The 401(k) loan calculator will show you how much a 401(k) loan will cost you over your working lifetime. Because of the huge financial repercussions of borrowing from a 401(k), a 401(k) loan should only be used in dire financial circumstances such as avoiding a foreclosure or bankruptcy.
The calculator assumes you will not be contributing to your 401(k) while you pay back the loan. Due to reduced paychecks, most borrowers cannot afford to continue contributions. This further limits your retirement savings. If your employer matches your contributions, you lose the match as well.