Mulch Calculator
A good layer of mulch suppresses weeds, locks in soil moisture through dry spells, and gives borders and beds a neat, finished look — whether you choose bark, woodchip, or compost mulch depends on whether you want a long-lasting decorative layer or something that feeds the soil as it rots down. Getting the depth right matters too: too thin and weeds push through, too thick and you risk smothering plants. Enter the area you want to cover below and we'll work out how many bags you need.
Work out how much mulch you need
Enter the area you want to cover and choose a mulch depth.
Topping up an established border
Enter an area above to see how much mulch you need.
Recommended products
A few well-reviewed options to help you prepare and maintain your beds.
- Recycled plastic
- Large gardens
- 466L
Currently unavailable
Pros
- Three-stage composting chambers
- Click-together assembly, no tools required
- Large 466L capacity suits big gardens
Cons
- Bulky — needs a dedicated corner
- Plastic
- Small gardens
- 190L
£69.98
Pros
- Rotating drum speeds up decomposition
- Elevated design deters pests and rodents
- No digging or turning by hand
Cons
- 190L capacity may be tight for larger households
- Recycled plastic
- Small gardens
- 220L
£54.99
Pros
- Large 220L capacity at a low price
- Front hatch for easy compost access
- Lockable, air-tight lid
Cons
- Basic single-chamber design
- No aeration vents
Frequently asked questions
How often should I mulch?
Most UK gardens benefit from a fresh layer of mulch once a year, typically in spring after the soil has warmed up or in autumn to protect roots over winter. Bark and woodchip break down slowly, so topping up an existing layer to your target depth is usually enough rather than removing and replacing it entirely.
What is the best mulch for a UK garden?
Bark chippings are a popular all-rounder for borders and pathways, offering good weed suppression and a neat finish that lasts a couple of years. Woodchip is cheaper and great for informal areas but breaks down faster. Compost or well-rotted manure feeds the soil as it breaks down, making it a good choice around hungry plants, though it needs topping up more often.
Why does mulch depth matter?
Too thin a layer (under 5cm) lets light through and weeds still germinate. Too thick a layer (over 10cm) can smother plant roots, encourage rot around stems, and stop water reaching the soil properly. Sticking to 5-10cm depending on the job gives the best balance of weed suppression and moisture retention without harming plants.
Should I mulch right up to plant stems and tree trunks?
No — leave a small gap of a few centimetres around stems and trunks. Piling mulch directly against them traps moisture against the bark, which can lead to rot and encourages pests to shelter there.
How this calculator works
The calculator multiplies the area you want to cover by the depth you choose to get the total volume in cubic metres, then converts that to litres (1m³ = 1,000 litres). It then divides the total litres by the standard UK mulch bag size of 75 litres and rounds up to the nearest whole bag. The result is the number of 75L bags you need to order.
The depth options are based on common UK gardening practice: 5cm for topping up an established border, 7.5cm for new beds where weed suppression is the main priority, and 10cm for freshly dug or newly planted beds that need maximum moisture retention and weed protection while plants establish.
Mulch types and their uses
| Mulch type | Best for | Depth | Lifespan | Notes |
|---|---|---|---|---|
| Bark chippings | Borders, pathways | 5-7.5cm | 2-3 years | Neat finish, good weed suppression |
| Woodchip | Informal areas, trees | 5-10cm | 1-2 years | Cheap, breaks down faster, good for soil |
| Compost / manure | Vegetable beds, hungry plants | 5-7.5cm | 1 year | Feeds soil as it breaks down |
| Gravel / stone | Alpine plants, Mediterranean | 5cm | Permanent | Doesn’t break down, good drainage |
| Leaf mould | Woodland borders, shade | 5-10cm | 1-2 years | Free if you make your own, excellent soil conditioner |
Worked example: A 20m² border at 7.5cm depth
A border 20m² in area mulched at 7.5cm depth needs a volume of 20 × 0.075 = 1.5m³, which is 1,500 litres. Dividing by the standard 75L bag size gives 1,500 ÷ 75 = 20 bags exactly. If you chose the 10cm depth for a new bed instead, the volume would be 20 × 0.1 = 2m³ = 2,000 litres, requiring 27 bags.
Bark chippings are the most popular choice for a decorative border. A 75L bag typically covers about 1.5m² at 5cm depth, or about 1m² at 7.5cm depth. For a 20m² border at 7.5cm, you’d need roughly 20 bags of bark chippings, which at around £4-£6 per bag works out to £80-£120 for materials. Bulk bags (usually 500L-1m³) are cheaper for large areas but you need storage space.
