Best Seed Starting Kit for Garden Planning: Size It to Your Seed Count

Most people buy a seed-starting setup by eye. Start from how many plants you need, and the number of cells, trays, mats, and lights follows.

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The Short Answer

Work out how many transplants you need, add a buffer, and divide by cells per tray. For many home gardens that lands at one or two standard trays — a size where an all-in-one kit (tray, cell insert, and humidity dome) is convenient. Add a heat mat if you’re starting warm-season crops like tomatoes and peppers, and a grow light unless you have a very bright, south-facing window.

Step 1: Turn Plants Into Cells

Only start indoors what you’ll transplant. Crops like beans, carrots, radishes, and beets are usually direct-sown, so they don’t need cells at all.

Cells needed = transplants needed + a buffer for seedlings that fail or come up weak. A 20–25% buffer is a reasonable starting point. You may sow two seeds per cell and thin to one; that changes your seed count, not your cell count.

Worked example	Transplants	With 25% buffer
Tomatoes (one 8×2 bed)	4	5
Peppers	8	10
Lettuce	36	45
Total cells	48	60

The Plant Spacing Calculator gives you transplant counts per bed, and the Seed Calculator tells you how many seeds to buy after germination losses.

Step 2: Turn Cells Into Trays

Most seed-starting gear is built around the standard “1020” tray, roughly 10 × 20 inches. Inserts divide it into cells:

Insert	Cell size	Best for
128-cell	Small	Lettuce, onions, and other small starts you’ll transplant young
72-cell	Medium	A general-purpose default for most vegetables
50-cell or fewer	Large	Tomatoes, peppers, and squash kept indoors longer

In the example above, 60 cells fit one 72-cell tray. If your larger plants need bigger cells, split the list: one tray of larger cells for tomatoes and peppers, one of smaller cells for lettuce.

Step 3: Size the Heat Mat and Light to the Trays

Heat mat: buy by tray count; single-tray mats are sized for one 1020 tray. Warm-season seeds such as tomatoes and peppers germinate more reliably in warm soil. A mat with a thermostat controller lets you set a temperature instead of guessing, and you can move trays off the mat once seedlings are up.
Grow light: check that the light’s coverage area matches your tray footprint. Seedlings under too little light stretch and flop. Follow the manufacturer’s recommended hanging height, and put the light on a timer so day length stays consistent.
Mix and labels: use a fine seed-starting mix rather than garden soil, and label every row with variety and sowing date.

An all-in-one kit makes sense if you…

Need one or two trays’ worth of transplants
Are starting seeds indoors for the first time
Want a humidity dome and matching tray without piecing parts together

Buy parts separately if you…

Need several trays, or mixed cell sizes
Already own a shelf and lights
Mostly grow direct-sown crops — you may only need a single small tray

Common Questions

How many seed trays do I need?

Divide the cells you need (transplants plus a buffer) by the cells per insert, and round up. Sixty cells fit one 72-cell tray; 150 cells need three.

Do I really need a heat mat?

Not for everything. Cool-season crops like lettuce often germinate fine at room temperature. A mat matters most for warm-season crops in a cool house.