How Many Tomato Seeds Do You Really Need?

The short version: divide the number of tomato plants you want by your seed's realistic germination rate, then add a 10 to 20 percent buffer for weak seedlings — that's the whole formula, and rounding up matters more than it seems.

Tomato seed is inexpensive, so it's tempting to just buy a packet and sow generously. But if you've ever ended up with 4 surviving seedlings when you wanted 12, or 40 when you wanted 15, you already know that "generously" isn't a plan. The seed-count math for tomatoes is straightforward once you separate three variables: how many plants you actually want, how reliably your seed germinates, and how much buffer you need for the seedlings that sprout but don't make it to transplant size.

The Base Formula

The core equation is: seeds needed equals target plants divided by germination rate. If you want 12 tomato plants and your seed packet is rated at 90 percent germination, that's 12 divided by 0.9, which equals 13.3, so you'd sow 14 seeds. Round up, always — a fraction of a seed doesn't exist, and rounding down leaves you short if even one seed underperforms.

Why Germination Rate Isn't Fixed

The percentage on the packet reflects a lab test done near the packing date, not a guarantee for however long the seed sits in your drawer. Tomato seed stored in a cool, dry place holds close to its original germination rate for about 3 to 4 years, but drops roughly 10 to 15 percentage points per year after that if stored poorly. A packet rated 90 percent when fresh might realistically perform at 70 percent after 4 humid summers in a garage. If you're using older seed, test it first: sow 10 seeds on a damp paper towel and count how many sprout in 7 to 10 days to get your real, current rate.

Building In a Damping-Off Buffer

Germination rate only tells you how many seeds sprout, not how many survive to transplant size. Damping-off, a fungal issue in overly wet seed-starting mix, typically claims another 5 to 15 percent of seedlings even under decent conditions. So the fuller formula is: seeds needed equals (target plants divided by germination rate) multiplied by 1.15 for a 15 percent survival buffer. For that same 12-plant target at 90 percent germination: 12 divided by 0.9 equals 13.3, multiplied by 1.15 equals 15.3, so sow 16 seeds.

Two Seeds Per Cell Changes the Odds

Instead of sowing one seed per cell and hoping, many growers sow 2 seeds per cell and thin to the strongest seedling. The math behind this: if a single seed has a 90 percent germination chance, the odds that at least one of 2 seeds in the same cell germinates is 1 minus (0.1 times 0.1), which equals 1 minus 0.01, or 99 percent. That 9-point jump in reliability per cell is why professional growers rarely bank on single-seed sowing for anything they can't easily resow.

From Seeds to Packets

A standard tomato seed packet holds anywhere from 20 to 50 seeds depending on variety and brand — check the packet, since heirloom and specialty varieties often ship fewer. If you need 16 seeds total and your packet contains 25, one packet covers you with 9 seeds to spare for a resow if a tray gets knocked over or a cutworm gets ambitious. If you're growing 5 different varieties at 3 plants each, that's 5 separate germination calculations, since older or rarer seed often has a lower rate than a fresh hybrid packet.

A Full Worked Example

Say you want 6 Brandywine plants from a 2-year-old packet you're guessing is down to 75 percent germination, plus a 15 percent survival buffer. Seeds needed equals (6 divided by 0.75) multiplied by 1.15, which is 8 multiplied by 1.15, equals 9.2, so sow 10 seeds. If the packet originally held 20 seeds, you've used half of it and still have a reserve for a second sowing attempt if the first one struggles.

Round Up, Every Time

Tomato seed math comes down to one adjustable formula: target plants, divided by realistic germination rate, times a survival buffer of roughly 1.1 to 1.2. Test old seed before trusting the packet's original number, sow in pairs when reliability matters, and always round up. It costs a few extra seeds to get this right, and costs a whole month of lost growing season to get it wrong.