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Seed Vault Storage Life Calculator
Estimate how long saved seed keeps from storage temperature and humidity, then plan germination rate, seeding density and a replant year.
Seed Vault Storage Life Calculator
Chlorova · updated for 2026
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Storage life follows the two rough rules behind this model, that seed life roughly doubles for every ten degree drop in temperature and for every one percent fall in moisture content, and both rules break down outside the range they were derived, below freezing or above about ten percent moisture. The figures are population averages, and a jar of old seed may germinate at ten percent or ninety. A germination test on ten seeds on a damp paper is cheap and is the only real answer before you commit a season, and cap values here exist because an unchecked doubling rule otherwise predicts a century of viability.
Seed storage arithmetic is built on two rules of thumb that every seed saver learns early, and then applies far too confidently. Cool doubles life, dry doubles life again, and together they turn a two year onion seed into something that outlives a teenager. This calculator uses those rules with the published base longevity of each crop to estimate how long your jar will keep, what germination it should still have, how thickly to sow it and the year you should replenish the reserve.
Estimating How Long a Jar Will Keep
- Choose the seed type to load its base life.
- Enter the storage temperature you really have.
- Give the relative humidity around the jar.
- Set germination when the seed was fresh.
- Enter the years it has already been stored.
- Count the cells you plan to sow this year.
- Give the seed count in one packet.
- Read the predicted life before the verdict.
- Check expected germination before sowing thin.
- Run a ten seed test to confirm the guess.
- Re run it for a freezer to see the gain.
- Note the replenish year in your plan.
Doubling Rules, Moisture and a Half Life
Two doubling rules carry the whole model. Life roughly doubles for every ten degrees the storage temperature falls from seventy, and it doubles for every one percent the seed moisture drops below eight, so a jar at 65 F and forty percent humidity gives a factor of 1.41 from temperature and nothing from dryness.
Seed moisture at equilibrium tracks the air around it at about one fifth of the relative humidity, which is why forty percent air means roughly eight percent seed and why a desiccant matters more than a cold shelf. That predicted life is then used as a half life for vigor, so germination falls by half across the full expected span. The twenty five year cap exists because an uncapped doubling rule happily predicts a century, and no honest seed bank promises that.
Worked Example: Two Year Old Tomato Seed in a Hall Closet
- Choose tomato, with a base life of 3.5 years.
- A 65 F closet gives a temperature factor 1.41.
- Forty percent humidity means 8 percent moisture.
- The dryness factor is therefore exactly 1.00.
- Predicted storage life is 4.95 years total.
- Two of those years are already behind this particular jar.
- Germination falls to about 68.0 percent now.
- Sow two seeds in every cell rather than the usual one.
- Twenty cells therefore need 40 seeds in total.
- At 25 a packet that is 1.60 packets sown.
- Plan to replenish the reserve by the year 2031 at latest.
- A freezer at 20 F would cap the life out at 25 years.
Base Longevity by Seed Type
Typical storage life for each crop dried to about eight percent moisture and kept near seventy degrees, which is the starting figure the two doubling rules then adjust.
| Seed type | Base years | Drier is urgent | Notes |
|---|---|---|---|
| Tomato | 3.5 | No | Wash and dry slowly |
| Pepper | 3.0 | No | Dry flesh before storing |
| Bean and pea | 2.5 | No | Dries on the plant |
| Brassica | 3.5 | Yes | Small seed, cool jar |
| Squash and melon | 3.0 | No | Wash, dry, big seed |
| Lettuce | 3.0 | Yes | Light sensitive and small |
| Carrot and celery | 2.0 | Critical | Short lived, seal it |
| Onion and leek | 1.0 | Critical | Buy fresh or freeze dry |
Key Terms
- Base longevity
- Expected life of a seed type at 70 F and eight percent moisture.
- Equilibrium moisture
- The water content seed settles at for the humidity around it.
- Desiccant
- Silica or similar material that keeps a sealed jar dry.
- Germination test
- A counted sprouting trial on damp paper before sowing.
