Soil & Compost Calculator
Filling a raised bed or border with the wrong amount of soil is an easy way to overspend or run short halfway through a job. Whether you're topping up with topsoil, compost, or a multi-purpose mix, getting the volume right first means you can compare buying loose bulk bags against standard 50L or 70L bags with confidence. Enter your bed or border dimensions below and we'll do the maths for you.
Enter your raised bed or border dimensions
We'll work out how much soil or compost you need, in litres, m³ and standard bags.
Enter a length, width and depth greater than zero to see how much soil you'll need.
Recommended products
A few well-reviewed options for composting and filling 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
Topsoil or compost for a raised bed?
Most raised beds do best with a blend rather than one or the other: a good general mix is around 50-60% topsoil for structure and weight, combined with 40-50% compost or well-rotted manure for nutrients and drainage. Pure multi-purpose compost alone can hold too much water and settle quickly, while pure topsoil alone can be too dense and low in nutrients for vegetables.
How deep should soil be for vegetables?
Leafy crops and shallow-rooted veg like salad and herbs are fine with 15-20cm of soil, but most vegetables — beans, brassicas, courgettes — do better with 25-30cm. Root vegetables like carrots and parsnips need at least 30-40cm of loose, stone-free soil to grow straight and to size.
What is the difference between topsoil, compost and multi-purpose compost?
Topsoil is the mineral-rich soil dug from the ground, used mainly to fill volume and provide structure. Compost (garden or soil improver) is decomposed organic matter added to boost fertility and improve drainage or moisture retention. Multi-purpose compost is a lighter, more nutrient-dense growing medium, often peat-free, designed for containers and seed starting rather than bulk-filling large beds.
Why does my finished bed need less soil than I expected?
Loose soil and compost settle and compact by around 10-20% once watered in, so it's worth buying slightly more than the raw calculated volume, or planning to top up after the first few waterings.
How this calculator works
The calculator multiplies your bed or border length by width by depth to get the volume in cubic metres, then converts that to litres by multiplying by 1,000 (since 1m³ = 1,000 litres). It then divides the total litres by the bag size you choose — 50L or 70L — and rounds up to the nearest whole bag, since you can’t buy part of a bag.
It’s worth buying slightly more than the exact calculated volume. Loose soil and compost settle and compact by around 10-20% once watered in, especially if it’s been delivered loose rather than in bags. If you’re filling a deep raised bed (over 30cm), consider adding an extra 10% on top of the calculator’s bag count to allow for settling and to bring the bed up to the right level after the first few waterings.
Soil types and what to use where
| Material | Best for | Typical depth | Notes |
|---|---|---|---|
| Topsoil | Filling volume, structure | 20-40cm | Mineral-rich, heavy, low nutrients alone |
| Multi-purpose compost | Containers, seed starting | 15-30cm | Light, nutrient-dense, not for deep beds alone |
| Garden compost / soil improver | Improving existing soil | 5-10cm top layer | Organic matter, boosts fertility and drainage |
| Ericaceous compost | Acid-loving plants | 15-30cm | For rhododendrons, camellias, blueberries |
| Manure (well-rotted) | Vegetable beds, heavy feeders | 10-20cm | High nutrients, must be well-rotted or it burns roots |
Worked example: A 2m x 1m x 30cm raised bed
A raised bed 2m long, 1m wide and 30cm deep has a volume of 2 × 1 × 0.3 = 0.6m³, which is 600 litres. Using 50L bags, you need 600 ÷ 50 = 12 bags exactly. Using 70L bags, you need 600 ÷ 70 = 8.57, rounded up to 9 bags.
For a vegetable bed, a good mix is 60% topsoil and 40% compost. That means 360 litres of topsoil (7 x 50L bags or 6 x 70L bags) and 240 litres of compost (5 x 50L bags or 4 x 70L bags). Buying in bulk bags (usually 500L-1m³) is often cheaper for large beds, but you need somewhere dry to store any leftover soil.
Common mistakes when buying soil and compost
- Buying pure compost for a deep raised bed. Multi-purpose compost is too light and airy for a 30cm-deep bed — it will settle and compact, leaving you with a bed that’s half the depth you planned. Mix it with topsoil for volume and structure.
- Forgetting that bulk bags are measured in litres or cubic metres, not by weight. A 1m³ bulk bag of topsoil weighs around 1.2-1.4 tonnes, so you need a hard-standing area for delivery and a way to move it to the bed. Don’t order a bulk bag if you can’t get the vehicle close to the garden.
