viability of washed yeast

Does anyone know or have any sources for determining viability of washed yeast? I would like to think it is equal to a fresh vial at maybe 90% but am not sure

This question has been asked dozens of times. But unfortunately, unless you invest into a microscope and some plates, there is no guidelines. There are WAY too many variables... strain, age, fermentation conditions, washing conditions, storage conditions, mutations, infections, etc, etc.

From personal experience, I can only tell you that my washed yeast always behaves better than a fresh smack pack. Regardless I always make a starter.

You should make a starter, its really the only easy way to verify viable yeast at the home brewing level... I agree with BigB though, my washed yeast has always been more robust, etc than a new pack, even after 2-3 months in the fridge.

Like everyone else as said, the only way I test viability, is to make a starter.

So really then, with yeast viability swinging by possibly as much as 30% (impossible to know w/out lab equipment) the whole MrMalty calculator idea is a rough guideline at best, it seems

The only way to tell the viability is to pitch it in a new batch and judge it based off of airlock activity. That is the ONLY reliable sign of fermentation and yeast health.

but on a more serious note, I had the same problem with the Mr.Malty calculator thing as well. Simply eyeballing the % of trub and the yeast viability seems to be so inaccurate to the point of nonsense. I think saying pitch 1/3 of the yeast that you got from washing would be more accurate.

But making a starter will help negate those guesses and ensure the right amount of yeast