Amount of dry yeast needed...

[Anthonie]

I am sure this has been asked on here before but I can't find it.... I am getting ready to brew a IIpa and have read that two packets of dry yeast should be used. My first question is why this is?

I usually brew smaller batches (3gal) based on what my stove top can handle and typically do one packet of dry yeast for all of my brewing. So should I be adjusting the amount of yeast based on the size of the brew?

I also was thinking about making four gallons and putting one gallon in a separate fermenting vessle to try different dry hopping. So again, would this need less yeast and is there a way to determine how much less?

Thanks ahead of time for your help

 

[Maylar]

Starting gravity and batch size determine pitch rate. Here is a calculator, which I'm told is a bit conservative -
http://www.brewersfriend.com/yeast-pitch-rate-and-starter-calculator/

 

[Silver_Is_Money]

The answer may depend upon the selected yeast as well as the chosen fermentation temperature, but for 4 gallons I would only re-hydrate and pitch 1 pack. If you are going to sprinkle straight from the pack, you may want to consider 2 packs, but again this depends upon the yeast and the temperature. For Nottingham dry yeast you can likely get by just fine with one pack no matter how you choose to pitch it. All bets are off for other dry yeasts.