Seed Germination Calculator
Test germination rates and plan how many seeds to sow.
Use ToolFree calculator
Plug Tray Calculator
Cells to sow and plug trays to buy for your target transplants, adjusted for germination, plus tray cost.
Plug Tray Calculator
Chlorova · updated for 2026
Calculator
Quick samples
Click “Calculate” to see your results.
The germination allowance assumes you sow one seed per cell and that the rate on your seed packet holds true, but older or cool-soil lots sprout lower, so a stubborn variety is worth sowing a spare flat beyond this figure. Reused trays and mixed germination across species also shift the real cell count, and a domed humid tray sprouts better than an open one on a cold windowsill. Treat the tray count as a plan, then thin or re-sow any blank cells, and remember that a few extra trays are cheaper than a short set of transplants at planting time.
Start too few cells and you face planting day with empty gaps; start too many and you drown in leggy seedlings with nowhere to put them. This plug tray calculator turns the number of transplants you actually want into the cells to sow and the flats to buy, fattened for the germination rate you expect, then prices the trays. Choose a 128-, 72-, 50-, or 32-cell tray, enter your target plant count and germination, and the seed-starting setup is decided before you fill so much as one cell with mix.
Planning a Seed-Starting Setup
- Pick the tray cell count that suits the size of the transplant you want to grow.
- Enter how many finished, healthy transplants you actually need for the garden.
- Set the realistic germination rate that your particular seed will give you.
- Enter the price you pay for one single plug tray of the chosen size.
- Read the total number of cells you must sow to hit that plant count.
- Check how many whole plug trays that many cells will end up filling.
- Note the total tray cost so you can budget the season's seed-starting.
How the Tray Count Is Worked Out
Because not every seed sprouts, the cells you must sow first inflate the target plant count by dividing it by the germination rate, so a hundred transplants at eighty-five percent becomes about a hundred and eighteen cells. That sown-cell figure is then divided by the tray's cell count and rounded up to whole flats, since a partial tray still has to be set out and filled.
The cost is simply that rounded tray count times the price you pay per flat. The tray picker supplies only the cells per tray, which is why one plant target quietly drives the sow count, the flats, and the bill together. A lower germination rate is the number most people forget to raise, and it fattens the whole order faster than tray size does, so an honest or cautious rate beats an optimistic one printed on a two-year-old packet.
Worked Example: One Hundred Transplants in Seventy-Two Cell Trays
- You want about a hundred strong transplants ready to set out in the garden beds.
- Your current unopened seed packet tests at roughly eighty-five percent germination.
- So the cells you need to sow rise to ceil(100 ÷ 0.85) ≈ 118 cells.
- You are planning to use the standard seventy-two-cell plug trays.
- The flats needed are ceil(118 ÷ 72) = two whole plug trays.
- At eight dollars for each tray the total cost is two × $8 = $16.
- Those few extra sown cells quietly cover blank cells and a spare or two.
- A stronger germination rate would likely trim the whole requirement to a single flat.
Trays for a Hundred Transplants
Whole flats needed to raise a hundred transplants at eighty-five percent germination for each tray size, matching the cell counts the picker applies. Change the tray in the calculator and the sow count, flats, and cost re-solve off that one cells-per-tray figure.
| Tray | Cells per tray | Trays for 100 plants |
|---|---|---|
| 128-cell | 128 | 1 tray |
| 72-cell | 72 | 2 trays |
| 50-cell | 50 | 3 trays |
| 32-cell | 32 | 4 trays |
Key Terms
- Plug tray
- A molded flat of individual cells used to start seedlings in separate soil blocks.
- Cell
- One planting pocket in a tray, holding a single seed and its developing transplant.
- Germination rate
- The share of seed that sprouts, so a lower rate needs extra cells sown to hit the plant count.
- Transplant
- A seedling grown in a cell then set out in the garden at the right size.
- Dome lid
- A clear cover that holds humidity over a tray to lift and even out germination.
Starting Exactly the Right Number
- Sow to your germination rate, not your wish, since an optimistic hundred percent is what leaves a flat of empty cells and short planting.
- Match cell size to crop, because tiny salad and herbs crowd a 128-cell flat while tomatoes need the room of a 32-cell plug to avoid root-bound check.
- Tray size barely changes total cells, only the flat count, so choose it for root room and handling rather than to save money on trays.
- Reused and warped trays seed unevenly and drain poorly, so a fresh flat or two is a cheap hedge against a patch of stubborn blanks.
- Grow a few spares on purpose, since leggy, damaged, or pest-hit seedlings get culled before planting, and extras beat bare gaps in the bed.
Common Mistakes & Fixes
- Sowing exactly the plant count with no germination cushion, then coming up short at planting time.
- Starting big fruiting crops in fine 128-cell flats where the roots bind and the starts check.
- Trusting an old packet's printed rate, then facing a flat of blanks from seed that barely sprouts.
- Buying trays by the flat count alone and ignoring that cell size drives how healthy a plug gets.
- Overlooking spare cells, so a single leggy or eaten seedling becomes a permanent gap in the row.
When to Use This (and When Not To)
Use it at seed-starting time, when you know how many transplants a bed needs and want the cells, flats, and cost before you sow. It counts plug trays and cells, not how many seeds a packet holds or how far a row will run, so for direct-sowing use the seed-packet and direct-seeding row calculators. To size the soil mix that fills your flats, the potting-mix recipe calculator works out the volume of growing medium you will need to make up.
Plug Tray Calculator: Frequently Asked Questions
How many cells do I need to start 100 transplants?
Divide your plant target by the germination rate, since not every seed sprouts. A hundred transplants at eighty-five percent germination means sowing about one hundred and eighteen cells, which the calculator rounds to whole trays for whichever cell count you choose.
What size plug tray should I use?
Pick cell size by crop, not convenience: tiny salad and herbs suit a 128-cell flat, most vegetables want a 72-cell tray, and tomatoes and squash need the room of a 32-cell plug so their roots do not bind. The total cells barely change, only the number of flats you set out.
Do I need more trays if germination is low?
Yes, a lower germination rate fattens the sow count before it is split into flats, so seventy percent seed needs noticeably more cells than ninety percent seed for the same plant target. Enter an honest rate and let the calculator carry it through to the tray count.
Should I count on one seed per cell?
This model assumes one seed sown per cell, which is standard for plug trays and keeps thinning simple. If you drop several seeds per cell for a stubborn crop, the plants per tray change even though the cell count does not, so adjust your target to match how you actually sow.
Is it better to grow spare transplants?
Usually, because seedlings get leggy, nipped, or damaged before they reach the ground, and a few extras beat bare gaps in a finished bed. Rounding up to whole trays already leaves some headroom, but for a precious or expensive crop, plan one extra flat on purpose.
How many seedlings fit in one tray?
The usable plants are the cell count multiplied by your germination rate, since not every pocket sprouts. A seventy-two-cell tray at eighty-five percent yields roughly sixty-one transplants, so divide your target by that yield to see the flats you will really need.
Sources & Further Reading
- Seed Starting in Plug Trays and Flats — University of New Hampshire Extension. Basis for the germination-adjusted cell and tray math used here.
- Choosing Cell Size for Vegetable Transplants — Cornell Cooperative Extension. Underpins the crop-to-tray-size guidance on this page.
- Germination Rates and Testing Home Seed — Penn State Extension. Source of the germination-allowance cautions in this calculator.
Chlorova Gardening Team
Builds, checks and documents every tool we publish
Chlorova editorial review
Checked against extension-service publications
Formula verified against published references · Updated