How does Reserve or National Guard retirement transfer over to federal service?

I had about 10 years active and 4 reserve which equated to 11+ years on my retirement buy-in. That included boot, A School, going to war for summer vacations, etc. The HR types figured it all out from my pile of DD-214s and Annual Reserve Statements. Since I had to go FERS, that meant they calculated what 3% of all my base pay was. Turned out I needed to pay $10 Grand which I did over a two year period with a payroll deduction to avoid the interest kicking in later. In any case, buy-in early is paying more that the whole amount, every year. Killer investment when you crunch the effective rate. If you go the Fed route, make sure they calculate your "effective date" for leave purposes. It steps up from 4 hours a pay period to 6 hours after 3 years and 8 hours after 15 years. The sooner you start scraping in the larger numbers, the more fun time you can have. With 8 hours, you have to force yourself to not lose it.

You can “buy back” your military time and have it counted for federal retirement. Your guard and reserve time will have to be computed for active duty time.

When I started at the USPS my active duty time counted towards my leave accrual time. So from day 1 I got 8 hours a pay period. I never bought back my time because reasons and I wish I had.

You can buy back active duty time you have accumulated in the GUARD/Reserve and it can to applied to your Federal time in service. Search the OPM website for more details. Keep a copy of all paper work if you do buy time back.