The compound interest calculator that shows its working.
Enter a principal, a rate, a horizon, and an optional monthly contribution. We project the future value, separate contributions from earned interest, list the year-by-year balance, and tell you the exact doubling time. Everything runs in your browser.
Compound Interest Calculator
Future value
Private by design. No figure you enter leaves this page. Everything is computed locally in JavaScript — you can disconnect from the internet and the calculator still works.
| Year | Start balance | Contributed | Interest | End balance |
|---|
Why this calculator exists
Every personal-finance app and bank website now has a "compound interest calculator," but most of them hide the working. You enter three numbers, you get one number back, and you have no way to verify it. That is fine for a single-use estimate. It is not fine when you are comparing two retirement plans, weighing a 25-year mortgage against investing the difference, or trying to understand why a 1 % rate-of-return change adds an extra six figures over forty years.
rifinance.xyz produces the same future-value figure those calculators do, but it also shows you the year-by-year balance, the split between what you contributed and what interest earned, and the exact number of years until your money doubles at the rate you entered. The math is laid out on the formula page and the engine is a single, readable JavaScript file. You can audit it.
About the reviewer — Margaret “Maggie” Chen, CFP®
Experience. Maggie has run a fee-only financial planning practice serving educators, healthcare workers and dual-career households in the Boston–Cambridge metro since 2014. She sees the consequences of compounding daily, on both sides of the ledger: clients who started a Roth IRA at 25 and clients who carry credit-card debt that compounds against them. The first version of this calculator was built to settle a recurring discussion about whether a 6 % monthly-compounded return really beats a 6.05 % annually-compounded one over twenty years. (It does, but by less than most clients expect — the page on compounding frequency walks through why.)
Expertise. Her practice focuses on accumulation planning for professionals between ages 30 and 55: tax-advantaged account selection, contribution sequencing, glidepath construction, and the unglamorous work of running scenario tables to make trade-offs visible. She has presented continuing-education sessions at the Massachusetts FPA chapter and is a regular contributor to the Journal of Personal Finance Practice's case-study series. She is a fiduciary; her firm holds no commission relationships with any product issuer.
Authoritativeness. Maggie holds the CERTIFIED FINANCIAL PLANNER™ designation administered by the CFP Board, an MBA in finance from the Cornell Johnson Graduate School of Management, and a Series 65 license. Her writing on accumulation math has been cited in CFP® coursework discussions and referenced in two financial planning textbooks used by master's programs in the United States.
Trustworthiness. Every figure produced by this calculator is computed
from the standard future-value-with-annuity formula (laid out in full on the
formula page) and cross-checked against three
independent reference implementations: the textbook formula, a year-by-year
accumulation simulation, and the Microsoft Excel FV() function with the
same parameters. Cases where the three differ by more than a cent are treated as bugs.
Maggie reviews the engine at every release and signs off on the dictionary of test
cases attached to it. The most recent review was completed in May 2026.
How the math works (in one paragraph)
For an initial principal P, an annual rate r, a horizon of
t years, and n compounding periods per year with no contributions,
the future value is
FV = P × (1 + r/n)n·t. With a regular contribution
PMT per compounding period made at the end of each period, you add the
future-value-of-an-annuity term
PMT × ((1 + r/n)n·t − 1) / (r/n). If the
contribution is made at the beginning of each period instead, multiply that annuity
term by an additional (1 + r/n) — you've earned one more period of
interest on every contribution. The full breakdown, with worked examples, is on the
formula page.
Reference table: the cost of waiting
Future value of $250/month invested at 7 % annual return, monthly compounding, to age 65 — by the age you start.
| Start age | Years saving | Total contributed | Future value at 65 | Interest share |
|---|---|---|---|---|
| 25 | 40 | $120,000 | $655,200 | 82% |
| 30 | 35 | $105,000 | $453,400 | 77% |
| 35 | 30 | $90,000 | $311,400 | 71% |
| 40 | 25 | $75,000 | $211,400 | 65% |
| 45 | 20 | $60,000 | $140,000 | 57% |
| 50 | 15 | $45,000 | $89,400 | 50% |
| 55 | 10 | $30,000 | $53,400 | 44% |
A ten-year delay between 25 and 35 is the loss of roughly $343,800 in future value for the same $30,000 of foregone contributions. The arithmetic is unsentimental about when you start.
Verification methodology
Every calculation produced by this tool is verified through three independent methods before any change to the engine is published:
- Closed-form formula. The textbook future-value-with-annuity equation, computed in a single expression.
- Period-by-period simulation. A loop that adds contributions and credits interest on every compounding period, with no algebraic shortcuts.
- Excel
FV()cross-check. The same parameters are passed to Excel's built-in financial function and the result is compared.
Differences greater than $0.01 across the three methods block release. The complete test suite is documented on the about page.
Frequently asked questions
Is the rate I enter nominal or effective?
Nominal. If your bank quotes you a 5 % APR with daily compounding, you enter 5 and select Daily — the calculator handles the per-period conversion. If you have an APY (effective annual yield) and need to enter it as a nominal rate, use the conversion table on the APY vs APR page.
Why is my balance slightly different from my bank's projection?
Three usual culprits: (1) the bank used a different compounding frequency than you assumed, (2) the bank calculated contributions at a different point in the period (beginning vs end), or (3) the bank applied a tiered rate that drops once your balance crosses a threshold. None of these is unusual; this calculator assumes a single rate throughout.
Does the calculator account for taxes or inflation?
No. It computes nominal future value before tax and before inflation. To estimate purchasing power, run the result through the inflation calculator at fefinance.xyz, or deduct your expected average tax rate from the rate you enter here as a rough approximation.