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Mulch calculator
Mulch, bark, wood chip and compost are spread to a depth over a bed, so the quantity is a volume, and the volume is sold either loose by the cubic metre or, for most garden jobs, bagged by the litre.
Enter the bed length and width in metres and the mulch depth in centimetres.
This calculator turns a bed into both. Enter the bed length and width in metres and the depth you want in centimetres; the tool works out the volume in cubic metres and litres and divides by your chosen bag size to count the bags. The litre figure is the useful one at the garden centre, where bark and chip come in 50 or 70 litre bags and the cubic metre is what a landscape yard quotes for a loose load. Depth is what makes mulch work, and getting it right is the whole point of measuring rather than guessing. A layer of about 50 to 75 mm suppresses weeds by blocking the light they need to germinate, holds moisture in the soil through dry spells, and evens out the temperature around roots, while still being open enough to let rain soak through. Thinner than 50 mm and weed seedlings push straight through and the soil dries out; much thicker than 75 mm on a planted bed and the mulch can stop air reaching the roots and keep the surface so wet that stems rot. The maths makes the depth choice concrete: the same bed mulched at 75 mm needs half as much again as at 50 mm, so the depth you pick is the single biggest lever on how many bags you carry home. One detail the volume does not capture is that mulch is kept clear of plant stems and tree trunks, left in a shallow ring rather than piled against the bark, because bark held against a wet mulch collar rots, so a bed with established plants uses a little less than the bare-area figure suggests. People use this before mulching borders, vegetable beds, the ground under trees and shrubs, or a play area, and before pricing a loose load against bags. For an irregular bed, split it into rectangles, work out each area, and add them before the depth is applied; a rough curved border can be measured as its average length times its average width. The estimate covers the mulch itself, not edging or the weed-control membrane some people lay under a decorative bark, which is measured to the bare bed area instead.
How it works
- Measure the bed length and width in metres and enter them.
- Enter the depth in centimetres, usually 50 to 75 mm for weed control and moisture.
- Set your bag size in litres, commonly 50 or 70 litres for bagged bark or chip.
- The tool finds the volume in cubic metres and litres from the area and depth.
- It divides the litres by the bag size and rounds up to whole bags.
bags = ceil( length x width x (depth / 100) x 1000 / bag litres )
Length times width gives the bed area in square metres. Multiplying by the depth in metres (the centimetre figure divided by 100) gives the volume in cubic metres, and multiplying by 1000 converts that to litres, since one cubic metre is 1000 litres. Dividing the litres by the bag size gives the bags in exact terms, and rounding up reflects that bags are sold whole. For a loose load, read the cubic-metre figure and order to the yard increment.
- length, width
- bed dimensions in metres
- depth
- mulch depth in centimetres, divided by 100 for metres
- 1000
- litres in a cubic metre
- bag litres
- size of one bag, commonly 50 or 70 litres
Mulch depth and what it does
| Under 50 mm | too thin | weeds break through, soil dries |
| 50 to 75 mm | the working range | suppresses weeds, holds moisture |
| Over 100 mm on plants | too deep | can starve roots, rot stems |
| Bagged bark or chip | 50 or 70 litres | the common bag sizes |
Worked example
A 4 by 2 metre border mulched 75 mm deep, using 50 litre bags of bark: the bed is 8 m2, which at 0.075 m deep is 0.6 m3, or 600 litres, of mulch. At 50 litres a bag that is 12 bags. Drop the depth to 50 mm and the same bed needs 0.4 m3, 400 litres, or 8 bags, a third less, which is why the depth is worth settling before you count bags. A landscape yard would quote the 0.6 m3 figure for a loose load instead of the bag count.
Key facts
- Mulch quantity is a volume: bed area times the depth you spread.
- A 50 to 75 mm layer suppresses weeds and holds soil moisture.
- Bagged bark and chip come in 50 or 70 litre bags; one cubic metre is 1000 litres.
- Depth is the biggest lever: 75 mm needs half as much again as 50 mm.
Tips
- Settle the depth first, since it decides most of the order.
- Keep mulch a few centimetres clear of stems and trunks to stop rot.
- Compare a loose cubic-metre load against bags once a bed gets large.
- Mulch onto damp, weeded soil so it seals in moisture rather than dryness.
Frequently asked questions
How deep should mulch be?+
About 50 to 75 mm is the working range. That is enough to block light from weed seeds, hold moisture in the soil and buffer root temperature, while staying open enough for rain to pass through. Below 50 mm weeds break through and the soil dries; above 75 mm on a planted bed the mulch can starve roots of air and keep the surface wet enough to rot stems.
Litres or cubic metres, which do I use?+
Both describe the same volume. Garden centres sell bagged bark and chip by the litre, usually in 50 or 70 litre bags, so the litre figure and the bag count are what you need there. A landscape yard or loose delivery is priced by the cubic metre, so use that figure when comparing a tipped load against bags for a larger bed.
Why does the depth change the order so much?+
Volume is area times depth, so depth scales the whole quantity. Going from 50 to 75 mm adds half as much again, and going to 100 mm doubles it against the 50 mm figure. Because the area is fixed by the bed, the depth you choose is the main thing that decides how many bags you buy, which is why it is worth settling first.
Do I mulch right up to the plants?+
No. Keep mulch a few centimetres clear of plant stems and tree trunks, in a shallow ring rather than a mound piled against the bark, because bark held against wet mulch can rot. This means a bed full of established plants uses slightly less than the bare-rectangle figure, so on a densely planted border you can treat the estimate as a safe maximum.
What about an irregular or curved bed?+
Split it into rectangles, work out each area, and add them before applying the depth. A rough curved border can be measured as its average length times its average width, which is close enough for a mulch order. For a circle, use the radius squared times 3.14 for the area. Then read the litres and bags as normal.
Does this include a membrane or edging?+
No, it counts the mulch only. A weed-control membrane laid under decorative bark is measured to the bare bed area, the same area figure the tool already shows. Edging is bought by length to the bed perimeter. Compost dug in as a soil improver rather than spread on top is a different job, though the volume maths is the same.
Things to watch
- Piling mulch against stems and trunks can rot the bark, so leave a clear ring.
- A layer thinner than 50 mm lets weeds and drought through.
- This counts the mulch only, not a weed membrane or bed edging.
Last updated: 2026
This is an estimate for general guidance, not financial, tax, legal or medical advice. Figures can change and individual circumstances vary. Always confirm with an official source before making decisions.
Built and maintained by Vikas Dulgunde. Editorial standards.