How to estimate soil and compost volume
Volume starts with inside dimensions and actual fill level, then converts to bag litres or bulk cubic metres.
Back to calculator ↑Measure shape and fill level
For a rectangular planter, measure inside length, inside width and the depth you will actually fill, all in centimetres. For a straight-sided circular planter, measure inside diameter and fill depth. The formulas use the internal space occupied by growing medium; outside dimensions overstate the capacity, and full wall height overstates it when you leave room below the rim.
A tapered pot, raised base, reservoir, stones or other inserts do not match the simple shapes closely. If you know a reliable filling volume from the container maker or your own measurement, choose known litres instead. Deducting a separate object by eye can be more misleading than entering a tested capacity.
Convert centimetres to litres and bags
Divide cubic centimetres by 1,000 to get litres. A rectangular planter measuring 100 × 50 × 30 cm inside holds 150 litres at that fill depth. With a 5% extra allowance, the estimate is 157.5 litres. At 25 litres per bag, 157.5 ÷ 25 = 6.3, so buy seven whole bags. The same quantity equals 0.1575 cubic metres for bulk ordering.
For a cylinder, halve the inside diameter to get radius, then use π × radius² × fill depth ÷ 1,000. Apply your allowance after calculating the shape volume. If you buy bags, use the stated litres per bag and round up once at the end. Bulk output remains an exact cubic-metre estimate because suppliers may set their own minimum order quantities.
Compost is part of the total volume
If you enter a compost percentage, the tool divides the total required volume. For example, 20% of 100 litres means 20 litres of compost and 80 litres of other soil or growing medium. It does not add 20 litres on top of a 100-litre fill. With bag purchasing, each component rounds to whole bags separately, so the total bought can exceed the exact calculated fill.
This percentage is your own planned mix, not a recommended universal recipe. The University of Minnesota Extension distinguishes container potting mixes from raised-bed mixes; plant needs, drainage and bed conditions influence the choice. Check guidance for the crop or ornamental plant you are growing before deciding the material and compost share.
Buying checks and common mistakes
Check the actual litres printed on each bag and whether the product is intended for containers or raised beds. Growing media can settle and the final fill level can change after watering. The adjustable extra allowance covers some uncertainty, but it is not a promised settlement rate. For a large job, confirm how the bulk seller measures and delivers cubic metres.
Do not confuse centimetres with metres or treat a 25 kg bag as 25 litres. Mass and volume differ, and moisture changes weight. Use volume stated on the package. Blank prices remain unknown; a known zero can be entered as zero. For a bulk compost share, the calculator shows the separate cubic metres but does not invent a compost price.
Common questions
What about a tapered pot?
Use a known capacity from the maker or a direct measurement; a straight cylinder can overestimate a tapered pot.
Is compost added on top of the soil volume?
No. Entered compost share replaces an equal part of the total volume, reducing the soil portion.
Can I buy by kilograms?
This tool uses litres and cubic metres because mass per volume depends on material and moisture. Ask the seller for the relevant conversion.
Sources and calculation guidance
Check these against your chosen product’s label and instructions.