I usually brew 1.7-gallon batches these days. I use a proportional amount of yeast. 5 gallons requires a full pack of dry yeast for ales, or maybe 2 packs for a lager, right? So for a 1/3 size batch, I use 1/3 pack for ales or maybe for a lager I'd use one full pack. Dry yeast lasts for years and years and years. I've used opened packs that have been in the refrigerator for 2 or 3 years without any trouble at all. I just fold over the corner and tape it shut. People who haven't tasted my beer will always whine about my practices but in my experience it works awesome. Super cheap 'n' easy. Try it for yourself and you'll see for yourself.