There are two useful monthly numbers: the amount your budget can sustain and the amount a goal requires. Compare them before committing to a savings target. The Savings Goal calculator works backwards from a target and timeline; your own cash flow shows whether that number is realistic.
Start with what your budget can support
Suppose your monthly take-home pay is $4,000. Regular essentials cost $2,700, and you set aside $500 for irregular bills. That leaves $800 per month for your savings goals, or 20% of take-home pay. This is an illustration, not a universal percentage to follow.
If a goal requires $1,200 per month, the gap is $400. Review the target, timeline and expenses you can change. A higher assumed investment return reduces the modeled monthly amount, but it does not create spendable cash or guarantee the goal. Keep short-term cash needs and longer-term investments separate when deciding where to save.
Monthly savings to reach a goal
Assuming contributions at the end of each month, compounding monthly:
| Goal | Time | At 5% | At 7% | At 10% |
|---|---|---|---|---|
| $250,000 | 20 years | $608 / month | $480 / month | $329 / month |
| $500,000 | 30 years | $601 / month | $410 / month | $221 / month |
| $1,000,000 | 30 years | $1,202 / month | $820 / month | $442 / month |
Notice what the return does: to reach $1,000,000 in 30 years, you need $1,202/month at 5% — but only $442/month at 10%. The higher expected return more than halves the required savings rate. That's compound interest doing the heavy lifting.
The flip side: what $1,000 a month becomes
Working forward instead — $1,000 saved every month for 20 years:
| Return | You contribute | Final value | Growth |
|---|---|---|---|
| 5% | $240,000 | $411,034 | $171,034 |
| 7% | $240,000 | $520,927 | $280,927 |
| 10% | $240,000 | $759,369 | $519,369 |
Which return should you use?
- 5% — a lower illustrative return scenario used to test how much more you may need to save.
- 7% — Caspenda’s illustrative nominal annual return default (not a forecast). Historically the S&P 500 has averaged roughly 10% a year in nominal terms since 1926 and about 7% after inflation, per long-run data compiled by NYU Stern’s Aswath Damodaran (Source: NYU Stern historical returns).
- 10% — a higher illustrative return scenario. Do not treat it as a forecast or planning guarantee.
Past performance never guarantees future results. If you plan with 7% and the market delivers less, you'll have to save more later — that's why a small cushion matters.
Sources & assumptions
- Scenario rates: 5%, 7%, and 10% are model inputs used to show sensitivity. The SEC's Investor.gov compound-interest calculator also treats interest rate as an estimated input and supports testing a variance range. Source: Investor.gov.
- Historical return context: the long-run S&P 500 figures referenced above come from NYU Stern's historical-return dataset; past returns do not predict future results.
- Inflation: BLS CPI is a standard source for historical U.S. consumer-price changes. Source: BLS.
- Caspenda math: monthly savings figures assume end-of-month contributions and the stated compounding assumptions. See Methodology.
The takeaway
Compare the monthly amount your budget can support with the amount your goal requires. If they differ, adjust the target or timeline and test a range of return assumptions. Use the Savings Goal calculator for the target, then check purchasing power with the Inflation calculator.