Originally Posted by DustoffVT

You need 20 years LEO (6c) coverage. Under 6c your retirement is calculated at 1.7%/ year x the average of your high 3 years. Any years beyond 20 pay at the normal rate of 1%. You can buy back military time to tack on to retirement, which can add 1% per year of active duty service. The cost of the buy back is 3% of your military earnings.

It is a complex deal with many rules and details. Google FERS 6c and I'm sure OPM has a guide online somewhere.