FAQ

Help! My Kerbals are not using any oxygen!

As it says on the longer Description page of the wiki (go read it now), Kerbals do not need Oxygen while on Kerbin if a few conditions are met.

Isn't this a little too easy? One container holds enough to last a Kerbal for almost a year!

Yes, a normal sized container does last 300+ days for a single Kerbal. Trips to the Mun or Minmus are short and are easy to provide for. Trips to other planets are much longer and will require more supplies. I calcuated that a round-trip to Duna would take ~1130 Kerbin days (including the wait for a return window). Sending 6 Kerbals would require adding 2.4 large containers or about 4.5 tons. That may not sound like much but a Mk1-2 Command Pod + PPD-10 Hitchhiker Storage Container is only 6.5 tons, so your life support supplies will increase the mass required for your crew by ~70%. And a round-trip to Jool would take ~3023 Kerbin days.

All of my math is available in my spreadsheet

And you should not send a single Kerbal on an inter-planetary mission. He would get quite lonely.