<prompt> Human: This might be a really, really dumb question but Ive never actually done this before. If I wanted to contribute the annual maximum to my 401k 18,500 in 2018, how do I make sure that I hit that amount by next year?Is it as simple as just figuring out my annual take-home income, and figuring out the percentage that 18,500 is? For example, if I make 100k pre-tax annually, then 18,500 100,000 18.5, so I tell my employer to contribute 18.5 from each of my paychecks?What happens if I started contributing late in the year? Can I contribute one-off amounts for a few paychecks to catch up? What happens if I legit put down at the start of the year that I want to contribute 33 of my paychecks to my 401k and as a result, I hit the legal maximum in the middle of the year? Is my employer supposed to automatically detect this and stop my contributions, or will I have to pay a penalty at tax time next year?  <response>: Is it as simple as just figuring out my annual take-home income, and figuring out the percentage that 18,500 is? For example, if I make 100k pre-tax annually, then 18,500 100,000 18.5, so I tell my employer to contribute 18.5 from each of my paychecks?More or less, yes. What happens if I started contributing late in the year? Can I contribute one-off amounts for a few paychecks to catch up?You can usually adjust your contribution percentage whenever you like. Or else you could just divide 18,500 by your remaining pay for the year. What happens if I legit put down at the start of the year that I want to contribute 33 of my paychecks to my 401k and as a result, I hit the legal maximum in the middle of the year? Is my employer supposed to automatically detect this and stop my contributions, or will I have to pay a penalty at tax time next year?Your payroll department will usually cut you off at the contribution cap, but you can always verify this with them. 