"Start investing early" is advice everyone's heard. It's also advice that rarely comes with an actual number attached, which makes it easy to nod along and still put it off another year. So here's the number: what does a ten-year delay actually cost you, in dollars per month, for the exact same end goal?
The setup: $1,000,000 by age 65, at a 7% return
Using the same future-value-of-annuity formula behind the compound interest calculator (deposit equals target divided by the growth factor for monthly deposits at a given rate and number of months), here's what it takes to reach $1,000,000 by age 65, assuming a 7% average annual return the whole way:
Starting at age 25 (40 years to grow): about $381/month. Over those 40 years you'd put in $182,880 of your own money, with the remaining $817,120, about 82% of the final total, coming purely from growth.
Starting at age 35 (30 years to grow): about $820/month. Over those 30 years you'd put in $295,200 of your own money, with $704,800, about 70% of the final total, coming from growth.
The actual cost of waiting ten years
Waiting until 35 instead of starting at 25 means needing to save about $439 more every month to land on the exact same $1,000,000 goal. That's more than double the monthly deposit, for ten fewer years of saving overall but the identical end result. And it's not because you're saving less money overall, you actually end up putting in more of your own cash ($295,200 versus $182,880) for a smaller share of the final total coming from growth.
That's the mechanical reason early investing works so well: growth compounds on growth. A dollar invested at 25 has ten more years to compound than a dollar invested at 35, and each of those extra ten years is applied to an already-growing balance, not just the original dollar. The gap isn't really about how much you save, it's about how long your money has to work before you need it.
What if you already waited?
If you're past 25, this isn't really a reason to feel behind, it's a reason to use real numbers instead of guessing. The same calculator works for any starting age. If you're 35 now, $820/month at 7% for 30 years still gets you to the same $1,000,000. If that number doesn't fit your budget, the calculator also has a mode that runs the math forward instead: pick a monthly deposit you can actually afford and see what it grows into by a certain age, rather than working backward from a fixed goal.
What this doesn't account for
These numbers are in nominal dollars, meaning they don't adjust for inflation. $1,000,000 forty years from now will not buy what $1,000,000 buys today, so the real purchasing power of that goal is smaller than the number suggests. The 7% return is a commonly used long-run historical average for a diversified, stock-heavy portfolio, not a guaranteed outcome, real returns vary significantly from year to year and can be negative in any given year. This also models end-of-month deposits with monthly compounding at one fixed rate for the entire period, it doesn't account for which specific account type you use (a 401(k), an IRA, or a regular taxable brokerage account all have different tax treatment that this tool doesn't model).
What if you're past 35 already?
The same math runs in both directions. If you're 45 and want $1,000,000 by 65, that's 20 years to grow, and the required monthly deposit at 7% climbs well past the 30-year number, since there's simply less time for growth to do the heavy lifting. That's not a reason to give up on the goal, it's a reason to either adjust the target, extend the timeline past 65, or accept a larger monthly commitment. The calculator handles any combination of current age, target age and return rate, so you can see your specific number instead of assuming the 25-vs-35 example applies directly to you.
It's also worth remembering that very few people invest the exact same fixed amount every single month for 30 or 40 years straight. Raises, job changes and life events all shift what's realistic year to year. Treat these numbers as a target to aim for and periodically re-check, not a rigid schedule that has to be hit to the dollar every month without exception.
Why monthly compounding changes the shape of the growth
Each month, your account doesn't just earn a return on the money you've put in, it earns a return on the money you've put in plus every return it's already earned in prior months. Early on, most of your balance is your own contributions and growth is a small slice. Years later, the balance is large enough that growth itself contributes more dollars in a single year than your actual deposits do. That's visible directly in the numbers above: at 25, 82% of the final $1,000,000 comes from growth rather than your own deposits. At 35, with ten fewer years for that effect to build, growth's share drops to 70%. The extra decade isn't just extra deposits, it's extra time for growth to become the dominant force in the account.
The takeaway
For the same $1,000,000 goal at the same 7% return, starting at 25 instead of 35 cuts the required monthly deposit by more than half, from about $820 down to about $381. The ten years aren't just time, they're doing real financial work that no later contribution can fully replace. Run your own target, age and return rate through the compound interest calculator to see what starting today, instead of next year, is actually worth for you.
7% is a commonly used long-run historical return assumption for a diversified stock-heavy portfolio and is not guaranteed; past performance does not predict future returns. See the SEC's investor.gov resources for background on the compound interest formula used here.
This is an educational estimate, not financial advice. See the disclaimer for more.
More money math in my book. Check out Money Math for Your 20s.