Why I built another finance calculator
Most online calculators show you the monthly payment but not why you are paying so much in the early years. I went through that wall myself in 2020 — I refinanced a mortgage, looked at the new monthly number, felt clever, and then six months later realized I had paid down almost no principal. The amortization curve was doing exactly what amortization curves do; nobody had bothered to draw it for me.
I built these tools after losing roughly $3,200 on that refinance — not from rates, but from closing costs I waved through because the broker said 'don't worry, you'll make it back in eighteen months.' I never made it back, because I sold the house in fourteen. Every calculator on this site exists because I, or someone close to me, ate a number we did not understand.
This is not a financial advice site. It is a 'show me the math' site. The difference matters: advice tells you what to do, math tells you what is going to happen if you do it. We are firmly in the second camp.
What we calculate vs what we do not
We compute mathematical scenarios. We do not predict markets. We do not recommend products. We will never tell you which broker to use, which coin to buy, or whether you should refinance — those decisions depend on your tax situation, your job stability, your risk tolerance, and a dozen other inputs we do not collect and would not want to.
Our compound interest calculator uses the standard discrete and continuous formulas. There is no patent on math: the formula `A = P(1 + r/n)^(nt)` is in every introductory finance textbook. What we add is a clean amortization view, a visible 'inflation-adjusted' toggle, and a chart that shows you the gap between the nominal and real returns over your time horizon. The default assumptions are ours; the math is the discipline's.
For loans and mortgages, we run the same formulas your bank does (PMT and the standard amortization schedule). If our number does not match your bank's quote, the difference is almost always in: (1) the day-count convention, (2) PMI or insurance escrows that the bank rolls into the payment, or (3) origination fees baked into the principal. We list those line items separately so you can spot the source of the gap.
How we picked our default values
For long-term equity returns, our default is 7% real (after inflation). That number comes from the Damodaran NYU Stern dataset of historical S&P 500 returns, not from a pop-finance blog quote. The full historical nominal average since 1928 is around 9.7%; subtracting roughly 2.7% long-run U.S. inflation gives us the 7% real default. We list both numbers in the tooltip so you can override.
For inflation, our default is 2.5% in the U.S. and 2.0% in Korea, updated quarterly when the U.S. Bureau of Labor Statistics CPI-U release and the Bank of Korea inflation report come out. Past defaults included 2.1% (early 2021) and 4.3% (mid-2022); we do not silently change the value behind your back. The default field shows the value and the date it was last reviewed.
For mortgage rates, we do not bake in a default. The mortgage rate environment moves too fast and varies too much by credit score and loan-to-value ratio for a single number to mean anything. The field starts empty and asks you to paste the rate from your actual quote.
A note on crypto calculators
The crypto tools are deliberately mechanical. The DCA simulator runs your past purchases against historical prices we cache from public price feeds. The staking calculator multiplies your stake by the published validator yield and a slashing-risk discount you can adjust. The mining calculator estimates revenue from current network difficulty and pool fee, minus the electricity cost you enter.
None of these tools predict price. They cannot. We say it on every crypto page because crypto returns have historically had standard deviations several times higher than equities, which means a 'projected' return for crypto is closer to a guess than a forecast. Use them for scenario planning, not for sizing a position.
We also explicitly do not surface a 'recommended' coin or pool. The space changes too fast for our content to stay accurate, and frankly, recommending crypto products to strangers is a category of harm we want no part of.
YMYL disclaimer (in plain English)
This site is for education and self-modeling. The numbers it produces are mathematical outputs based on the assumptions you enter. They are not financial advice, tax advice, or a recommendation to buy or sell anything. We are not registered financial advisors, brokers, or tax professionals.
Tax rules, contribution limits, and rate environments change. Our defaults are reviewed quarterly, but you should always cross-check anything that drives a real-money decision against an authoritative source: the IRS, your country's tax authority, your retirement plan administrator, or — for anything non-trivial — a licensed professional who knows your situation.
If you find a calculator that gives you a result you cannot explain, please tell us. The page footer has a contact link. We have rewritten three calculators in the past year because a careful reader caught an edge case we had missed.