Measure the space available for new soil
A raised bed’s advertised outside size can overstate the space it can hold. Measure the inside length and width, then the inside height. In the topsoil and raised bed calculator, headroom is the space you intend to leave below the rim.
A 2 × 1 m interior filled to 35 cm holds 0.7 m³. Entering a 40 cm wall height without the 5 cm headroom would add 0.1 m³ that is not part of that fill plan.
Count existing materials once
Lower filler and existing soil above it occupy different parts of the bed. If filler is 10 cm deep and soil above it is 5 cm, deduct both from the target height. Do not enter their combined height again in the existing-soil field.
The calculator makes a geometric deduction; it does not recommend a filler recipe or establish whether the existing material is suitable for the intended plants. Those choices require their own assessment.
Use the surface-layer mode for a different question
Adding topsoil over an area is not the same as filling an empty container. State the desired layer and the usable existing portion within that layer. The ground beneath it should not be subtracted simply because it already exists.
Use separate zones for depths or bed shapes that differ. Check the sketch for overlapping rows, because the tool cannot infer where independently entered beds sit on the site.
Read bag and bulk quantities separately
For a 0.44 m³ target, nine 50 L bags provide 0.45 m³. A supplier selling in 0.25 m³ increments would instead supply 0.5 m³. The difference is purchase rounding, not another soil allowance.
Record the units printed on the bag. US dry quarts are not interchangeable with liquid quarts. The report keeps volume units explicit. Use the mulch calculator for a distinct surface mulch layer and the volume calculator for other geometric volumes.