The short answer
Your investment grows by whatever return it earns, minus fees, compounded over the years you leave it alone, plus anything you keep adding. Three things move the result the most: how long you stay invested, how much you put in, and the return. Fees and inflation then decide how much of that you keep.
None of these is known in advance except what you put in. That's why a good projection is a range, not a single number.
A lump sum on its own
Put $10,000 into an investment that earns a steady 7% a year with no fees. After 20 years it's worth about $38,696.84. You never added a cent, yet the balance nearly quadrupled.
The pace picks up over time. In this case, growth passes the original $10,000 a little past year 10. That's compounding at work: each year's gain is figured on a bigger balance than the year before.
Monthly investing on its own
Now skip the lump sum and invest $300 a month at the same 7% for 20 years. You put in $72,000 and end with about $152,260.91. Growth makes up a bit more than half of the final balance.
Regular deposits take longer to snowball, since the newest dollars have barely started earning. Even so, in this example the monthly habit ends up worth about four times the lump sum's result. Combine the two, add a 0.25% fee, and $10,000 plus $300 a month reaches about $184,522.20.
Why a late start costs more than the missed deposits
Keep the same plan and end date, but start five years late. The result falls to $117,898.31, a gap of $66,623.89. You skipped $18,000 of deposits, so the other $48,623.89 is growth that never happened.
Raising your contribution over time helps close gaps like that. In the same plan, raising the monthly amount 3% each year lifts the 20-year result to about $223,127.76.
- Start now: $184,522.20
- Start in 3 years: $141,977.02
- Start in 5 years: $117,898.31
- Start in 10 years: $69,798.14
Picking a return you can defend
The return is the input people most like to stretch. In the example, going from 7% to 9% adds about $54,000. Going down to 5% takes away about $41,000. If a plan only works at the high number, it isn't a plan yet.
Match the rate to what you'll own. Investor.gov ties time horizon and risk tolerance to how you split money between stocks, bonds and cash. Money you need in three years can't sit through a deep drop and wait for a recovery the way 30-year money can.
Fees, inflation and checking your own numbers
A 0.25% yearly fee costs about $6,436 over 20 years in the example, and a 1% fee costs about $24,336. Inflation doesn't take money out of the account, but at 2.5% a year, $184,522 in 20 years buys roughly what $112,609 buys today.
Want to run your own figures? Our investment growth calculator lets you change one input at a time. It shows the yearly balances, a lower and higher return, and the cost of waiting side by side.
Frequently asked questions
How long does it take to double an investment?
At a steady 7% a year, $10,000 doubles in a little over 10 years. Higher returns shorten that, and fees stretch it out.
Is it better to invest a lump sum or monthly?
A lump sum has more time in the market, so it grows more if returns are positive. Monthly investing spreads purchases over time. This guide's numbers assume steady returns and can't settle which wins in a real market.
How much of my final balance will be growth?
It depends mostly on time. In the 20-year example, growth is about 56% of the balance. Over 30 years it rises to about 71%.
Do I need to account for taxes?
Usually, yes. Taxes depend on the account type and where you live, so the projections here are before tax. Check the rules for your account before you plan around the result.
References
These sources support the method or guidance used for Investment Calculator. Verify time-sensitive rules at the source.
Try the calculator
Open Investment Calculator, enter your scenario, and compare its supporting rows with this guide's method and checks.