Why You Should Never Sow Exactly What You Want
The number on the seed packet is a lab number. A packet that says 85 percent germination was tested under ideal warmth and moisture, and your kitchen windowsill or cold spring soil rarely matches that. On top of that, not every seed that sprouts becomes a transplant: damping-off fungus, leggy growth, and accidents during pricking out all take a toll. So if you want 12 tomato plants and you sow exactly 12 seeds at 85 percent germination, you should expect only about 10 sprouts and maybe 8 or 9 survivors. The fix is simple and is what every nursery does: over-sow on purpose, then thin to the strongest seedlings.
The Seeds-to-Sow Formula
The real chance any single seed turns into a keeper plant is the packet germination rate, knocked down by how old the seed is, then multiplied by how many seedlings survive to transplant. Divide your target by that combined success rate and round up.
seeds to sow = target plants / (germination% x age factor x survival%)
Worked example: you want 12 plants, the packet says 85 percent, the seed is one year old (about 0.9), and tomatoes survive transplant at roughly 85 percent. Combined success is 0.85 x 0.9 x 0.85 = 0.65, so you sow 12 / 0.65 = 18.5, rounded up to 19 seeds. At 2 seeds per cell that fills 10 cells, and you should expect roughly 15 sprouts and 12 keepers, exactly your goal.
Seeds Per Cell and Thinning
Sowing 2 or 3 seeds per cell is insurance against a dud in any single cell, but it is not free plants: once a cell has two seedlings up, snip the weaker one at soil level rather than pulling it, which protects the roots of the keeper. The calculator counts every sown seed toward your total, then tells you how many cells or pots to fill so your seed-starting trays and shopping list line up with reality instead of optimism.
Frequently Asked Questions
How many seeds should I plant per hole?
For most vegetables and flowers, sow 2 to 3 seeds per cell or hole and thin to the single strongest seedling once they have their first true leaves. Large, reliable seeds like beans, peas, and squash can go one per cell because their germination is high and individual seeds are valuable. Snip the extras at soil level with scissors instead of pulling so you do not disturb the keeper's roots.
What does germination rate actually mean?
Germination rate is the percentage of seeds that sprouted in a controlled lab test, usually printed on the back of the packet, for example 85 percent. It is a best-case figure measured under ideal warmth and moisture, so your real-world results are almost always a bit lower. That gap is exactly why this calculator multiplies the packet number down by seed age and seedling survival before telling you how many to sow.
Do old seeds still germinate?
Often yes, but the rate falls every year and depends heavily on storage. Seeds kept cool, dark, and dry last far longer than seeds left in a hot garage or humid shed. Onions, parsnips, and spinach fade fast and are best used within a year or two, while tomatoes, peppers, beans, and most squash can stay viable for four to six years. When in doubt, run a paper-towel germination test on ten seeds before committing your whole packet.
How do I run a quick germination test?
Dampen a paper towel, lay 10 seeds on it, fold it over, and seal it in a labeled zip bag in a warm spot. Check after the days listed for that crop and count how many have sprouted: 8 of 10 means about 80 percent germination. Plug that real number into this calculator instead of the packet figure for the most accurate seeds-to-sow count.
Practical Guide for Seed Germination & Sowing Calculator
Match your over-sowing to how precious the seed is. With a cheap packet of 200 lettuce seeds, sowing 50 percent extra costs nothing and saves a reseeding trip. With a 10-seed packet of a rare heirloom tomato, you want to maximize every seed: run a germination test first, sow into individual cells with bottom heat, and baby each seedling rather than relying on volume. The calculator's success-per-seed number tells you which situation you are in at a glance.
Timing multiplies your germination problems. Cold soil is the number one reason real-world germination falls short of the packet, because most warm-season crops barely sprout below 60 F and rot if they sit wet and cold for too long. A seedling heat mat that holds the tray around 70 to 75 F can lift real germination by 20 points or more, which directly lowers how many seeds you need to sow. Pair this tool with a frost-date or seed-starting-date calculator so you start at the right moment.
Plan for thinning before you sow, not after. Decide up front that you will keep the single strongest seedling per cell and treat the rest as insurance, then size your tray to the cell count this calculator gives you. Gardeners who feel guilty about thinning end up with crowded, leggy, root-bound seedlings that all underperform. A clean snip and a confident keep-the-best rule produces fewer but far stronger transplants.
Quick Checklist
- Use your packet germination rate, or run a 10-seed paper-towel test for a real number.
- Over-sow on purpose so you have keepers to spare, then thin to the strongest.
- Sow 2-3 seeds per cell for small crops, one per cell for big reliable seeds.
- Store leftover seed cool, dark, and dry, and label it with the year.