How much growing medium fits?
Measure the inside of the bed and the actual depth you intend to fill. Wall height and usable soil depth are not necessarily the same.
Illustrative 2.4 × 1.2 m bed filled 30 cm deep: 22 nominal 40 L bags, rounded up. This is volume, not the load on a roof or balcony.
How this works, and what it leaves out
Litres = inside length (m) × inside width (m) × fill depth (cm) × 10. Bag count rounds up the stated volume. Irregular shapes, existing fill, settling, compaction and the composition of the mix need separate allowances. No conversion to weight is made.
How to use this tool
- Measure the inside length and width in metres. Exclude any space that will remain occupied by existing fill.
- Enter the depth of growing medium in centimetres. This can be lower than the wall height.
- Enter the volume printed on the bags you are considering. The bag count rounds the calculated volume up to whole bags.
A 2.4 × 1.2 metre bed filled 30 centimetres deep
The calculation is 2.4 × 1.2 × 0.30 = 0.864 m³, or 864 litres. With nominal 40-litre bags, 864 ÷ 40 = 21.6, so the estimate rounds up to 22 bags. Those figures describe a rectangular volume, before any allowance for settlement or a differently shaped base.
What the result means
If you are topping up an existing bed, calculate the added depth rather than buying enough to fill the whole structure again. For an irregular bed, divide the shape into rectangles and add their volumes. Settling, the mix composition and the material already in place need a separate judgement.
Questions before you start
Should I use the wall height as the depth?
Only if that is the depth you actually intend to fill. Leave out any unfilled space above the soil and account separately for material already in the bed.
Will this tell me whether a balcony can carry the bed?
No. Volume is not structural loading. Wet material, the frame, stored water and people all contribute to load. This calculator makes no assessment of a balcony, deck or roof.
Put it in context
Use these guides alongside the tool for the choices a calculation or decision form cannot make for you.