About BoostCalc

BoostCalc is a free, browser-based calculator that helps US savers and investors visualize how consistent contributions, time, and interest rates combine to grow wealth — and how inflation quietly erodes that growth. This page explains what the tool does, how to use it, and how every number on the result screen is produced.

What the calculator does

Given four inputs — starting balance, monthly contribution, annual interest rate, and time horizon — the calculator returns:

Future balance. The projected total value of your account at the end of the time horizon, before any optional tax adjustment.
Total contributions. The cumulative amount you put in (starting balance plus all monthly contributions).
Interest earned. The difference between future balance and total contributions — the portion that growth produced for you.
Inflation-adjusted ("real") value. What the future balance would buy in today's purchasing power, using your chosen inflation rate.
After-tax estimate (optional). A simple approximation that applies your chosen tax rate to total interest earned.
Year-by-year breakdown. A stacked bar chart showing contributions vs. accumulated interest for every year of the projection.

How to use it — a 30-second walkthrough

Enter your starting amount. What you have today in the account you're modeling. Leave at 0 if you're starting from scratch.
Enter your monthly contribution. The amount you plan to add every month. Even small numbers compound meaningfully over decades.
Pick an annual rate. Type one in, or use the preset chips (HYSA, CD, S&P 500 real, S&P 500 nominal) for typical US benchmarks.
Choose a time horizon. Use the slider or type a year value (1–60).
Optional — toggle inflation and tax adjustments. Inflation is on by default at 3%; the tax estimate is off by default.
Share or save. The "Copy shareable link" button encodes all your inputs into the URL so you can bookmark or share a scenario.

The math behind every result

Internally, the calculator converts your annual rate and chosen compounding frequency (annual / monthly / daily) into a single effective monthly growth factor, then iterates twelve months per year:

      effective_annual_rate = (1 + rate/n)n − 1

      monthly_growth = (1 + effective_annual_rate)1/12 − 1

      balancet+1 = balancet × (1 + monthly_growth) + monthly_contribution

      real_value = future_balance / (1 + inflation)years

This produces the same result as Microsoft Excel's FV() function when you match its parameters, and matches the long-form compound interest formulas published by the US Securities and Exchange Commission's investor education materials.

Where the preset rates come from

Avg savings 0.4% — the long-run national average APY on regular US savings accounts as reported by the FDIC.
HYSA 4.5% — representative recent APY for high-yield savings accounts at major US online banks. This rate moves with the Federal Reserve.
5-yr CD 4% — representative recent APY for 5-year certificates of deposit at FDIC-insured institutions.
S&P 500 real 7% — the widely cited long-run inflation-adjusted return of the S&P 500 index over multi-decade periods.
S&P 500 nominal 10% — the long-run return before inflation, again over multi-decade periods.

These are general benchmarks, not promises. Actual rates change frequently and depend on the specific account or fund you choose.

Design principles

Clarity over hype. The inflation-adjusted figure is shown by default because it represents what your future balance will actually buy.
Privacy by design. The calculator runs entirely in your browser. The site requires no account, sends no inputs to any server, and stores nothing about you. Third-party ads on the site may use cookies — see the Privacy Policy for details.
Education, not advice. BoostCalc explains concepts and provides a calculator. It does not recommend specific products, accounts, or investments, and it does not know your personal situation.

What this site is not

BoostCalc is not a licensed financial advisor, broker, tax preparer, or fiduciary. Every figure on the site is a general estimate intended for educational purposes. For decisions involving your own money — choosing accounts, planning retirement, managing taxes — consult a licensed professional who can review your full situation.

Data sources and accuracy

Rate benchmarks referenced on this site are drawn from widely reported public data, including the Federal Reserve (interest rate environment), the FDIC (deposit insurance, national averages), and long-run S&P 500 total return statistics. Rates change frequently. Results from any compound interest calculator — including this one — are illustrative projections, not guaranteed outcomes.

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