Mulch Calculator — Bags & Coverage

Mulch volume is calculated as area multiplied by depth. The RHS recommends applying organic mulch such as bark, wood chip or garden compost to a depth of 50 to 75 mm, which is 5 to 7.5 cm. That depth suppresses weeds, retains moisture and insulates roots without smothering the soil. Gravel mulch is usually sufficient at 25 to 40 mm. Going deeper, up to 100mm, buys stronger weed suppression where a bed has been badly infested.

The calculator shows volume in cubic metres and litres along with the number of bags needed. Standard retail bags hold 50 to 80 litres, which is 0.05 to 0.07 m³ and covers only 0.7 to 1 m² at 75mm. Bulk bags hold approximately 0.5 to 1.0 m³, so one 0.7 to 0.9 m³ bag covers around 12 m² at that depth. A loose truck load of 3 to 15 m³ is cheaper per cubic metre but needs access. Bark weighs roughly 300 to 400 kg per m³ loose, while composted material is heavier at 500 to 700 kg per m³.

Enter the bed area and the depth you want and the totals follow, along with bags or bulk bags and an estimated cost. A 20 m² border at 75mm comes to 1.5 m³, so about 2 bulk bags. Keep mulch 5 to 10 cm clear of plant stems and tree trunks, because material banked against bark holds moisture there and causes rot.

Material choice in 2026 sets the cost. Decorative bark mulch is £45 to £90 a bulk bag and wood chip is cheaper and more rustic at £30 to £60. Composted bark at £55 to £100 is the premium option, since it improves the soil as it breaks down. Garden compost costs nothing if you make it yourself, or £30 to £50 a bulk bag commercially. Coco husk at £40 to £90 a bag is light with decent eco-credentials. Stone or slate chippings at £60 to £140 last far longer but add nothing to the soil. Black plastic mulch is cheap in commercial growing and poor environmentally.

The gains from a good mulch layer are measurable. At 75mm and above, weed growth drops by 70 to 90%, and summer watering falls by 30 to 50% because the soil below stays damp. Mulch insulates roots against temperature extremes, slows erosion on slopes and stops rain splashing soil onto foliage, which cuts disease. Organic materials such as bark and compost feed the soil as they rot and give earthworms a habitat, so the biology improves year on year. Beds also simply look tidier with a clean mulched edge.

Mid-spring, from March to May, is the moment to apply, when the soil is moist but warming. Hot dry summer days lock in heat and trap dryness, frozen ground is a waste of material, and midsummer mulching just buries weeds that are already growing. Weed the bed first, water it if the soil is dry, then spread 75 to 100mm around the plants without letting it touch the stems. Organic mulches decompose by 20 to 50% a year, so top up annually, and never build a mulch volcano against a tree trunk, since the bark rots and pests move in.

Some jobs want a specific material. Composted bracken or pine needles make an ericaceous mulch for rhododendrons, azaleas and blueberries. Straw is the traditional cover under strawberries, keeping the fruit off the soil. Living mulches such as clover and vetch are green manure crops sown between rows and dug in at the end of the season. Glass nuggets last well and look striking but do nothing for the soil, and rubber mulch is cheap and synthetic with the environmental questions that go with it. Choose on appearance, soil benefit and how long you want it to last.