How to Fill a Raised Bed

1. Decide the finished level and calculate the empty volume

Measure the bed on the inside. Decide where you want the finished soil surface to sit, then subtract any depth already occupied by usable soil or a deliberate lower fill. The calculator can do this subtraction for rectangular, square and circular beds.

2. Check what the bed is sitting on

Drainage and the lower layer depend on construction. RHS recommends raised beds on free-draining ground where possible. For hard surfaces it advises drainage holes or weep holes. Shallow beds over soil can use the ground below as part of the rooting environment, while deeper beds can be managed differently.

3. A very deep frame does not mean premium growing medium to the bottom

RHS specifically notes that for beds deeper than 50cm, lower-value rubble or subsoil can be used in the base to improve drainage and reduce the amount of good soil required, with removed turf and soil reused where appropriate. That is a practical reason to calculate the fresh growing layer rather than automatically multiplying the full frame height.

4. Choose the growing medium for the crop and site

RHS gives a general mixture for many raised beds using topsoil, organic matter and sharp sand, but it also changes the proportions for crops that need extra drainage and for beds built on hard surfaces. That alone shows why a universal 50/50 or 70/30 internet ratio should not be hard-coded into a quantity calculator.

Think about the existing soil, bed depth, drainage, crop, local conditions and the actual products available. Use the Topsoil Calculator and Compost Calculator separately if you have decided on your own material split.

5. Avoid excessive compaction

Fill in manageable layers and firm only enough to remove large voids. The aim is not to ram the bed solid. Soil structure and drainage matter, and newly placed material can settle after watering and time.

6. Leave room to settle and top up

RHS recommends filling a raised bed to around 5cm below the rim, firming it and leaving it to settle for a couple of weeks before topping up if needed. That is a useful planning principle because a perfectly brim-full fresh fill may not remain at the same level.

Worked example

A 2m × 1m bed that needs 400mm of fresh growing material has a volume of 0.8m³, or 800 litres. Two identical beds need 1.6m³. If the frames are 600mm deep but 200mm is already deliberately occupied, the calculation is still based on the remaining 400mm rather than the full 600mm.

Buying bags, bulk bags or loose material

Small retail bags are easy to handle but a large bed can consume dozens. Bulk bags and loose deliveries vary in stated volume and weight, so compare supplier information against the calculated litres or cubic metres rather than assuming a universal bag capacity.

Common mistakes

1. Measuring the outside of the timber or masonry.
2. Filling to the physical rim instead of deciding a finished level.
3. Using one fixed soil recipe regardless of crop and drainage.
4. Ignoring material already in the bed.
5. Forgetting settlement and top-up after the first fill.