Grow Light Shelf Capacity Calculator
A shelf’s physical tray count sets an upper bound on seedling capacity. Usable light coverage can be smaller. This calculator checks rectangular fit; it does not evaluate light intensity.
Try your own numbers
4 nominal 10 × 20 trays in a uniform orientation
Check the whole seasonMeasure the useful rectangle
Measure inside posts and lips, not the outside rack dimensions. Leave room for cables, watering access, and anything else that occupies the shelf. Use your actual tray dimensions; “1020” is a nominal label and products can have larger outside rims.
Check both orientations
A 10 by 20 inch rectangle can sit with either edge along the shelf. Compare each uniform orientation. This simple estimate does not search mixed-orientation nesting or allow trays to hang past the shelf edge. Physical confirmation beats a theoretical area calculation.
Treat lighting as a separate constraint
A four-tray physical fit does not guarantee four trays receive useful light. Enter only the tray positions you judge usable under your fixture. Seedling Space models capacity you provide; it does not recommend fixture intensity, electrical layouts, or safe shelf loading.
Worked example
A clear 48 × 24 inch rectangle holds four nominal 10 × 20 trays in one orientation. The area ratio alone suggests 5.76, which is not a usable whole-tray layout.
These calculations describe space and time, not guaranteed plant performance. Use your actual equipment and crop-specific growing guidance.