Common mistakes when mulching
- Mulching too early in the year. Wait until spring soil has warmed up (usually April-May in the UK) before mulching borders. Mulching cold, wet soil traps moisture and slows soil warming, which delays plant growth.
- Using fresh woodchip directly on delicate plants. Fresh woodchip can temporarily rob soil of nitrogen as it breaks down, which weakens nearby plants. If you must use fresh chip, keep it away from plant stems and consider adding a nitrogen-rich fertiliser first.
- Piling mulch against tree trunks or plant stems. This is one of the most common mulching mistakes. Leave a 5-10cm gap around the base of plants to prevent moisture against the bark, which causes rot and encourages pests like voles and weevils.
- Forgetting to weed before mulching. Mulch suppresses weeds, but it doesn’t kill existing ones. Weeding first ensures the mulch stays weed-free for longer and prevents perennial weeds from pushing through the layer.
- Using the wrong mulch for the job. Gravel is excellent for Mediterranean plants and alpine beds, but it’s a poor choice for a woodland border where leaf mould or compost would feed the soil. Match the mulch to the planting style.
What to look for when buying mulch
For bark chippings, look for a product labelled as “decorative bark” or “bark chippings” rather than just “woodchip.” Decorative bark is usually screened to remove fines and small particles, giving a cleaner, more uniform finish that lasts longer. Woodchip is cheaper but contains more fines and breaks down faster, making it better for soil improvement than visual appeal.
If you’re buying in bulk, check whether the product is sourced from sustainable UK forestry. The FSC logo indicates the wood comes from responsibly managed forests. Avoid mulch made from recycled pallets or treated timber, which may contain chemicals or metal fragments harmful to plants and soil life.
Finally, consider how you’ll transport the bags. A 75L bag of bark weighs around 20-25kg when dry, but can be heavier when wet. If you have a long garden path or steps, buy smaller bags or arrange for delivery to the back door. Bulk bags (500L-1m³) are the most economical option for large areas, but you need a hard-standing area for the delivery vehicle and a way to move the bag to the border.
More questions answered
How often should I mulch?
Most UK gardens benefit from a fresh layer of mulch once a year, typically in spring after the soil has warmed up or in autumn to protect roots over winter. Bark and woodchip break down slowly, so topping up an existing layer to your target depth is usually enough rather than removing and replacing it entirely.
What is the best mulch for a UK garden?
Bark chippings are a popular all-rounder for borders and pathways, offering good weed suppression and a neat finish that lasts a couple of years. Woodchip is cheaper and great for informal areas but breaks down faster. Compost or well-rotted manure feeds the soil as it breaks down, making it a good choice around hungry plants, though it needs topping up more often.
Why does mulch depth matter?
Too thin a layer (under 5cm) lets light through and weeds still germinate. Too thick a layer (over 10cm) can smother plant roots, encourage rot around stems, and stop water reaching the soil properly. Sticking to 5-10cm depending on the job gives the best balance of weed suppression and moisture retention without harming plants.
Should I mulch right up to plant stems and tree trunks?
No — leave a small gap of a few centimetres around stems and trunks. Piling mulch directly against them traps moisture against the bark, which can lead to rot and encourages pests to shelter there.
Can I use grass clippings as mulch?
Grass clippings can be used as a mulch, but only in thin layers (2-3cm) and only if they’re from a lawn that hasn’t been treated with weedkiller. Thick layers of grass clippings mat together, become slimy, and prevent water reaching the soil. They’re best used on vegetable beds where they break down quickly and add nitrogen.
Should I put a membrane under the mulch?
A weed-suppressant membrane under mulch is useful on new beds or areas with persistent weed problems, but it isn’t essential on established borders where the soil is already weed-free. If you do use a membrane, choose a permeable woven type that lets water through — solid plastic sheets trap water and prevent air reaching the soil.
Does mulch attract slugs and snails?
Bark and woodchip mulch can provide hiding places for slugs and snails during the day, especially in damp UK conditions. If slugs are a problem in your garden, keep mulch a few centimetres away from the base of vulnerable plants and use organic slug control methods around the edges of mulched beds.
When is the best time of year to mulch?
The best time is late spring (May-June) after the soil has warmed up, or in autumn (October-November) to protect roots over winter. Mulching cold, wet soil in late winter traps moisture and delays soil warming, which can slow plant growth. Autumn mulching is particularly effective at protecting plant roots from frost heave.