Yeast Calculator
Estimate liquid yeast viability over time, calculate pitching rate and packs needed, plan yeast starters, and rehydrate dry yeast correctly for a clean, complete fermentation.
About Yeast Viability, Pitching & Starters
Healthy yeast is the single biggest factor in producing clean beer. Pitching the right amount of viable cells reduces esters and fusels, prevents stuck fermentations, and suppresses contamination. This calculator addresses the four most common yeast-management decisions a homebrewer faces.
Viability: how fast does yeast die?
Liquid yeast (Wyeast Activator, White Labs PurePitch, slants) loses viability continuously after packaging. The widely-cited rates are 21% per month at room temperature and 7% per month refrigerated. A pack stamped at 100 billion cells has roughly 80 billion after 6 months refrigerated — and only ~20 billion after 6 months at room temperature. Always check the manufacture (not expiry) date.
What is the right pitching rate?
The accepted targets, popularized by George Fix and adopted by Mr Malty, are:
- Ale: 0.75 million cells/mL/°P
- Lager: 1.5 million cells/mL/°P
- High gravity (above 1.060): add ~1 M cells/mL/°P per 0.010 SG above 1.060
- Cold-conditioned lager: consider 1.75 M cells/mL/°P for diacetyl control
Required cells = rate × volume (mL) × °Plato. For a 20 L ale at 1.050 (~12.4 °P): 0.75 × 20,000 × 12.4 ≈ 186 billion cells — about two fresh packs or one fresh pack plus a starter.
How starters work
A starter grows yeast on a small volume of low-gravity wort (typically 1.030-1.040 / 8-10 °P) before pitching. The growth achieved depends on the inoculation rate (cells pitched per mL of starter) and how well the starter is aerated. Stir plates, which continuously introduce oxygen and keep yeast in suspension, achieve the highest growth factors. The Kai Troester model, used in this tool, estimates grown cells from the inoculation rate and total extract.
Dry yeast: rehydrate or sprinkle?
Manufacturers (Fermentis, Lallemand, Mangrove Jack's) recommend rehydrating dry yeast in warm sterile water at 30-35 °C for 15-30 minutes before pitching. This preserves ~95% of viable cells. Sprinkling directly onto wort can kill 35-50% of cells due to osmotic shock. Use 10 times the yeast mass in water (e.g. 11.5 g yeast → 115 mL water).
Target Cells by Batch Size
| Batch (L) | OG (SG) | °P | Ale target (B) | Lager target (B) |
|---|
| 10 | 1.050 | 12.4 | 93 | 186 |
| 20 | 1.050 | 12.4 | 186 | 372 |
| 20 | 1.080 | 19.3 | 290 | 579 |
| 40 | 1.050 | 12.4 | 372 | 744 |
B = billion cells. Assumes 0.75 M / 1.5 M cells/mL/°P. Plato converted from SG via ASBC polynomial.
Frequently Asked Questions
How fast does liquid yeast lose viability?
Liquid yeast (smack packs and slants) loses roughly 21% of viable cells per month when stored at room temperature, and about 7% per month when refrigerated. A pack stamped at 100 billion cells loses viability quickly, so older packs may need a starter or an extra pack.
What pitching rate should I use for ale vs lager?
The widely-accepted target is 0.75 million cells/mL/°Plato for ales and 1.5 million cells/mL/°Plato for lagers (double, because lagers ferment colder and slower). For high-gravity beers above 1.060, increase the rate by roughly 1 million cells/mL/°P for each 0.010 over 1.060.
How many cells are in a yeast pack?
A standard Wyeast Activator or White Labs vial contains about 100 billion (1×10¹¹) viable cells when fresh. Dry yeast sachets (11.5 g) typically contain 200-350 billion cells depending on the strain and manufacturer, with reduced loss over time if stored cool and dry.
When do I need a yeast starter?
You need a starter whenever one pack does not deliver enough viable cells for your batch size and gravity — typically for lagers, batches above 1.060 SG, or larger volumes (above 20 L / 5 gal). A stir-plate starter can grow yeast 4-6× from a single pack.
How do I rehydrate dry yeast?
Sprinkle the dry yeast onto 10 times its weight of sterile water at 30-35°C (e.g. 11.5 g yeast into 115 mL water), let it rehydrate undisturbed for 15-20 minutes, then stir gently and wait another 5-10 minutes. Never use hot water above 40°C and avoid rehydrating directly in the wort, which can kill a significant fraction of cells due to osmotic shock.
What is a yeast growth factor in a starter?
The growth factor (also called inoculation rate) is the ratio of new cells grown to the inoculum pitched. At an inoculation rate of 25-100 million cells/mL, yeast grows roughly 4-6× on a stir plate with oxygen. The Kai Troester / Mr Malty models are commonly used to estimate final cell counts from starter volume and original gravity.
Does under-pitching cause off-flavors?
Yes. Under-pitching stresses yeast, leading to higher esters, fusel alcohols, diacetyl, sulfur compounds, stalled fermentations, and higher final gravities. Over-pitching is less common at the homebrew scale but can strip flavor and produce thin beers with low ester character.