You know where you are and where you want to be. Pick a growth rate and see how long the trip takes, with milestones along the way.
This is the future value formula turned around so it solves for time. Because time sits in the exponent, we need logarithms to get it out:
Turning $10,000 into $100,000 at 8% a year.
Ten times over takes a bit more than three doublings, and at 8% each doubling takes about nine years.
You have $150,000 today and want $1,000,000, assuming 7% a year and no new contributions.
Adding money along the way would get you there a lot sooner.
You’ve saved $40,000 toward $60,000 and expect 5% a year.
That’s about 8 years and 4 months.
With compound growth, time is an exponent: you raise (1 + r) to the power of t. Logarithms are simply the tool for pulling an exponent back down so you can solve for it.
Then you’ve already made it, and the calculator will say zero years.
No. It assumes one starting balance growing on its own. Regular contributions would shorten the timeline, and the Investment Growth Calculator can show you by how much.
The math is exact for the numbers you enter. Real returns change from year to year, though, so think of it as “how long, if things go roughly like this,” not a promise.
Doubling time is just the special case where your target is twice your starting amount. The Doubling Time Calculator also shows how close the Rule of 72 gets.