- Not accounting for the volume of existing soil or rubble at the bottom of the bed. If you’re filling a raised bed on top of existing soil, the new soil only needs to fill the depth above ground level, not the full depth of the bed frame.
- Using the wrong compost for the job. Ericaceous compost is essential for acid-loving plants like rhododendrons and blueberries, but it’s a waste of money for vegetables or general bedding plants. Check the plant’s requirements before buying.
- Ordering exactly the calculated volume with no spare. A 10% extra allowance covers settling, and it’s useful for topping up after heavy rain or for filling gaps around plant roots.
What to look for when buying soil and compost
For topsoil, look for a BS3882:2015 specification if you want guaranteed quality. This British Standard covers topsoil for landscaping and ensures it’s free from contaminants, has the right pH for most plants, and contains a reasonable amount of organic matter. Budget topsoil from unknown sources can contain builders’ rubble, weeds seeds or even broken glass.
For compost, check whether it’s peat-free. Most UK garden centres now stock peat-free multi-purpose compost by default, and many professional growers prefer it because it’s more sustainable and often performs better in the long run. If you’re growing acid-loving plants, make sure the compost is specifically labelled ericaceous — standard multi-purpose compost is usually neutral or slightly alkaline.
Finally, think about delivery. A 1m³ bulk bag of topsoil delivered to your door is usually the cheapest way to fill a large bed, but you need a hard, level surface for the delivery vehicle and a way to move the bag to the bed. If you don’t have vehicle access, stick to 50L or 70L bags that you can carry or wheelbarrow.
More questions answered
Topsoil or compost for a raised bed?
Most raised beds do best with a blend rather than one or the other: a good general mix is around 50-60% topsoil for structure and weight, combined with 40-50% compost or well-rotted manure for nutrients and drainage. Pure multi-purpose compost alone can hold too much water and settle quickly, while pure topsoil alone can be too dense and low in nutrients for vegetables.
How deep should soil be for vegetables?
Leafy crops and shallow-rooted veg like salad and herbs are fine with 15-20cm of soil, but most vegetables — beans, brassicas, courgettes — do better with 25-30cm. Root vegetables like carrots and parsnips need at least 30-40cm of loose, stone-free soil to grow straight and to size.
What is the difference between topsoil, compost and multi-purpose compost?
Topsoil is the mineral-rich soil dug from the ground, used mainly to fill volume and provide structure. Compost (garden or soil improver) is decomposed organic matter added to boost fertility and improve drainage or moisture retention. Multi-purpose compost is a lighter, more nutrient-dense growing medium, often peat-free, designed for containers and seed starting rather than bulk-filling large beds.
Why does my finished bed need less soil than I expected?
Loose soil and compost settle and compact by around 10-20% once watered in, so it's worth buying slightly more than the raw calculated volume, or planning to top up after the first few waterings.
Can I reuse existing soil from another part of the garden?
Yes, if it’s good quality. Existing topsoil from a patio or decking excavation can be reused in a raised bed, but you may need to mix in compost or manure to restore nutrients. Avoid reusing soil that has been contaminated by builders’ rubble, chemicals, or persistent weeds like Japanese knotweed or ground elder.
How often should I replace or top up soil in a raised bed?
Raised bed soil doesn’t need replacing every year. Top up with 5-10cm of fresh compost or manure each spring to replenish nutrients, and do a full replacement every 3-5 years if the bed is heavily cropped. Container-grown plants need fresh compost every 1-2 years because nutrients are exhausted faster in confined root space.
Should I put a membrane at the bottom of my raised bed?
A weed-suppressant membrane at the bottom of a raised bed on top of existing soil or hardcore helps prevent weeds growing up into your bed and stops soil washing out through the base. Use a permeable membrane so water can drain through, not a solid plastic sheet, which will create a waterlogged bog. If you’re building on a hard surface like concrete, a membrane isn’t necessary but you should ensure there are drainage holes in the sides.
What is the best soil pH for a vegetable garden?
Most vegetables grow best in slightly acidic to neutral soil, pH 6.0-7.0. Potatoes, carrots and tomatoes tolerate a slightly wider range, but brassicas (cabbage, kale, broccoli) prefer a slightly alkaline pH of 6.5-7.5. You can buy a simple soil pH testing kit from most garden centres. If your soil is too acidic, add garden lime; if it’s too alkaline, add sulphur or use ericaceous compost for acid-loving crops.