As a military retiree I have some knowledge here, but am hesitant to even begin typing it out. I’ll try to constrain myself on detail.
There are actually currently four military retirement systems and they have different rules.
If you joined before Sep. 8 1980 (this was me), you are part of the “Final Pay” system. This means that if you meet the years of service requirements, you get a lifetime salary based on a formula that uses your final pay as its basis. This means if you got a rank promotion right before retiring, you get to retire with that as your baseline.
The formula is as follows:
You multiply your final monthly pay by 2.5% for each year of military service. The minimum to get benefits is 20 years. At the minimum, 20 x 2.5% = 50%, and at 40 years you hit 100%.
The next system is for people who joined between Sep 8 1980 and July 31 1986. This system is called the “High 36” system. The High 36 system is identical to the Final Pay system, except instead of using your final pay as the basis, it calculates the basis by averaging the highest 36 months of pay during your entire military career. What this functionally does is means if you got a rank promotion and retired a month later, you don’t get a lot of benefit, so you generally have to serve in your highest rank for at least three years for it to fully weight your retirement. The percentages for years of service is the same as for Final Pay, and also has a minimum service years of 20.
The next system is the CBS/REDUX OR High36 system, for people who joined between Aug 1 1986 and Dec 31 2017. This system is basically an OR option, one option is to simply retire using High 36 rules. The other option is you can take a bonus “CBS”, at 15 years of service. This is $30,000. If you take the $30,000 bonus at 15 years of service, you then can retire after 20 years of service under a REDUX system–the REDUX system is the same as High 36, except instead of the 2.5% per year of service multiplier, it is reduced with a convoluted formula. The formula is for EACH year LESS than 30 years of service, your total benefit is reduced by 1%. As a quick example, the basic formula for 20 years, again, nets you 50% retirement, under REDUX, you lose 1% for each year less than 30 years–so 10%, so now you retire at 40% pay instead of 50% pay. If you go to 30 years you essentially neutralize the REDUX. FWIW I think this system is only beneficial if you are really certain you are going past 30 years, and even then I was always suspicious of it, that $30k bonus is subject to taxes, so it’s really $21k.
The next and current system is the “BRS” or Blended Retirement System. This system is a dual-component system. The first component is identical to the High 36 system, you get a life annuity based on years of service and a multiplier EXCEPT under BRS, it isn’t 2.5% per year of service–it is 2%. To “compensate” for that lost .5%, service members in the BRS get in the TSP, which is like a government 401k–and the Department of Defense actually contributes a matching payment to each of your TSP contributions. You are automatically enrolled in the TSP at a 1% contribution rate, and the government matches your contributions dollar for dollar up to 4%. It is up to the servicemember how much to contribute to the TSP, but most prudent people recommend you absolutely do no less than 4% to get the full match, and most say you should consider putting in around 10%.
The TSP is actually a much better system for a large % of the military, because a large % of the military doesn’t serve to that 20 year, many people do 1-2 enlistments and are out. The TSP funds are portable, you take them with you when you leave the service, no matter how short or how long you’re in service.
Note that people in the military before BRS was the norm, can also join and contribute to the TSP–but they are not eligible for matching funds because they aren’t in the BRS.
The other final difference with the BRS system is you can “lump sum” your guaranteed annuity, there is a complex formula where basically you can reduce your annuity benefit by like 50% in exchange for a lump sum payout. While opinions vary, my understanding is most people think the lump sum payout is not a good option for most people, but there are cases where it can be.