Mulch & Bark Calculator

Q: How much mulch do I need per square metre?

Multiply the area by the depth of mulch required. For weed suppression and moisture retention, a 75 mm depth is recommended. For a 20 m2 border at 75 mm depth: 20 x 0.075 = 1.5 m3. A bulk bag (jumbo bag) of bark mulch typically contains approximately 0.7-0.9 m3 loose, so you would need 2 bulk bags. Bagged bark mulch usually comes in 60-70 litre bags (0.06-0.07 m3). Loose bark mulch is the most economical for large areas - order in full loads where possible.

Q: What depth of mulch should I apply?

For general weed suppression and moisture retention, apply mulch to a depth of 50-75 mm. Thinner layers (under 50 mm) allow light through and do not suppress weeds effectively. Avoid applying mulch deeper than 100 mm as this can exclude air and water from the soil and cause root rot in established plants. Keep mulch 50-100 mm away from the base of tree trunks and shrub stems - a mulch mound against the stem (called volcano mulching) traps moisture and encourages rot and disease.

Q: What is the difference between bark mulch, wood chip, and garden compost?

Bark mulch is made from shredded or chipped tree bark and decomposes slowly, making it a long-lasting mulch for ornamental borders - it typically lasts 2-4 years before needing topping up. Wood chip is chipped whole wood (including sapwood) and decomposes faster - it is better for paths and play areas than ornamental borders as it can temporarily lock up soil nitrogen as it breaks down. Garden compost is fully decomposed organic matter - it is a soil conditioner rather than a mulch and should be dug in or lightly forked into the soil surface.

Q: Can I use mulch to suppress weeds?

Bark mulch applied at 75 mm depth significantly reduces weed growth by blocking light reaching the soil surface. For best results, remove all existing weeds (including roots) and lay a weed-suppressing membrane before applying mulch. The membrane prevents weeds coming up from below while the mulch blocks light from above. Some persistent weeds such as bindweed and couch grass will still grow through. Refresh the mulch layer every 2-3 years as it decomposes to maintain the weed-suppressing depth.

Q: What mulch is best for a play area?

For children's play areas, use play-grade bark or wood chip certified to BS EN 1177 (Impact Attenuating Surfacing). This standard specifies the critical fall height that the surface can protect against. A 300 mm depth of BS EN 1177 certified bark chip provides protection for falls from up to approximately 3 m. The depth must be maintained as the chip compacts and decomposes over time - inspect and top up at least annually. Play-grade bark must be free of splinters, sharp edges, and toxic species.

Q: How often do I need to top up mulch?

Bark mulch in ornamental borders typically needs topping up every 2-3 years as it decomposes into the soil (which also improves soil structure and fertility). In high-rainfall areas or on heavy clay soils where decomposition is faster, top up every 1-2 years. Wood chip decomposes faster than bark and may need annual topping up. Gravel and slate decorative mulches do not decompose but can be lost through displacement and will need topping up every 3-5 years. Weed membrane under all mulch types extends the life of the mulch layer.

Volume, bag count, and cost for any garden mulch or ground-cover material across multiple beds and borders.

Material

Volume/bag: -

Depth (mm)

75 mm is the recommended minimum for effective weed suppression. 100 mm for problem areas.

Overage / Waste (%) 10% recommended to account for uneven ground and settling.

Include weed membrane Landscape fabric under the mulch - calculated in m² (total area + 10% overlap).

Delivery format

Beds & Borders

Bed / border name Length (m) Width (m)

Or enter total area directly (m²) - overrides grid above

Job Costing (optional)

Mulch cost per bag

Weed membrane per m²

Labour

Rate x total area

Delivery (flat fee)

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Results

Total Area
-
m²

Volume (inc. waste)
-
m²

Bags to order

Weed Membrane

m²

Bed Breakdown

Bed / Border	Dimensions	Area (m²)	Net vol (m²)	With waste (m²)

Frequently Asked Questions

How much mulch do I need per square metre?

Multiply area by depth. A 20 m² border at 75 mm depth: 20 x 0.075 = 1.5 m³. A bulk bag holds approximately 0.7-0.9 m³ loose, so 2 bulk bags. Bagged bark: approximately 60-70 litres (0.06-0.07 m³) per bag. Loose delivery is most economical for large areas.

What depth of mulch should I apply?

For weed suppression and moisture retention: 50-75 mm. Under 50 mm lets light through - weeds will grow. Over 100 mm can exclude air and water, causing root rot. Keep mulch 50-100 mm away from plant stems and tree trunks - piling it against them traps moisture and causes rot.

What is the difference between bark mulch, wood chip, and garden compost?

Bark mulch: shredded tree bark, slow to decompose, lasts 2-4 years - ideal for ornamental borders. Wood chip: chipped whole wood, decomposes faster, better for paths and play areas (can lock up soil nitrogen temporarily). Garden compost: fully decomposed organic matter - a soil conditioner, not a mulch; dig or fork into the soil surface.

Can I use mulch to suppress weeds?

Yes - 75 mm of bark mulch significantly reduces weeds by blocking light. For best results: remove all weeds first (including roots), lay a weed-suppressing membrane, then apply mulch on top. Persistent weeds like bindweed and couch grass can still grow through. Refresh every 2-3 years as bark decomposes.

What mulch is best for a play area?

Use play-grade bark or wood chip certified to BS EN 1177 (Impact Attenuating Surfacing). A 300 mm depth of certified bark chip protects against falls from up to approximately 3 m. Inspect and top up annually as the chip compacts and decomposes. Must be free of splinters, sharp edges, and toxic species.

How often do I need to top up mulch?

Bark mulch in ornamental borders: every 2-3 years. High-rainfall areas or heavy clay soils: every 1-2 years. Wood chip: faster decomposition, may need annual topping up. Gravel/slate: does not decompose but displaces over time - top up every 3-5 years. A weed membrane underneath extends the life of all mulch types.

Mulch Depth Guide

Depth	Best for
50 mm	Light decorative cover, annual top-up
75 mm	Standard weed suppression (recommended minimum)
100 mm	Problem weed areas, tree bases, new borders
150 mm	Paths, play areas, heavy-duty weed control

Don't mulch right up to plant stems - leave a 100 mm gap to prevent collar rot.

Material Guide

Bark chips (fine): Neat, long-lasting 2-3 years. Best for beds and borders.
Bark chips (medium/coarse): Larger pieces, slower to break down, great for paths.
Wood chip: Raw chipped wood, cheaper, breaks down faster - good soil improver.
Cocoa shell: Attractive, chocolatey smell, nitrogen-rich. Toxic to dogs - avoid if pets present.
Compost: Improves soil as well as suppressing weeds. Re-applied annually.
Straw: Cheap, good for veg beds. Short-lived - biodegrades in one season.
Rubber mulch: Very long-lasting, good for play areas and paths. Not for edible gardens.
Gravel / slate: Permanent cover. Use for drought-tolerant schemes and Mediterranean planting.

Tips

Lay weed membrane first for best results - peg every 0.5 m and overlap joins by 150 mm
Wet the ground before laying mulch in dry weather
Top up bark mulch every 1-2 years as it breaks down
Compost and wood chip improve soil structure - they're not just weed suppressants
Bulk loose loads are significantly cheaper per m² than bagged for large areas
Allow for slight settling - freshly applied mulch compresses by about 10-15%

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Mulch & Bark Calculator

How much mulch or bark do I need for my garden?