Yeast Calculators and Final Gravity

So I have started using liquid yeast and have been using yeastcalc to figure out my starters and such. I am a a little confused on how they know what the end product should be. You put in a OG but no FG so how can they tell you what amount of yeast to use? If the calculator doesn't know how the yeast attenuates or what FG you are shooting for, how do they work?
Maybe I am asking a stupid question but can't figure it out.

P.S. Anyone want to chime in on if they use Kai or Jamil's. I know Kai is a regular on here, and I have looked at both of their data it just seems like a large discrepancy.

They don't bother with the FG because it is irrelevant to determining the pitching rate. A yeast starter calculator is only trying to determine whether you are pitching enough yeast based on the amount of sugars that will be in the beer, hence only being concerned with the OG. How much the yeast attenuates is up to the yeast and not relevant to how much should be pitched. Hope that helps. Also, keep in mind that these yeast calculators aren't exactly precise, they are just tools to help you approximate how many yeast cells you have, and how many you'd have after you have prepared the starter. As far as which ones people use, I think you'll find that the majority on this website use www.mrmalty.com as I see it come up very often in posts. Personally, I use this: http://www.brewersfriend.com/yeast-pitch-rate-and-starter-calculator/

FWIW, I have used both yeastcalc.com and mrmalty.com. You get different numbers from both. So that tells me right there that these pitching rate calculators are an approximation. I use both when I get ready to brew or to get a starter ready. I get it in the ball park. Each time I manage to get below the final gravity I was shooting for. But like BlackGoat said. The yeast work on the sugars in the wort. So the calculations are going to be based on the OG. There is more to final gravity than the yeast attenuating all the way. It could be how fermentable you made the wort, temperatures high or low. There is no calculator for that. Just ranges.

Og and FG are just estimates.

You can pitch a ton of yeast but when the fermentable sugars are eaten up then the yeast has nothing else to eat. more yeast cannot eat sugar that is not there. So basically the OG and FG is determined by the fermentability of the wort. mash low and have a very fermentable wort will make the FG lower. Mash high and have a less fermentable wort and you will have a higher FG.

beergolf said:

Og and FG are just estimates.

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OG is probably less of an estimate. I guess at the time you are making your starter it is in most cases an expectation. Mind you it is possible to tune the OG to what you want but like you say the FG is definitely more rubbery.
As has been stated many times around here Yeast Suppliers attenuation rates are very flexible and you can easily end up well outside their published range.

I like Yeastcalc myself.

My own little test on Kai's numbers show it is right on the money.
I do however have this lingering worry about too high a growth rate in starters and what effect it may have on the beer that is produced from the starter.
I wonder at what starter growth rate something would show up in the beer?

StusBrew said:

FWIW, I have used both yeastcalc.com and mrmalty.com. You get different numbers from both. So that tells me right there that these pitching rate calculators are an approximation.

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That or they are recommending different pitching rates, measuring live yeast using a different model, or there could be differences in other calculations.

Back to OP, your FG is going to have far more to do with wort composition and fermentation schedule than anything else, even yeast strain or pitching rates. An FG approximation is not in any way related to optimal pitch rates.