How much can you put into a 401k?

The limit for contributions buy anyone under 50 is $18,500 in 2018 and $19,000 in 2019.
Starting at age 50, you can contribute an additional $6000 each year.

I vaguely remember a limit of 50% of your gross income, but I doubt anyone whose income was low enough to hit that limit first can afford to contribute that much.

depends on your age. over 50 you can put in more. Last I looked it was about 18 or 23 thousand.

Correct answers so far.

I just want to add that your employer can contribute matching funds money up to 15% of your taxable earnings.

Obviously you can't magically make your employer contribute more than they offer, but 15% is their cap if they choose to go that high.

$18,500/year.