When I use those online compound interest calculators to estimate the value of my NW over time, they always ask if you are compounding monthly or yearly. How do I find out with my 401K, Roth, etc., which way they are compounded? Are they generally compounded one way or another, usually?

Those calculators are just giving you ballpark estimates based on your mathematical inputs.

IRL what you'll experience is wide swings in your portfolio as the markets go up and down (assuming you have most of your assets in equities - either index/mutual funds or individual stocks). Even if you carry bonds your yields will change month-to-month. Most stocks will also pay our dividends quarterly

in other words, when using those calculators for predicting how much money you will have in "x" years tehre's no "best" way. They are more accurately used if you have a debt to someone (or they have a debt with you) and you want to calculate how much is owed. For most debts (mortgages, credit cards, etc) debt is compounded monthly.

Yes, I knew they were pretty rough approximations. Do you know of any better ways to calculate how much money I might have in a certain number of years? I'd like to play around with figuring out when I might be able to FIRE. I know there's that FIRE calc thing, but it seems really complicated.

well, FIRE calc and cfiresim are two of the best calculators out there that use real historical data to model a portfolio's performance.

Simply assuming a uniform return over long-ish periods will only give you ball-park estimates because you'll never actually have that kind of steady, non-volatile situation.

This is one of those things you can't really predict or control. Either run some cFIREsim simulations and see at what point a given percentage of scenarios (say, 50% cross above your FI number), or just use the simplistic compound interest calculators and input pessimistic and optimsitic return scenarios to give you an idea.

Ultimately what might happen or what has happened won't matter - only what will happen. That's impossible to know, so just keep plugging away until you're done.