How to use the yeast pitch rate calculator
- Enter the batch volume and choose gallons or liters, then the original gravity as specific gravity (such as 1.050) or degrees Plato.
- Enter a pitch rate in million cells per milliliter per degree Plato. Brewing guidance often uses about 0.75 for ales and 1.5 for lagers, and you can change it.
- Enter the cells in a fresh pack, its age in months and the viability lost each month.
- Read the cells needed, the number of packs to use, the cells those packs hold and the pitch rate you would actually reach.
Formula
degrees Plato = -616.868 + 1111.14 SG - 630.272 SG^2 + 135.997 SG^3
cells needed (billions) = pitch rate x volume in mL x degrees Plato / 1000
viability = 100% - loss per month x age in months (not below 0)
packs = cells needed / (cells per fresh pack x viability), rounded up
The polynomial turns specific gravity into degrees Plato for the usual range of beer worts. A US gallon is exactly 3.785411784 liters.
Five gallons of a 1.050 ale
A specific gravity of 1.050 is 12.39 degrees Plato. Five US gallons is 18,927.06 mL. At 0.75 million cells per milliliter per degree Plato, the batch needs 0.75 x 18,927.06 x 12.39 / 1000 = 175.8 billion cells.
A pack with 100 billion cells at manufacture, two months old and losing 20 percentage points a month, has 60 percent viability, so 60 billion living cells. The batch needs 175.8 / 60 = 2.93 packs, rounded up to 3 packs, which hold 180.0 billion living cells and give a pitch rate of 0.768 million per milliliter per degree Plato.
Why pitch rate matters
Too few yeast cells make fermentation slow and stress the yeast, and too many can leave the beer thin. Pitch rate guidance scales the cell count with the volume and the gravity, because a stronger wort needs more yeast to ferment cleanly. The rate is a rule of thumb that differs between ales and lagers and between brewers, which is why the page lets you set it.
Yeast also dies slowly while it sits. A pack that is a few months old has fewer living cells than the count printed on it, which is the reason for the viability step. The loss per month is an assumed straight line. Real packs vary with the strain and with storage.
What it leaves out
The page does not model yeast starters, which grow more cells before pitching, nor dry yeast by weight, nor cell growth in the wort. It only answers how many packs of a given kind give a target count. Follow the maker's guidance for the yeast you use. To estimate alcohol from your gravity readings, use the ABV Calculator, to correct a reading for temperature, the Hydrometer Temperature Correction Calculator, and to swap yeast types in a recipe, the Yeast Conversion Calculator.
Assumptions and limits
- The pitch rate, cells per pack and viability loss are numbers you enter. The values in the page are examples.
- Viability falls in a straight line with age and cannot go below zero.
- Specific gravity must be above 1.000 and at most 1.150, and Plato above 0 and at most 35.
- The number of packs is rounded up to whole packs.
- Nothing typed here leaves your browser.
Frequently asked questions
What pitch rate should I use?
Many brewers use about 0.75 million cells per milliliter per degree Plato for ales and about 1.5 for lagers. Your yeast supplier may recommend something else, and the page lets you change it.
Why does the page give a Plato figure for my gravity?
Pitch rates are usually stated per degree Plato. The page converts specific gravity to Plato with a standard polynomial so you can use either unit.
How do I account for a starter?
The page does not model one. A starter increases the number of cells before pitching, so work out the cells it would give from a starter calculator and use that figure in place of the pack count.
Why does the answer say I need more than one pack?
A single pack holds a fixed number of cells, usually fewer than a full batch needs at the pitch rate. The page rounds up so the target count is met or exceeded.