$500/mo contributions + $5,000 initial at 7.0% annual return, compounded monthly.
Investment vs Interest
Breakdown of your projected future value.
Year-by-Year Growth
Balance vs total invested over time.
Year-by-Year Breakdown
Balance, total invested, and interest earned per year.
| Year | Balance | Total Invested | Interest Earned |
|---|---|---|---|
| 1 | $11,558 | $11,000 | $558 |
| 2 | $18,590 | $17,000 | $1,590 |
| 3 | $26,130 | $23,000 | $3,130 |
| 4 | $34,215 | $29,000 | $5,215 |
| 5 | $42,885 | $35,000 | $7,885 |
| 6 | $52,181 | $41,000 | $11,181 |
| 7 | $62,149 | $47,000 | $15,149 |
| 8 | $72,839 | $53,000 | $19,839 |
| 9 | $84,300 | $59,000 | $25,300 |
| 10 | $96,591 | $65,000 | $31,591 |
| 11 | $109,770 | $71,000 | $38,770 |
| 12 | $123,901 | $77,000 | $46,901 |
| 13 | $139,054 | $83,000 | $56,054 |
| 14 | $155,303 | $89,000 | $66,303 |
| 15 | $172,726 | $95,000 | $77,726 |
| 16 | $191,409 | $101,000 | $90,409 |
| 17 | $211,442 | $107,000 | $104,442 |
| 18 | $232,923 | $113,000 | $119,923 |
| 19 | $255,958 | $119,000 | $136,958 |
| 20 | $280,657 | $125,000 | $155,657 |
Compound Interest Calculator — Investment & Savings Growth
About this tool
Free Compound Interest Calculator with Monthly Contributions
This compound interest calculator lets you estimate how savings or investments grow over time when interest compounds on top of itself. Enter an initial investment, set a monthly contribution amount, choose an annual interest rate, and select how many years to invest. The calculator shows the total future value, how much you invested in total, and how much came from compound interest alone.
Compound interest is one of the most powerful forces in long-term investing. The key difference from simple interest is that each period, interest is calculated on the growing balance, not just the original amount. This creates exponential growth, especially at higher rates and longer time horizons. A $500 monthly contribution at 7% annual return over 30 years produces over $567,000 — more than $387,000 of which is pure interest earned, despite only investing $180,000 total.
This calculator supports four compounding frequencies: monthly, quarterly, semi-annual, and annual. Most savings accounts, CDs, and index fund projections use monthly compounding. Some bonds and structured products use quarterly or semi-annual. You can switch between them to see how frequency affects the end result. For most realistic scenarios, monthly compounding produces the most accurate model for recurring investment accounts.
The tool includes S&P 500 average, conservative, aggressive, and HYSA 2025 quick preset buttons. These are useful starting points — the S&P 500 has returned roughly 10% annually on average over the long run, while high-yield savings accounts in 2025 pay around 4% to 4.5%. The Rule of 72 estimate in the sidebar tells you how long it takes for your money to double at the current rate, without doing any math.
All calculations run privately in your browser. Your investment amount, contribution level, rate assumption, and compounding preference are never uploaded. Use this tool alongside the SIP Calculator for India-focused planning, or the Mortgage Calculator and Rent vs Buy Calculator for housing decisions.
Features
- Calculate future value of lump sum + monthly contributions with compound interest
- Choose monthly, quarterly, semi-annual, or annual compounding frequency
- See total invested vs interest earned split with a visual stacked bar
- Inflation-adjusted value to evaluate goals in today's purchasing power
- Interactive year-by-year chart with hover tooltip showing balance and interest
- Rule of 72 doubling time estimate based on your current rate
- Quick presets: S&P 500 avg (10%), HYSA 2025 (4.5%), conservative, aggressive
- Year-by-year breakdown table showing balance, invested, and interest earned
- Download the projection table as CSV for spreadsheet analysis
- Copy a clean investment summary to clipboard for sharing or notes
- Auto-saves inputs to browser storage so you can return without re-entering
- Runs fully in browser — no account, no upload, no tracking of financial data
How to Use This Compound Interest Calculator
- 1Enter your initial investmentAdd the starting amount you have available to invest. This could be existing savings, a windfall, or an opening deposit in a new account. The calculator compounds this amount from day one alongside your ongoing contributions.
