Just realized a lot of people don't actually understand what happens at the end of the year with their 401k contributions. There's this thing called a true up that employers do, and it's actually pretty important to know about.

So here's the deal. When your employer offers matching contributions on your 401k, they're basically saying they'll match what you put in, usually up to a certain percentage of your salary. Sounds straightforward right? The problem is how they actually process it.

Most employers calculate what they owe you on an annual basis, but they pay out the match with each paycheck instead of doing one big lump sum at the end. Makes sense from a cash flow perspective. But this creates a weird situation if you don't contribute evenly throughout the year.

Let me give you an example. Say you make 72k a year, 6k per month, and your employer matches up to 5% annually. That means they owe you 3,600 total for the year. Normally if you contribute 300 per paycheck, they match 300 per paycheck, and after 12 months you're both at 3,600. Perfect.

But what if you accelerate your contributions? Say you throw in 3,000 per month instead. You hit the annual contribution limit in July and stop. Now you've only contributed for 7 months, so your employer only matched for 7 months too. They paid out 2,100 when they actually owe you 3,600. That's a 1,500 shortfall.

Or imagine you only contribute in January when you get a bonus. You throw the whole thing at your 401k. Your employer matches up to 5% of that paycheck, but they still owe you 5% of your full annual salary. Again, mismatch.

This is where the true up comes in. At year end, employers that promised annual matching contributions are supposed to reconcile what they actually owe versus what they've paid. If they're short, they true up your account to make the difference. It's basically their way of saying we promised annual matching and we're going to deliver on that promise, even if the timing got weird.

Not all 401k plans need to do this though. If your employer calculates matches on a pay period basis instead of annually, they only owe you a match for the specific paychecks you contributed to. No true up needed unless there was an error.

The main takeaway is that if your employer offers matching contributions, make sure your plan actually does a true up at year end. It's an easy thing to overlook but it could mean real money in your retirement account.