Yeast Viability Calculator
Viable cells remaining in a liquid yeast pack based on its age.
Fresh-pack count, e.g. 100 B
Days since manufacture
-% viable
Viable cells now-
How it works
Yeast cells die off steadily in storage. The Mr Malty model assumes a near-linear decline of about 0.7% of viability per day from the production date, which works out to roughly a fifth of the cells lost each month.
viability % ≈ 100 − 0.7 × days (≈ 21% loss/month, floored at 0)
Sources: viability decline model from Mr Malty.
A rule of thumb - storage temperature, strain and handling all matter. Cells kept cold and undisturbed hold up better than the average; warm shipping does worse.
Frequently asked questions
- How fast does liquid yeast lose viability?
- Refrigerated liquid yeast drops roughly 0.7% viability per day - about 21% per month. A pack three months old can be down near 40% viable, which is why old packs need a starter or a bigger pitch.
- Where do I find the production date?
- Liquid yeast packs are stamped with a manufacture or "best by" date. Use the days since manufacture (not best-by) for the most accurate viability estimate. If only a best-by date is shown, count back to the production date if the maker publishes the shelf-life window.
- Can I still use old yeast?
- Often yes - just pitch more of it or build a starter to restore the cell count. Use the "viable cells now" figure here to decide how many packs or how large a starter you need.