Measure rim to rim
Use the tray’s longest outside points, including flanges and handles. Do not rely only on the “1020” name.
Seed-starting station capacity planner
How many seed trays fit your shelf?
Check the trays, lights, heat mats, domes and growing room together—before you buy or assemble the rack.
Runs in your browserNo signupEvery dimension editable
4 trayswhen the clear dimensions work
Quick answer: A four-foot rack can hold four 1020 trays per tier only when its clear space is wide and deep enough for the trays’ actual outside dimensions. “1020” is a category, not one exact size.
Build your station plan
Measure once. Know what fits.
1
Your shelf
Use the advertised dimensions, then subtract obstructions.
2
Your tray
Measure rim to rim. Presets are only a starting point.
A five-minute measuring guide
Measure the space the tray can actually use.
Find the clear shelf
Measure between posts and inside raised shelf lips. Record anything that reduces the rectangular space.
Check the vertical stack
Add tray height, dome or plant height, fixture thickness and your intended light gap.
Use real footprints
Enter illuminated coverage and heat-mat dimensions—not just product names or nominal fixture lengths.
Why exact measurements matter
Two “1020” trays can be different sizes.
One widely sold heavy-duty 1020 tray is listed at 21.1 × 11 inches. Another is listed at 21.44 × 10.94 inches. That difference can decide whether two trays fit between posts or whether a row clears a shelf lip.
TrayTally keeps every measurement editable because the object in your hand is the final authority.
21.1 × 11 in
21.44 × 10.94 in
Same category. Different outside dimensions.
Transparent by design
How TrayTally calculates capacity
Physical fit
We subtract the post and lip losses you enter, then test rows and columns of trays in both orientations—including useful mixed layouts. Only complete trays count.
Coverage
Light and mat capacity are calculated separately from shelf capacity. Heat mats use a clearly disclosed 90% footprint tolerance to reflect common nominal 1020 mats.
Vertical clearance
Opening height minus tray, dome or plant, fixture and light-gap dimensions gives the remaining clearance. Negative means a collision.
What the result does not claim
This is a geometric planning aid—not horticultural, structural or electrical advice. Confirm product clearances, rack capacity, power requirements and growing recommendations with the relevant manufacturer.
Common planning questions
Before you order the rack
How many 1020 trays fit on a 48 × 24 shelf?
Often four—two across and two deep—but only if the clear space between obstructions fits the actual outside dimensions plus any handling gap. Enter your measurements above to verify it.
Can four 1020 trays fit on a 48 × 18 shelf?
Usually not fully supported within an 18-inch-deep rectangle: two tray widths generally need about 22 inches. Some gardeners accept overhang, but TrayTally counts only trays fully inside the usable shelf area.
Does the calculator choose a grow light?
No. It checks whether trays fit inside the footprint you enter. It does not infer PPFD, DLI, spectrum or crop suitability from wattage or fixture size.
Why does heat-mat capacity use a tolerance?
Many mats sold for 1020 trays are slightly smaller than the tray’s outer rim. We treat a mat footprint at least 90% of the tray dimensions as one compatible heating position and show that assumption beside the result.
Does sharing a plan upload my measurements?
No. The plan is encoded into the shareable URL. The calculator runs in your browser and does not require an account or save your plan to a database.
Plan every tray, light, mat and inch.