Compound Interest Calculator — Long-term Savings Growth
Enter a starting balance, a monthly contribution, an annual rate of return, and a number of years. CalcWize compounds the figures month-by-month and shows the final balance, the share that's your contributions vs. the interest earned, plus a year-by-year visual of how the gap widens.
Why monthly contributions matter so much
Time-in-market beats lump sums for most people. A $200/month contribution compounded for 30 years at 7% becomes around $245k — and only $72k of that is your money. The rest is the eighth wonder of the world doing its thing.
What rate of return to use
Long-run real returns on diversified equity portfolios sit around 6–8% annually (after inflation it's 4–6%). Cash and bonds are lower. Pick a rate that matches what you're actually invested in — not what you wish you were.
A worked example: starting age matters
Saving $300/month at 7% for 40 years lands at roughly $787,000. Starting ten years later — same $300/month for 30 years — only reaches $367,000. The difference is more than two times the outcome for a third less time. That's the cost of waiting.
Real vs. nominal returns
A 7% return in a 3% inflation world is only 4% in real (today's purchasing power) terms. The numbers CalcWize shows are nominal — what your statement will say. To see what they're worth in today's money, deflate by your inflation assumption, or use the Retirement Planner's "today's money" toggle.
Common mistakes
Plugging in the recent five-year US stock-market return is the classic over-estimate — bull markets distort. Use a long-run figure (50+ years across global equities) and lean conservative. People also forget tax: returns inside a tax-free wrapper (ISA, TFSA, Roth) compound un-taxed; everything else gets taxed annually or on withdrawal.
Frequently asked questions
- What rate of return should I use?
- Pick a long-run figure that matches what you are actually invested in. Diversified equity portfolios have historically returned roughly 6–8% a year before inflation; cash and bonds are lower. When in doubt, lean conservative.
- Are the results before or after inflation?
- Before. The figures are nominal — what a statement would show. To see today’s purchasing power, subtract your inflation assumption from the rate, or deflate the final balance by expected inflation.
- Does it account for tax?
- No. Money inside a tax-free wrapper (ISA, TFSA, Roth) compounds untaxed, while taxable accounts are reduced by tax on gains or income. Treat the output as a pre-tax projection.
How we calculate it
Balances compound monthly: each month the running balance grows by the monthly rate (annual ÷ 12) and your monthly contribution is added on top. The final balance is the future value of your starting amount plus the future value of the stream of contributions.
What it doesn't do
- Volatile single-asset projections (this is a smooth-rate model)
- Tax-advantaged retirement projections that need country-specific contribution caps (use the Retirement Planner)
Last reviewed: 2026-05