Investment Growth Calculator
Project investment growth with regular monthly contributions. Enter a starting amount, monthly deposit, return rate and years to see your final balance, total contributions, interest earned and a growth chart.
How to use this tool
- Enter starting amount, monthly contribution, annual return rate and years in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your final balance and the full breakdown beneath it.
Regular investing plus compounding is how most long-term wealth is built. This calculator adds your monthly contribution every month, compounds monthly at your expected return, and charts your balance against what you actually put in — so you can see the gap that growth creates.
⚠ This tool provides general estimates for education only and is not financial, tax or legal advice. Figures may not reflect your situation — verify with a qualified professional.
Formula
Balance after m months = P × (1 + i)m + C × ((1 + i)m − 1) ÷ i
When i = 0: Balance = P + C × m
Where P = starting principal, C = monthly contribution, i = monthly rate (annual rate ÷ 12), m = total months.
Total Contributed = P + C × m; Interest Earned = Final Balance − Total Contributed.
How it works
This calculator models the future value of an investment that receives regular monthly contributions, using the standard future-value-of-an-annuity formula compounded monthly. The principal grows at the monthly rate while each monthly contribution also compounds for the remainder of the term, producing a balance that reflects both the time value of money and the power of regular saving.
When the annual rate is zero, growth is purely additive (no compounding), so total balance equals total cash deposited. The chart plots balance vs. total contributed year by year, showing how the gap between the two lines represents cumulative investment growth.
Worked example
- Starting amount = $0; Monthly contribution = $100; Annual rate = 0%; Years = 1 (m = 12 months).
- Monthly rate i = 0% ÷ 12 = 0, so simple addition applies.
- Final Balance = $0 + $100 × 12 = $1,200.
- Total Deposited = $0 + $100 × 12 = $1,200.
- Interest Earned = $1,200 − $1,200 = $0.
Final balance: $1,200; Total contributed: $1,200; Interest earned: $0.
Common mistakes to avoid
- Entering an annual return rate instead of a monthly rate in implementations that expect monthly input -- always check whether the rate field is annual or monthly.
- Ignoring investment fees and expense ratios -- even a 1% annual fee compounded over 30 years can reduce the final balance by 20-25%.
- Treating the projected balance as guaranteed rather than as an expected value; actual returns vary year to year and the final balance can differ substantially.
Key terms
- Future value of an annuity
- The accumulated value of a series of equal periodic payments at a specified rate of return over a set number of periods.
- Monthly contribution
- A fixed amount added to the investment at the end of each month, in addition to the starting principal.
- Compounding monthly
- Interest is calculated and added to the balance each month, so subsequent months earn interest on prior interest.
- Principal
- The initial lump-sum amount invested at the start of the period, before any contributions or growth.
- Total contributed
- The sum of the starting principal plus all periodic contributions made during the investment period, excluding any earned interest.
Frequently asked questions
- Is the contribution added monthly?
- Yes — your monthly amount is added every month and compounds at one-twelfth of the annual rate.
- What return rate should I use?
- Long-run global stock returns have averaged roughly 7% after inflation, but future returns are not guaranteed. Try a range to see best/worst cases.