How Much Mulch Do I Need? The Depth Formula That Works
Bed size and depth converted to cubic yards, cubic metres or bags, with worked examples for US and UK gardens.
Quick answer: Multiply the bed's length by its width by the mulch depth, all in the same unit. A US bed 20 ft by 10 ft at 3 inches deep is 50 cubic feet, which is 1.85 cubic yards. A UK bed 6 m by 3 m at 75 mm deep is 1.35 cubic metres, or 27 fifty-litre bags.
Mulch is sold in units nobody thinks in. You measure a garden bed in feet or metres, then have to buy cubic yards, cubic metres or bags. Getting it wrong in either direction is annoying: too little means a second delivery charge, too much means a pile on the drive for a fortnight.
The formula, and the depth that matters
Length times width times depth, with everything converted to the same unit first. Depth is the step people fumble, because 3 inches is 0.25 feet and 75 mm is 0.075 metres.
US worked example. A bed 20 ft by 10 ft is 200 square feet. At 3 inches deep that is 200 x 0.25 = 50 cubic feet. There are 27 cubic feet in a cubic yard, so 50 / 27 = 1.85 cubic yards. Order 2.
UK worked example. A bed 6 m by 3 m is 18 square metres. At 75 mm deep that is 18 x 0.075 = 1.35 cubic metres. A 50-litre bag holds 0.05 cubic metres, so you need 27 bags. That is when the bulk bag starts looking sensible.
How deep to go
Two to three inches (50 to 75 mm) is the usual range for established beds. Three to four inches suppresses weeds better but can smother shallow-rooted plants and stop light rain reaching the soil. Keep mulch pulled back a few inches from tree trunks and plant stems, since piling it against bark holds moisture where you least want it. If you are topping up rather than starting fresh, measure the depth you already have and only calculate the difference.
Run your own numbers
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Open the Mulch CalculatorBags or bulk
Bagged mulch is convenient and expensive per unit. A standard US bag is 2 cubic feet, so 13.5 bags make a cubic yard. Compare the two properly: multiply the bag price by 13.5, then set that against the bulk price per yard plus the one-off delivery fee. At $4 a bag that is $54 a cubic yard, against $30 a yard with a $60 delivery charge, and the two lines cross at two and a half yards, where both come to $135. Below that buy bags, above it order bulk, and run the same sum with your own supplier's prices because the crossover moves with the delivery fee.
The practical limits are handling and space. A cubic yard of bark or wood chip is heavy, more so after rain, so a load that fits a trailer by volume can still sit over its weight limit. A bulk bag needs a drop point a vehicle can reach and a tarp if it will sit for more than a day.
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Use the Mulch CalculatorHow to use the mulch calculator
Bed length and width. Measure with a tape, not by pacing. Garden beds are usually smaller than people guess. Depth. Choose inches or millimetres and stay consistent. Units. Set cubic yards for a US supplier, cubic metres or litres for a UK one.
For an irregular bed, break it into rectangles, run each separately and add the totals. Curved edges are close enough if you treat the curve as a straight line through its middle. For a circular bed use pi times the radius squared: a 12 ft circle has a 6 ft radius, so about 113 square feet, and at 3 inches that is 28 cubic feet or just over a cubic yard.
Getting the area right first
Most mulch mistakes are area mistakes, not volume mistakes. If the shape is awkward, work out the footprint before you touch the depth. Our guide to calculating square footage covers the odd shapes, and the same length-by-width-by-depth logic drives working out how much concrete you need.
Common questions
How many bags of mulch are in a cubic yard? Thirteen and a half, using standard 2 cubic foot bags. Nine bags if they are 3 cubic feet. In the UK, twenty 50-litre bags make one cubic metre.
How much does a cubic yard of mulch cover? About 324 square feet at 1 inch, 162 at 2 inches, and 108 at 3 inches. Halve the depth and you double the coverage, which is the quickest mental check on any quote.
Should I remove the old mulch first? Usually not. Wood mulch breaks down into the soil, which is part of the point. Rake it level, check the remaining depth, and top up to your target. Remove it only if it has matted into a crust water cannot pass through.
How much extra should I order? Round up by about 10%. Beds are rarely perfectly rectangular, settling costs you a little depth, and running short by a tenth of a yard means paying a second delivery charge for a wheelbarrow of bark.