- Orthodox seed
- Seed that dries and stores well, which covers most vegetables.
- Half life
- Time over which viability falls by half in this model.
What Seed Banks Do Differently
- A working gene bank dries seed to five or six percent moisture before freezing it, because cold at a high moisture content is how cell structure is damaged rather than protected.
- Ordinary garden seed is orthodox, meaning it tolerates drying, but a few things such as oak acorns and some tropical crops are recalcitrant and die if you dry them, so a freezer is not a universal answer.
- Temperature swings matter more than the average, since a shelf that cycles from sixty to ninety each summer ages a jar faster than a steady seventy does.
- Light and air are the two variables most home storage ignores, and a sealed jar in the dark beats an open envelope in a bright cupboard even at the same temperature.
- The honest planning move is to grow the seed yearly rather than store it, because a reserve you never test fails exactly when you need it most.
Common Mistakes & Fixes
- Storing seed that still feels cool and damp in the palm.
- Freezing seed at room humidity instead of drying it first.
- Trusting a packet date instead of running a test.
- Keeping seed in a clear jar on a sunny shelf.
- Assuming one doubling rule applies forever and cold.
- Reusing a jar whose lid seal has long since gone.
- Storing onion family seed for a second season.
- Labeling a jar with the year rather than the batch.
- Sowing old seed at the fresh seed rate thinly.
When to Reach for This
Use it when building a seed reserve for a household, when deciding whether an old packet is worth a season of sowing, when choosing between a closet, a fridge and a freezer, and when planning how much seed to save this autumn for the years ahead. Pair it with the seed germination calculator to test what you have, the seed packet calculator to size a sowing, the seed starting date calculator for when to sow it, and the plant spacing planner to work out how many plants the reserve will actually grow.
Seed Vault Storage Life Calculator: Frequently Asked Questions
How long does saved seed really stay viable?
In an ordinary house cupboard, expect tomato, pepper, brassica and squash seed for three to five years, beans and lettuce about the same, carrot a shorter two, and onion family barely one. Cool and dry storage multiplies all of those, and a freezer with a desiccant turns most vegetable seed into a decade or more.
Does a freezer really double seed life?
More than double, provided the seed is dry before it goes in. The rule of thumb is a doubling for every ten degree fall, so moving a jar from seventy to twenty degrees is five doublings. Seed frozen while still damp is damaged by ice, which is why drying comes first and always.
How do I know the humidity inside a jar?
You do not measure it directly, you control it with silica gel cards and a good seal, then let the seed equilibrate for a couple of weeks. A useful target is thirty to forty percent relative humidity in the headspace, which corresponds to roughly six to eight percent moisture in most seed.
Should I sow old seed thicker or replace it?
Sow thicker, which is exactly what the seeding line here estimates, then thin once germination is proven. If the predicted germination is under thirty percent, spend the seed on a test rather than a row, and buy fresh for the season you actually need to eat from.
Why is the predicted life capped at twenty five years?
Because the doubling rules were derived from short term experiments and extrapolate absurdly, happily promising a century of viability to a cold dry jar. No seed bank reports vegetable seed at that figure, so the cap keeps the output inside the range where the rules have any company.
Are commercially packed seeds longer lived than my saved seed?
Often slightly, because they were dried and cleaned professionally and sometimes sealed under inert gas, but a well dried home batch of the same crop is not far behind. What matters most is that your seed went into the jar dry, which is the one step home savers most often rush.
Sources & Further Reading
- Seed to Seed — Sutton, J., Seed Savers Exchange. Base longevity by crop, drying and storage method.
- Harrington's Rules and Seed Storage Behavior — USDA Forest Service seed laboratory research. Temperature and moisture doubling rules and their limits.
- Home Saved Seed: Drying, Cleaning and Storage — Oregon State University Extension. Equilibrium moisture, desiccants and germination testing.
- Germination Tests for Old Seed Lots — University of Illinois Extension. Ten seed paper towel method and interpreting results.
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
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