Compound Interest Scenarios
Compare specific situations: $100 a month for ten years, $10,000 at 7%, or the time to half a million. The tables state their assumptions; related calculators let you adjust the inputs.
Monthly contribution scenarios at a glance
Every row assumes monthly compounding and deposits made at the end of each month, with no taxes or fees. The point of showing three rates is that the rate you assume matters more than almost anything else you can change.
| Scenario | You contribute | At 5% | At 7% | At 10% |
|---|---|---|---|---|
| $100/mo for 10 years | $12,000 | $15,528 | $17,308 | $20,484 |
| $100/mo for 30 years | $36,000 | $83,226 | $121,997 | $226,049 |
| $200/mo for 20 years | $48,000 | $82,207 | $104,185 | $151,874 |
| $500/mo for 20 years | $120,000 | $205,517 | $260,463 | $379,684 |
| $100/mo, age 25 to 65 | $48,000 | $152,602 | $262,481 | $632,408 |
| $500/mo, age 30 to 60 | $180,000 | $416,129 | $609,985 | $1,130,244 |
Read the first and last rows together. $100 a month for ten years produces about $5,300 of modelled growth on $12,000 contributed at 7%. The same $100 a month held for forty years turns $48,000 into roughly $262,000 at the same 7%. Nothing changed except the number of compounding periods.
Lump-sum scenarios
No further contributions — one deposit, left alone, compounding monthly.
| Starting amount | Rate | Years | Ending balance | Multiple |
|---|---|---|---|---|
| $1,000 | 5% | 10 | $1,647 | 1.6× |
| $5,000 | 7% | 20 | $20,194 | 4.0× |
| $10,000 | 7% | 30 | $81,165 | 8.1× |
| $50,000 | 7% | 20 | $201,937 | 4.0× |
| $100,000 | 7% | 20 | $403,874 | 4.0× |
The multiple column is the useful one. At a fixed rate and timeline the multiple is identical no matter how much you start with — $5,000, $50,000 and $100,000 all become almost exactly four times as much over twenty years at 7%. Compound growth does not care about the size of the balance, only about the rate and the time. Both the starting amount and the timeline affect the ending balance.
All scenarios
How to read a scenario page
- The headline number is one assumption deep. Change the rate by two points and the thirty-year figure moves by tens of thousands. Use the linked calculators to compare those assumptions.
- Contributions and growth are shown separately. If growth is smaller than contributions, the timeline is doing very little work yet — that is normal in the first decade.
- Nothing here is after tax or after fees. Both reduce the effective rate. A 0.9% platform-and-fund charge on a 7% return is not a rounding error over thirty years.
- Inflation is not deducted. To see what the number buys rather than what it says, use retirement in today's dollars or read nominal vs real return.
FAQ
Which rate should I use?
There is no correct answer, only a defensible range. Long-run broad equity averages are commonly quoted between 7% and 10% before inflation; cash and bonds are far lower. Running a scenario at both 5% and 7% tells you more than picking one and trusting it, because the gap between the two is the uncertainty.
Why do these results differ from another calculator?
Almost always because of compounding frequency or contribution timing. A tool that compounds annually will show less than one that compounds monthly, and one that assumes deposits at the start of each period will show slightly more than one that assumes the end. The difference is explained in contribution timing and compounding frequency.
Is a lump sum better than monthly contributions?
For a fixed total amount, money invested earlier compounds for longer, so a lump sum usually ends higher. That is a mathematical statement, not a recommendation — most people do not have the lump sum, and the monthly habit is what they can actually do.
Can I build my own scenario?
Yes. The main compound interest calculator takes any starting balance, rate, timeline, compounding frequency and contribution, and shows the year-by-year table behind the result.