How to use this calculator
Use published cell capacity when available. For a real purchasing estimate, fill one tray as you normally would and measure the loose compost used, then select the measured-tray method. That accounts for your firming and any compost above the dividers. For geometry, measure the inside of one rectangular cell at the bottom and at the actual fill line; do not use the outside dimensions of the entire tray. Enter usable loose compost already on hand, or zero when starting from scratch. Capacity and target purchases are separate scenarios using that same starting stock.
The formula
A worked example
Ten trays with 60 cells at 21 mL each contain 12.6 L of nominal cell space. An explicit 10% allowance gives 13.86 L, so one 20 L bag leaves 6.14 L relative to that plan. If your own fill trial instead uses 2 L of loose compost per tray, choose the measured method: ten trays with no extra allowance need 20 L. With 5 L already available in the nominal-cell example, fill three complete trays now, leaving 0.842 L. The separate ten-tray target is short by 8.86 L; buy one 20 L bag and retain 11.14 L after filling all ten.
Assumptions & limitations
All trays are identical. Stock must be usable compost suitable for this same planned fill; compare loose litres on the same basis as your tray estimate. The stock-capacity scenario does not consume compost before the separate target-order scenario. Nominal cell volume and loose bag volume are not interchangeable measures of firmed compost. The allowance is your input, not a universal correction; avoid adding the same allowance twice after measuring a tray. The geometry method assumes straight tapering walls and rectangular cross-sections; rounded corners, ribs, drainage holes and compost above dividers are excluded. This estimates volume, not a crop-specific compost recipe or sowing schedule.
Common questions
How many seed trays can I fill with leftover compost?
Enter the usable loose litres already on hand. The result divides that stock by one tray’s requirement including your allowance and rounds down to complete trays. The target-purchase result starts from the original stock, not from the leftovers after making that maximum batch.
How much compost do 60-cell trays need?
The cell count alone is insufficient. Use the actual mL per cell or measure one tray. A 60-cell tray with 21 mL cells has 1.26 L nominal capacity, but filling and firming can use more loose compost.
Can I use 40 L or 50 L compost bags?
Yes. Enter the stated bag volume. Bag quantities round up and the result shows the excess purchased volume.
Does taper make a difference?
Yes. A cell 4 × 3 cm at the fill line, 2 × 1 cm at the base and 6 cm deep holds 38 mL in this geometric model, rather than the 72 mL of a straight box with the same top size.
Supporting reference
The method above states this calculator’s assumptions. This reference provides related definitions and technical context.
Containerwise: CD60 cell count and published cell capacity ↗Formula and worked example checked against automated test cases. About our methodology.