Yeast Viability Calculator
Enter your yeast's form and age — get its real viability %, the actual viable cells behind the label, and how many fresh-equivalent packs it takes to match one fresh one. Built on a viability% = 100 − decay×days heuristic by a former New Belgium yeast specialist.
viability% = clamp(100 − decay × days, 1, 100). Liquid ≈ 0.70%/day · dry ≈ 4%/month · slurry ≈ 1.2%/day. All three assume cold, stable storage — see below.
Find out what's actually alive in the pack.
A yeast pack's label tells you what it shipped with, not what's left. Viable cells decline every day from manufacture or harvest, and the rate depends entirely on the yeast's form — liquid, dry, or harvested slurry decay at very different speeds. This tool takes the form and the age and gives you an honest estimate of the real viable-cell count behind that "100 billion" on the label, so you can decide whether to pitch it straight, build a starter, or buy fresh.
Everything runs client-side, right in your browser. Nothing is stored, nothing is sent — change any input and the result updates live.
The viability decay formula, explainedThree curves, one shape.
Viability% = clamp(100 − decay rate × days, 1, 100)
Each yeast form gets its own daily decay rate, applied as a simple linear model from day zero (100% viable, the moment it was packaged or harvested) down to a floor of 1% — real cultures never truly hit absolute zero, but by that point you should not be pitching it anyway. These are heuristics, not lab-verified curves for your exact pack: real viability loss isn't perfectly linear, and storage conditions swing it hard in both directions. Treat the output as a planning estimate, not a certificate of analysis.
liquid_viability = clamp(100 − 0.70 × days, 1, 100)
# Dry yeast — far more stable, ~4%/month refrigerated (≈50% at a year)
dry_viability = clamp(100 − (days ÷ 30) × 4, 1, 100)
# Slurry — harvested & refrigerated, faster and rougher than a lab pack
slurry_viability = clamp(100 − 1.2 × days, 1, 100)
# real cells and fresh-pack equivalence
real_cells = labeled_cells × (viability ÷ 100)
fresh_equivalent_packs = ceil(1 ÷ (viability ÷ 100))
Why the three forms decay so differently
Liquid yeast is a living culture suspended in liquid with no protective drying step — it's the least stable form, losing roughly a fifth of its viable population every month even refrigerated. Dry yeast goes through a manufacturing dehydration process that puts cells into a dormant, low-metabolism state; kept cool and sealed, that dormancy is remarkably stable, which is why dry yeast can sit in a fridge for the better part of a year and still pitch respectably. Slurry — yeast you harvested yourself from a fermentation — has neither the lab-controlled packaging of a liquid pack nor the drying of dry yeast, plus it's carrying trub, dead cells, and whatever stress the previous batch put it through, so it tends to decline faster and less predictably than either.
A liquid pack, 90 days old. Liquid decay: 100 − (0.70 × 90) = 37% viable.
A pack labeled 100 B cells is really only about 37 B viable cells.
To hit a 200 B pitch target you'd need roughly 5.4 such packs — or grow a starter from one and skip buying five more old ones.
Reading the fresh-equivalent number
The fresh-equivalent result answers a specific question: how many of this old pack would it take to equal one fresh one? At 37% viability that's 1 ÷ 0.37 ≈ 2.7, rounded up to 3 packs to be safe. That's the number worth staring at — if the honest answer is "just buy three packs instead of one," a starter is almost always the better play, both financially and for fermentation health.
The part no calculator farm writesWhen the standard number is wrong.
A linear decay rate is a useful default. It is not the truth for every pack in every fridge. Here's where it breaks.
Storage temperature dominates everything
The rates above assume consistent, cold storage — a homebrew shop fridge or your own, held near 34–38°F the whole time. That assumption breaks constantly. A pack that rode in a hot delivery van for two days, sat on a warm loading dock, or spent a week at room temperature before refrigeration can lose that much viability in weeks, not months. The "best by" date printed on the pack is a manufacturer's estimate under ideal, unbroken cold-chain conditions — it is not a viability guarantee for a pack whose actual thermal history you don't know.
The under-counted middle
The riskiest packs aren't the obviously old ones — those get replaced or boosted with a starter automatically. It's the pack that's only three or four weeks old but spent part of that time warm in transit. It looks fresh by the date, calculates as ~80% viable by this tool's cold-storage curve, and might actually be well below that. If you have any reason to suspect a broken cold chain, treat the calculator's number as optimistic.
Dry yeast is not liquid yeast — don't borrow its curve
This is the single most common modeling mistake homebrewers make: judging a dry yeast packet by the liquid pack's ~21%-per-month decay. Kept cool and sealed, dry yeast holds 90%+ viability for a year — the dehydration process is specifically what makes that possible. Apply the liquid curve to a dry packet and you'll conclude a six-month-old sachet is nearly dead, when it's likely still very healthy. Always select Dry in this tool rather than eyeballing a liquid-pack answer for it.
Viability vs. vitality: alive isn't the same as ready
Viability is the fraction of cells that are alive at all. Vitality is how healthy and metabolically ready those living cells are — their glycogen reserves, membrane condition, how fast they can get going. An old-but-still-viable pack can report a reasonable viability percentage and still ferment sluggishly, because the cells that survived are tired, not just fewer. Count and condition both matter; a viability estimate is necessary but not sufficient for predicting how a pitch will actually perform.
A starter resets the clock
The cells growing in a starter are new — they bud off whatever survived the old pack, at high viability, regardless of how old that original pack was. For anything estimating under about 60% viability, a starter beats buying more old packs almost every time: it's cheaper than five backup packs and it produces healthier cells than the pack ever will again on its own. See the yeast starter calculator to size one from wherever your pack lands today.
Notes from the labAt New Belgium, we never trusted a "best by" date as a viability number — it's a shipping promise, not a cell count. We ran viability checks on every pack and every propagation step, because the gap between the label and reality could be the difference between a clean fermentation and a stuck one. The lesson that travels to homebrew scale: a date tells you nothing about the truck that pack rode in on. If you're not sure how it was stored, don't trust the calculator's number blindly — count it, or hedge with a starter. An estimate is a starting point for judgment, not a replacement for it.
Stop guessing at viability — start logging it.
A viability estimate from age is a stand-in for a real count. The value compounds when you log actual viability checks against every pitch and every batch outcome — and see exactly which packs, ages, and storage habits produce clean fermentations. That's what a QC dashboard does automatically.