Mulch & Bark Calculator UK · metric

Litres, cubic metres, bulk bags and cost of bark mulch for beds and borders. Metric units and £ costs, updated for 2026.

Last reviewed July 2026 · Free · Nothing you enter is stored

How this calculator works

Volume is area × depth in metres to give cubic metres (1 m³ = 1,000 litres). At 5 cm you need about 50 litres per m². A standard bulk bag holds roughly 1 m³ (1,000 litres) and covers about 20 m² at 5 cm.

The waste allowance

There is no waste allowance on this calculator, because there is very little to waste. What comes off the pallet goes onto the bed, and anything left over gets spread elsewhere. The failure here runs in the opposite direction. People order generously, spread the surplus rather than store it, and end up with borders far deeper than they intended, which does real harm to the plants in them. The only rounding worth doing is up to the next whole bag, and the number to round on is litres rather than bags, because bag sizes in this country are not standardised in any way. Bark also settles noticeably over the first few weeks as it knits together and starts to break down, so a border laid at 75 mm in March will measure less than that by June. That is normal and not by itself a reason to top it up.

Litres, cubic metres and what a bulk bag actually holds

Mulch is sold by volume in this country and the arithmetic is simple. A cubic metre is 1,000 litres, so the litres you need are the bed area in square metres multiplied by the depth in millimetres. At 50 mm that is 50 litres per square metre; at 75 mm it is 75. A full cubic metre covers about 20 m² at 50 mm, or a little over 13 m² at 75 mm.

The Royal Horticultural Society puts the same figure a different way, saying that two bucketfuls, or one standard bag of around 70 litres, will cover one square metre of ground. That works out at roughly 70 mm deep, which lines up with the depth it recommends.

The trap is the phrase bulk bag, which where bark is concerned means nothing in particular. Rolawn sells its landscaping bark in bags of approximately 500 litres and 750 litres, quoting coverage of up to 10 m² and 15 m² respectively at 50 mm. Other suppliers sell approximate 1,000 litre and 2,000 litre bags. All of them are called bulk bags. Compare the price per litre rather than the price per bag, and read the litres before ordering, because the difference between a 500 litre bag and a 1,000 litre bag is the difference between covering half your border and all of it.

This is the reverse of the aggregate trade, where a bulk bag is sold by weight to an agreed minimum. Bark has no such convention, and the word alone tells you nothing about how much you are buying.

How deep, and what the RHS says

The Royal Horticultural Society recommends that biodegradable mulches be laid at least 5 cm deep and ideally 7.5 cm. Below 5 cm you are decorating rather than mulching. The layer will not hold moisture in the soil for long, nor keep enough light off germinating weed seed to be worth the money.

The RHS is equally clear about what goes underneath. Lay mulch over moist soil, after removing weeds, when the ground is not frozen. Mulching dry soil in a drought seals the dryness in, which is precisely backwards. The conventional British timing is mid to late spring, when the soil has warmed but still holds the winter rain, and again in autumn.

Perennial weeds need digging out first. Bindweed, couch grass and ground elder come straight up through 75 mm of bark and are far harder to get at afterwards. Low-growing annual weeds do not need removing at all; the RHS notes that a covering of mulch will smother them.

The one instruction worth memorising is the gap. Keep a 10 cm mulch-free circle around any woody stem. Mulch piled against the base of a tree or shrub causes rotting, and the RHS specifically warns against piling mulches up against the stems of woody plants, where prolonged contact softens the bark and leaves it open to disease.

What too much mulch does, and where it must not go

Depth against a stem is the damaging case, not depth over open soil. Bark is adapted to sitting in the air, not to being buried. Held permanently damp against a trunk it softens and decays, and the wounds that follow are an open door to fungal infection. The base of a woody plant should visibly flare outward where it meets the ground. If it goes into the mulch like a post into a hole, there is too much of it in the wrong place.

Very deep layers of fine material behave differently from the same depth of coarse chip. Finely textured mulch packs down and can start shedding rain rather than letting it through, which is the reverse of what it is there for. Coarse bark chips stay open and do not have this problem, which is why they are the usual choice where a thick weed-suppressing layer is wanted.

Then there is the house wall, and here British practice diverges sharply from the American advice about termites. Our concern is damp. The damp-proof course is required to sit at least 150 mm above finished ground level under Approved Document C, and piling mulch, gravel or soil up against the brickwork bridges it and lets moisture track into the wall above. Air bricks ventilating a suspended timber floor have to stay clear as well. Keep beds and mulch down and away from the wall, and check every few years that nothing has crept up.

Deep mulch is also a habitat, and in a British garden that mostly means slugs and snails, which shelter in a damp layer through the day and emerge onto whatever is planted in it at night. That is not an argument against mulching. It is an argument for keeping the layer at the recommended depth and pulling it back from soft new growth.

Topping up without compounding the depth

Borders are usually refreshed once a year, and this is where they quietly get into trouble. Adding a full 75 mm every spring onto a layer that has only partly broken down compounds season after season, and after four or five years the bed is 200 mm deep with the crowns of the shrubs somewhere in the middle of it.

Measure before ordering anything. Push a trowel in at a few points around the bed. If there is still 50 mm or more of usable mulch, the job is to break the surface crust up with a rake so that water gets through and the colour comes back, not to buy more. Where the layer has genuinely thinned to 20 or 30 mm, top up to the target depth and no further.

Where beds have already been built too high, pulling the excess back is an afternoon well spent, particularly around trees and along the house wall. Up until the point at which the damage is done, it is entirely reversible.

On material, the RHS lists home-made compost, leafmould, composted bark and well-rotted manure as usually the best options, and they do different jobs. Composted bark lasts longest on the surface and suppresses weeds best; garden compost and rotted manure break down faster and feed the soil as they go. Bark belongs on the surface rather than forked in. The claim that it robs the soil of nitrogen when incorporated is repeated everywhere and disputed among soil scientists, so we would not state it as fact. What is not in dispute is that the surface layer is what suppresses weeds and holds moisture, and digging it in does neither.

Bark and mulch coverage by depth

All of this follows from a cubic metre being 1,000 litres. Bag counts assume the 70 litre bag the RHS uses as its reference size; check the litres printed on whatever your supplier actually sells, because bag volumes vary widely.

Depths of at least 5 cm and ideally 7.5 cm, the 10 cm mulch-free circle around woody stems, and the 70 litre bag covering roughly one square metre are all from RHS advice on mulching. Coverage is arithmetic from a cubic metre being 1,000 litres. Bulk bag volumes are not standardised: Rolawn sells 500 and 750 litre bags quoting up to 10 m² and 15 m² at 50 mm, while other suppliers sell approximate 1,000 and 2,000 litre bags.

What goes wrong in mulched borders

Mulch heaped against stems. The RHS asks for a 10 cm mulch-free circle around woody stems. Bark held damp against bark softens it and invites rot and disease.

Buying a bulk bag without checking the litres. Bark bulk bags run from 500 to 2,000 litres depending on the supplier. Compare price per litre, not price per bag.

Adding a full depth every year. New mulch over old that has not broken down compounds season after season until the shrub crowns are buried. Measure what is there and top up only to the target depth.

Mulching dry or frozen ground. The RHS advice is to lay mulch over moist soil that is not frozen. Mulching in a drought seals the dryness in.

Mulching over perennial weeds. Bindweed, couch grass and ground elder come straight through and are harder to dig out afterwards. Annual weeds can be left to be smothered.

Letting beds creep up the house wall. The damp-proof course needs at least 150 mm of clearance above finished ground level, and air bricks must stay unblocked. Mulch banked against brickwork bridges both.

Frequently asked questions