Pending some gross typographical error in the Internal Revenue Code, when it comes to knowing what you need to know about 401k regulations, you have four numbers to memorize: 10, 50, 59 ½, and 70 ½. All the other numbers are subject to change. Some of those numbers are contribution limits; others are catch up contribution limits or required minimum distributions (RMDs). Generally, the numbers in flux will only ever go up as Congress occasionally revisits this aspect of tax law to index it for inflation, but as we said, the other four are here to stay.

The 10 is for ten percent. This ten percent is the fine you have to pay for early withdrawals. This applies, in fact, to hardship withdrawals from your 401k. While there are certain instances in which your 401k funds may be accessed without this penalty (the words Midwestern disaster area should be on the tip of your tongue; if they aren’t, your hopes of avoiding penalties are only hopes and not realities), you will generally have to pay ten percent on anything you withdraw from your 401k account before you reach retirement age. This is one of the 401k regulations that is firm and immoveable.

Another such of the 401k regulations concerns the number 50. At age 50 is generally when you can start making catch up contributions. While the amount of what is considered a permissible catch up contribution is likely to change with the slow change of the laws, it’s currently at five thousand and a half dollars. These catch up contributions are allowed so that you can flesh out your 401k at an age when you’re probably making more money than you were when you first opened it.

Fifty nine and a half is the next important number, and if you forget the rest, remember this one. The age of 59 ½ is the age at which penalties no longer apply. Those six months count; at that point, you’re permitted to start withdrawing from your 401k, subject to any 401k regulations imposed by your employer or provider, and to start using your retirement fund for retirement. Any withdrawals before fifty nine and a half will be subject to penalties (notably that ten percent penalty mentioned above); any that come after will not. However, no matter when you withdraw, you will have to pay tax on the withdrawal amount. The government set this up to be a tax-deferred plan, not a tax-free one.

The last number that is unlikely ever to change is seventy and a half. At that age, if you’ve maintained consistent employment all the way through, you have to start taking the required minimum distribution (RMD) from your account. At this point, the 401k regulations step in and say that no matter how hard a worker you are—no matter how great an executive—you can’t keep on stockpiling that cash. You’re welcome to keep working, but the government wants its taxes, so consult the IRS-provided table to figure out what you need to withdraw each year. That wraps up the four numbers that will likely never change in 401k rules and regulations.