- 2Set your monthly contributionEnter the amount you plan to add every month. Consistent contributions dramatically increase total growth because each dollar invested gets more time to compound. Even modest amounts like $100 or $200 per month create substantial wealth over 20 to 30 years.
- 3Choose an annual interest rateEnter the expected annual return or interest rate. Use the quick preset buttons to start with common benchmarks: S&P 500 historical average (10%), conservative portfolio (5%), aggressive growth (14%), or HYSA 2025 (4.5%). Adjust the number to match your actual account or expected return.
- 4Select investment durationSet how many years you plan to keep the investment. The chart and table update instantly as you drag the slider. Notice how extending from 10 to 20 years often more than doubles the final balance due to exponential compounding in later years.
- 5Pick a compounding frequencyChoose how often interest compounds: monthly, quarterly, semi-annual, or annual. Monthly compounding is standard for most investment and savings accounts. Quarterly is common for some bonds. Annual is good for quick comparisons. Monthly compounding produces slightly higher results than annual at the same rate.
- 6Review the breakdown and chartThe results show future value, total invested, interest earned, inflation-adjusted value, and wealth multiple. The chart shows both balance and total invested over time — the gap between the two lines is pure interest earned. Hover over the chart to see exact figures for any year.
- 7Download or copy your projectionUse the Download CSV button to export the year-by-year table for spreadsheet analysis. Use Copy summary to save the key figures for notes, financial planning documents, or sharing with an advisor.
Common Use Cases
Frequently Asked Questions
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, which only applies to the original amount, compound interest grows exponentially over time. The more frequently interest compounds, the faster the balance grows. This is the foundational principle behind long-term investing and savings growth.
The compound interest formula is: A = P(1 + r/n)^(nt), where A is the future value, P is the principal, r is the annual interest rate, n is the number of compounding periods per year, and t is the number of years. When regular contributions are added, the future value of each contribution is also compounded forward to the end of the investment period. This calculator handles all combinations automatically.
Monthly compounding is the most common for savings accounts, CDs, and many investment accounts. Quarterly compounding is used in some bonds and structured products. Annual compounding is used for simpler comparisons and some long-term projections. The difference between monthly and annual compounding grows larger over longer periods and higher interest rates, but for most practical planning purposes the impact is moderate.
For stocks and index funds, the US stock market (S&P 500) has returned roughly 10% annually on average over the long run, though this includes volatile years and is not guaranteed. For high-yield savings accounts in 2025, rates are around 4% to 5%. For bonds and conservative portfolios, 3% to 5% is a common planning assumption. For mixed portfolios, 6% to 8% is often used. This calculator includes quick preset buttons for common scenarios.
The Rule of 72 is a simple way to estimate how long it takes for an investment to double. Divide 72 by the annual interest rate. At 7%, money doubles roughly every 72 ÷ 7 = 10.3 years. At 10%, it doubles in about 7.2 years. At 4%, it doubles in about 18 years. This calculator shows the Rule of 72 estimate in the sidebar based on the current rate you enter.
Monthly contributions have a compounding effect on their own. Each dollar invested early has more time to grow, while later contributions have less time but still benefit from compounding until the end. Over long durations, regular contributions can multiply total wealth dramatically compared to a lump sum alone. This is why consistent investing habits matter more than timing the market for most long-term investors.
Yes. The inflation rate field lets you enter an expected annual inflation percentage. The calculator uses this to show an inflation-adjusted future value alongside the nominal figure. The inflation-adjusted value reflects approximately what the future balance would be worth in today's dollars, which is important for retirement planning, education funds, and any goal evaluated in real purchasing power.
No. This compound interest calculator runs entirely in your browser using JavaScript. Your initial investment, monthly contribution, interest rate, duration, and compounding frequency are processed locally. Nothing is uploaded or stored on a server. Inputs are saved in your browser's localStorage so they persist across sessions, and you can clear them any time with the Reset button.
This compound interest calculator is designed for the US audience and supports monthly, quarterly, semi-annual, and annual compounding. It uses USD formatting and is applicable for index fund investing, HYSA savings, CDs, 401(k) projections, and general wealth planning. A SIP (Systematic Investment Plan) calculator is India-focused and designed around mutual fund SIP investing in Indian rupees.