Yeast Viability Calculator

Liquid Yeast Viability Calculator

Liquid yeast (vials and smack packs) loses viable cells over time in the fridge. Enter the manufacture date to see roughly how many cells are still alive — and whether you need a starter.

Check the pack — most UK liquid yeast is ~100B at manufacture.
Estimated viability

Why viability drops: Yeast cells slowly die off in the pack even under refrigeration. Manufacturers typically pack liquid yeast at close to 100% viability, and it's roughly 79% by 30 days, and around 50% by 3 months — the industry rule of thumb this calculator is based on.

What to do with the result: Above ~85% viability, pitch the pack directly. Below that, consider making a yeast starter to build the cell count back up before brew day — especially for higher gravity beers. If a pack is badly out of date, it's often more reliable to switch to a fresh dried yeast sachet instead of stretching an old liquid pack.

Example: A 100B vial packaged 45 days ago → approx 71% viable ≡ 71 billion viable cells. A starter is recommended for most ales at this point.

This is a simplified estimate — actual viability varies by strain and storage temperature. Store liquid yeast in the fridge (not the door) and use it as fresh as possible for best results.

Browse our full range of home brew yeast. Building a starter or checking pitch rate? See the Homebrew Calculators hub for the Yeast Pitch Rate and Yeast Starter tools. Washing and reusing a slurry instead? Use our Washed Yeast Calculator.