Short answer: pitching rate is three multiplications and viability is one subtraction. Cells needed (billion) = rate × litres × °Plato; live cells in an ageing pack = pack count × (1 − 0.007 × days); and a stirred starter ends near MIN(volume × 200, pitched + 140 × volume) billion cells. Put those in a sheet and you will stop under-pitching — the single most common avoidable fermentation fault.
This takes use case 13 from the 20 brewing calculations in Excel pillar and adds the two things that decide whether you actually hit the target: an ageing pack’s real cell count, and how much a starter grows. It pairs with the data-driven view of yeast viability and vitality.
Step 1 — how many cells the beer needs
The target is a pitch rate in million cells per millilitre per degree Plato: about 0.75 for ales, 1.5 for lagers, higher for big beers. Multiply it out:
cells_billion = rate * litres * plato → =B2*B3*B4
A 23 L ale at 12 °P needs 0.75*23*12 = 207 billion cells. The same wort as a lager needs 1.5*23*12 = 414 billion — which is why lagers so often need a starter or multiple packs.
Step 2 — how many cells you actually have
A fresh liquid pack is ~100 billion cells, but it bleeds viability at roughly 21% a month, about 0.7% a day from the date stamped on it. So the live count is what matters:
viability = MAX(0,100-0.7*days)/100
live_cells = packs * 100 * viability
A pack stamped 45 days ago is at 100-0.7*45 = 68.5% viability — so two packs give 2*100*0.685 = 137 billion live cells, well short of even the ale target. The decay below is why “it’s only a few weeks old” is not the same as enough yeast.
Step 3 — size the starter to close the gap
If live cells fall short of the target, a starter grows more. A stirred 1.040 starter carries about 100 g of extract per litre and tops out near 200 billion cells per litre; growth runs around 1.4 billion new cells per gram of extract. Combine the cap and the growth:
ending_cells = MIN(starter_litres*200, live_cells + 140*starter_litres)
From the 137 billion live cells above, a 2 L starter ends at MIN(2*200, 137+140*2) = MIN(400,417) = 400 billion — comfortably past the ale target and into lager territory. Add a column for grams of dry malt extract (starter_litres*100) so your shopping list falls out of the same sheet.
Where the model gets loose
Two honest limits. First, growth depends on aeration and agitation — the 1.4-per-gram figure assumes a stir plate or frequent shaking; a still starter grows far less, and these formulas will overstate it. Second, a count is an estimate until you measure it; the only way to truly know your cell density is a haemocytometer and a microscope, or a measured pitch from a slurry. Use the sheet to get into the right order of magnitude and to stop habitual under-pitching, not to claim three-significant-figure precision the biology won’t honour.
The bottom line
Three formulas turn “pitch a pack and hope” into a sized decision: cells needed from gravity and volume, live cells after viability decay, and the ending count from a starter. Build them once, enter the date on the pack, and you will catch the under-pitch before it becomes a slow, estery, or stalled fermentation — not after.
Frequently asked questions
How do I calculate how much yeast to pitch? Cells required (in billions) = pitch rate × volume in litres × gravity in °Plato, where the rate is about 0.75 million cells per mL per °P for ales and 1.5 for lagers. In Excel that is =ratelitresplato, so a 23 L ale at 12 °P needs 0.75×23×12 ≈ 207 billion cells.
How does yeast viability change with age? Liquid yeast loses roughly 21% viability per month, about 0.7% per day from its manufacture date. Model it as =MAX(0,100-0.7*days)/100 to get the surviving fraction, then multiply the pack’s cell count by it to find how many live cells you are really pitching.
How big a yeast starter do I need? Estimate ending cells as =MIN(starter_litres200, viable_cells_pitched + 140starter_litres) billion. The first term is the density a stirred starter tops out at (~200 billion/L); the second is growth of about 1.4 billion new cells per gram of the ~100 g/L of extract in the starter.
Part of the Brewing Science & AI track.