Whether you found a packet at the back of your drawer or want to stretch last year’s seed order, knowing your germination rate is the single most important step before sowing. A quick paper towel test plus this calculator tells you exactly what to plant — and how many — to get the stand you want.
What Is Seed Viability?
Seed viability is the percentage of seeds in a batch that are alive and capable of germinating under the right conditions. Viability declines over time — slowly for crops like tomatoes and squash, quickly for onions and parsnips. A 90% viability rate means 9 out of 10 seeds should sprout; a 40% rate means less than half will.
What Is a Good Germination Rate?
For fresh, quality seed you should see 80–95% germination. Rates of 60–79% are acceptable with modest overplanting. Seeds testing at 40–59% are marginal — plant 2–3× as many and expect thin spots. Below 40%, buying fresh seed is almost always more cost-effective than the effort of planting extras.
Why Test Before Planting?
Planting seeds that won’t germinate wastes bed space, time, water, and fertilizer. Worse, you may not realize the failure until it’s too late to re-sow. A 10-minute paper towel test done 2–4 weeks before planting gives you time to order fresh seed or adjust your planting density before the season slips away.
When Does Viability Drop Fastest?
Onions, parsnips, and leeks decline sharply after just 1–2 years. Corn and most herbs lose viability after 2–3 years. Tomatoes, peppers, cucumbers, and brassicas typically hold strong for 4–5 years. Squash, melons, and beans can remain viable 5–6 years when stored cool and dry below 50°F in a sealed container.
Step 1 — Prepare
Dampen a paper towel until moist but not dripping. Count out exactly 10 seeds (or 20 for more accuracy). Spread them evenly across half the towel, fold the other half over, and slide into a labeled zip-lock bag.
Step 2 — Wait
Place the bag in a warm, dark spot (65–75°F). Check every 2–3 days to make sure the towel stays moist. Wait the full average germination window for your crop — 5–7 days for lettuce, up to 14 days for peppers and slow crops.
Step 3 — Count & Calculate
Open the bag and count every seed that has produced a visible root or sprout. Divide by the number of seeds you tested and multiply by 100. Enter those two numbers above — the calculator handles the rest.
Stored seed loses viability at different rates depending on crop type and storage conditions. This table shows typical ranges for seed kept in a cool, dry location.
Seed Viability FAQs
How do I test seed viability at home?
The paper towel method is the easiest and most reliable home test. Place 10 seeds on a damp paper towel, fold it over, seal in a zip-lock bag labeled with the date and crop name, and place in a warm dark spot (65–75°F). After the normal germination window for that crop, count how many produced a sprout or visible root. Divide by 10 and multiply by 100 to get your germination percentage.
What germination rate is too low to bother planting?
Below 50% germination is the general threshold where buying fresh seed becomes more practical than overplanting. You would need to sow 2–3× as many seeds, and even then you may end up with an uneven stand. For expensive or rare seeds, it can still be worth planting at high density. For common crops available at any garden center, fresh seed is usually the better investment of time and garden space.
How many seeds should I plant per hole?
With seeds testing at 80%+ germination: 1–2 seeds per hole. At 60–79%: 2–3 seeds per hole. At 40–59%: 3–5 seeds per hole. Always thin to one plant per spot after germination to avoid overcrowding. For very small seeds like carrots and lettuce that are direct-sown in rows, scatter more densely and thin to the proper spacing after emergence.
How long do vegetable seeds stay viable?
Viability varies a lot by crop. Short-lived seeds (1–2 years): onions, leeks, parsnips. Medium-lived seeds (2–4 years): corn, carrots, beets, herbs. Long-lived seeds (4–6 years): tomatoes, peppers, cucumbers, squash, melons, brassicas. The above ranges assume proper storage — cool, dry, and dark, ideally in a sealed container with a desiccant packet. Heat and humidity dramatically shorten shelf life.
Can I improve germination of old seeds?
You can improve conditions but cannot restore lost viability. Strategies that help marginally viable seeds include: pre-soaking larger seeds (beans, corn, squash) for 4–8 hours in warm water before planting; ensuring soil temperature is in the ideal range for the crop; planting slightly shallower than normal; and keeping seed beds consistently moist until emergence. The most reliable fix remains simply planting more seeds per hole.
How should I store seeds to maximize viability?
The two enemies of seed viability are heat and moisture. Store seeds in a sealed glass jar or airtight container in a cool location — the back of a refrigerator works well for most seeds. Include a small silica gel desiccant packet to absorb any residual moisture. Label each packet with the crop name and year purchased. Never store seeds in a shed, garage, or kitchen where temperature swings and humidity are high.
About This Seed Viability Calculator
This free tool is designed for home gardeners and small-scale growers who want to make smart use of stored or saved seed. Simply enter the results of a quick paper towel germination test — the number of seeds tested and how many sprouted — along with how many plants you want to grow. The calculator returns your germination percentage, a viability grade, a recommended seeds-per-hole count, and the total seeds you’ll need to reach your target plant population. The reference table below your results shows typical germination rates and viability windows for common vegetables to help you plan the whole season.
For timing your plantings around your local frost dates, try our Frost Date Calculator. To build the perfect seed-starting schedule, visit our Planting Date Calculator.