Tray capacity and usable transplant count are different numbers. A tray with 72 cells provides 72 growing positions, but an expected loss means you may need to start more cells than the final number of seedlings you want.
The Seed Tray Planner uses an expected usable cell percentage. It converts the transplant target into a filled cell count, rounds the tray purchase up and shows how many cells in those purchased trays remain unfilled. Seeds and medium are counted for the filled cells only.
A target of 150 transplants
For 150 transplants and an 85% expected usable cell share, 150 divided by 0.85 is about 176.471 cells. The tool rounds that up to 177 filled cells.
With 72 cells per tray, three whole trays are required. They provide 216 positions, leaving 39 positions unfilled in this plan. At one seed per filled cell, the sowing count is 177 seeds. If each filled cell takes 50 mL of medium, the batch requires 8.85 litres.
That medium result does not fill all 216 available positions. If you intend to fill every purchased tray, change your plan accordingly rather than expecting the calculator to treat unused capacity as planted automatically.
Usable cells are the basis of this estimate
Enter the proportion of filled cells that you expect to produce a usable transplant. This is not necessarily the packet germination percentage. Seedling losses after germination can also affect whether a cell produces a plant suitable for moving.
Increasing seeds per cell changes the number of seeds sown, but it does not automatically improve the usable cell percentage in the model. Multiple seeds may need thinning, and more seedlings inside one cell are not counted as several final transplants here.
Use the capacity of the actual tray, not a rough description such as large or nursery size. Cell volume can vary substantially between trays with the same cell count, so the medium per cell needs its own measurement or supplier specification.
Save the tray plan with its assumptions
The results help organise a sowing session: how many trays to have ready, how many positions to fill, and how much medium to mix. Keep the success assumption alongside those quantities when you save the CSV so a later batch can use your observed record instead.
For a chosen medium recipe, Potting Mix Ratio Calculator divides the litre target into ingredient parts. For a seed order based on separate germination and establishment percentages, Seed Quantity Calculator offers a different calculation. Do not apply both tools' loss allowances to the same count without checking what each target represents.