and bed edges vary.
Frequently asked questions
How many bags of mulch make a cubic yard?
A cubic yard is 27 cubic feet. That is 13.5 bags of 2 cu ft (so buy 14), or 9 bags of 3 cu ft.
How deep should mulch be?
Most beds do well with 2–3 inches (5–8 cm). Paths and play areas often use 3–4 inches.
Is bulk mulch cheaper than bags?
Usually it is, once you need more than about 2–3 cubic yards (1.5–2 m³). Delivery fees can change the balance for small jobs, so compare the totals.
Can I use this for gravel, soil or compost?
Yes. The volume and bag count work for any loose material sold by volume. Only the recommended depth differs.