You are 35 years old. You earn $75,000 a year. You contribute 6% of your salary to your 401k, and your employer matches 3%. It sounds like a solid plan. But will it actually be enough to retire at 65? Will you have $1 million? $2 million? Or will you fall short? The math is complicated. Your salary will likely grow. The stock market will fluctuate (but hopefully grow). Inflation will erode the value of your money. And compound interest works in non-linear ways that are impossible to calculate in your head. You could try to build a complex spreadsheet, guessing at future variables. Or you could use a 401k calculator to instantly see that your current path puts you on track for $1.4 million at age 65 (assuming an 8% return and 3% salary growth). A 401k calculator projects the future value of your retirement savings based on your current balance, contribution rate, employer match, and expected investment returns. It allows you to stress-test your retirement plan by changing variables—show...