How to measure a raised bed
Measure inside the frame, not outside. Sleepers and boards are thick, and on a small bed the gap between the outside and inside measurements is a big share of the soil. For a rectangular bed take the inside length (A) and the inside width (B). For a round bed or a planter ring, measure the inside diameter (A) straight across through the centre.
Then measure the depth to fill, from the ground (or the top of anything already in the bottom) to where the soil should finish, leaving a little room below the top edge so water soaks in. The depth is always in centimetres, even when the beds are in feet and inches; an inch is about 2.5 cm.
Filling several beds? Add each one with Add another bed. The calculator adds them together and gives one shopping list, so you can see whether a bulk bag makes more sense than a pile of small bags.
How deep to fill a raised bed
How deep the soil needs to be depends on what you grow. These are the depths of good soil that Rolawn and the RHS give for planting in open ground, and they make a sensible guide for filling a raised bed too. The calculator starts at the shallow end of Rolawn’s figure for vegetables, 30–40 cm; change it in step 2.
| Use | Depth of soil | Source | Apply |
|---|---|---|---|
| Flowers, shrubs and perennialsMost ornamental beds. | 15–20 cm | Rolawn, RHS | |
| VegetablesRoot crops need the most depth. | 30–40 cm | Rolawn |
Loose soil and compost settle once they are watered. Quality Garden Supplies suggests ordering 10–25% more than the measured volume, and the top of that range for raised beds, so that is where the Extra for settling slider starts. Slide it down if you are only topping up a bed that has already settled.
How much soil for raised bed kits and sleeper beds?
The sum is short: inside length × inside width × depth gives cubic metres, and each cubic metre is 1,000 litres. A bed 2 m long and 1 m wide, filled 30 cm deep, holds 0.6 m³, or 600 litres before any extra for settling. Raised beds swallow far more bags than people expect.
Kit beds, sleeper beds and round planters all work the same way. Use the inside measurements, add every bed, and this raised bed calculator gives one total for the whole plot. A round bed is worked out as a circle, so a ring 1 m across holds a little over three-quarters of what a 1 m square bed would at the same depth.
Topsoil or compost: which mix?
There is no published ratio of topsoil to compost for raised beds. We looked, and no UK supplier or gardening body gives a fixed split, so the calculator starts at 70% topsoil and 30% compost soil improver as a starting point you can change, not as a recommendation. Set Topsoil in the mix to whatever you plan to use; the rest is counted as compost soil improver.
The compost line is compost soil improver, the bulky organic matter Rolawn and others sell for mixing into soil, rather than multi-purpose potting compost. Quality Garden Supplies notes that mixes with compost settle more than topsoil alone, so a bed that is mostly topsoil holds its level better as the organic matter breaks down.
Rolawn’s guide to building a raised bed suggests filling it in alternate layers of topsoil and organic matter, then leaving it a few weeks to settle before you plant. That works with any mix: the calculator gives the total of each, and you decide how to layer them. If you buy a ready-blended bed mix, set the share to 100% and read the topsoil line as your blend.
A soil calculator for raised bed mixes
Topsoil and compost soil improver are both sold by volume, so the bags are worked out from litres. The topsoil line also shows tonnes, using a typical 1.3–1.7 t/m³ for moist screened topsoil, because weight is what matters for a delivery or a car boot. The bag count does not depend on that density, only the tonnes do. Soil improver is light and is shown in litres only.
Worked example: two raised beds
Here is the full calculation for a vegetable plot with one long rectangular bed and one small round bed, filled deeper than the default for root crops, with the starting mix of 70% topsoil and 30% compost soil improver and the settling allowance. Every line comes from the same code that runs the calculator above.
- Bed 1 3 m × 1.2 m = 3.60 m²
- Bed 2 π × (1 m ÷ 2)² = 0.79 m²
- Total bed all areas added together = 4.39 m²
- Volume to fill 4.39 m² × 0.4 m = 1.75 m³
- Plus settling 1.75 m³ + 25% = 2.19 m³
- topsoil 2.19 m³ × 70% × 1.5 t/m³ = 1.535 m³ = 2.30 t
- compost soil improver 2.19 m³ × 30% × 1,000 = 658 litres
On the list:
- 2bulk bags of topsoil (0.75 m³ each)1.53 m³ (2.30 t) needed
- 135 litre bag of topsoilto top up
- 1bulk bag of compost soil improver (0.75 m³ each)658 litres needed
Notice that the long bed does most of the work: the round bed adds less than you might think. Notice too how the list buys: a whole bulk bag is topped up with small bags only when a little is left over, and a job that needs more than half a bulk bag gets a bulk bag rather than a pile of small ones.
UK topsoil and soil improver bag sizes
Topsoil and compost soil improver are both sold by volume in the UK. Bulk bags start at 500 litres, and the largest jumbo bags need a lorry with a crane. The shopping list uses the first bulk bag in the table, topped up with bags of 35 litres for topsoil and 50 litres for soil improver.
| Product | Size | Good for |
|---|---|---|
| Bulk bag, 0.75 m³ | 750 litres | A common large bulk bag of topsoil or compost. |
| Bulk bag, 0.5 m³ | 500 litres | The standard bulk bag from several topsoil suppliers. |
| Jumbo bag, 1.2 m³ | 1.20 m³ (1,200 litres) | The largest bag; needs a lorry with a crane to deliver. |
| 35 litre bag | 35 litres | Small bag of topsoil or compost. |
| 30 litre bag | 30 litres | Small bag of topsoil. |
| 25 litre bag | 25 litres | Small DIY-store bag of topsoil. |
| 50 litre bag | 50 litres | The usual bag of compost or soil improver. |
If your bags are a different size, divide the litres in the result by the bag size and round up. Even a small bulk bag of moist topsoil weighs well over half a tonne, so have it dropped as close to the beds as the lorry can reach.
Common mistakes when filling a raised bed
- Measuring the outside of the frame. Sleepers and boards take up room. Inside measurements give the soil you actually need.
- Forgetting the settling. A freshly filled bed sinks as it is watered. Without the extra allowance the soil ends up well below the top edge within weeks.
- Mixing up litres and cubic metres. One cubic metre is 1,000 litres. A bed that needs half a cubic metre needs ten bags of 50 litres, not one.
- Assuming there is a correct ratio. No UK source publishes a fixed topsoil-to-compost split for raised beds. Pick the mix that suits what you grow and set it in step 2.
- Planting straight away. Rolawn suggests leaving a newly filled bed a few weeks to settle before planting. Keep a bag back to top it up.
What else you'll need
- Topsoil for borders or a lawn beside the beds: the topsoil calculator works it out by area and depth.
- Compost to top up existing beds each year: use the compost calculator for a mulch layer.
- Bark or wood chip for the paths between the beds: the bark calculator covers it.
- Plants to fill the bed: the plant spacing calculator counts how many fit at the right spacing.
Last checked 28 October 2026