About Ratewit
Ratewit is a small, independent site of free money calculators. This page explains who builds it, how every calculator gets its numbers, how they are tested and how to tell us when something is wrong.
Who we are
Ratewit is built and run by Jeep, an independent developer in Bangkok. Jeep writes the calculators, their formulas and the text on every page. No bank, lender or broker pays for the calculators or their results, and Ratewit asks for no email address or sign-up.
Many online calculators hide how they get their numbers or push a product. Each Ratewit calculator shows its formula and a worked example on the same page, so you can check the math yourself. The site is available in English, German and Japanese; the German and Japanese pages are written natively rather than machine-translated, and have not yet been reviewed by a second native speaker.
How we calculate
Every calculator uses a standard, published formula. The list below is generated from the same configuration the calculators run on, so it always shows what they actually do. Totals are calculated from unrounded values and rounded only for display.
Loan calculator
M = P × r ÷ (1 − (1 + r)−n)
P = amount borrowed, r = annual rate ÷ 100 ÷ 12, n = number of months
At 0%: M = P ÷ n
Total paid = M × n, total interest = M × n − P
Based on: the level-payment (annuity) loan formula
Car loan calculator
L = price − down payment − trade-in + sales tax
Sales tax = tax rate × (price − trade-in)
M = (L − B ÷ (1 + r)n) × r ÷ (1 − (1 + r)−n)
B = balloon payment (0 if none), r = annual rate ÷ 100 ÷ 12, n = number of months
At 0%: M = (L − B) ÷ n
Total interest = M × n + B − L, total cost = price + sales tax + total interest
Based on: the level-payment (annuity) loan formula; the balloon loan formula (the balloon is discounted to today and the rest repaid in level payments)
Compound interest calculator
FV = P × (1 + m)n + C × ((1 + m)n − 1) ÷ m
P = initial amount, C = monthly contribution, n = years × 12
Monthly compounding: m = annual rate ÷ 100 ÷ 12
Yearly compounding: m = (1 + annual rate ÷ 100)1/12 − 1
At 0%: FV = P + C × n
Based on: the future value of a lump sum plus monthly contributions paid at the end of each month
How we test
Every time the site is built, a self-test recalculates 402,904 cases. If a single displayed figure is off, the build stops and no page is updated.
- One implementation. The same code produces the result shown before any JavaScript runs and the live result in your browser, and the build checks that the two are identical.
- Hand-checked cases. Known results must appear exactly as the page shows them. For the loan calculator, for example, 10,000 at 7% over 60 months must show 198.01 a month and 1,880.72 in total interest.
- Schedules must add up. Hundreds of loan and car loan schedules, from the shortest to the longest allowed term and from 0% to the highest allowed rate, must end at exactly 0.00, or exactly at the balloon payment, and repay exactly the amount borrowed.
- Savings match the textbook formula. Hundreds of compound interest runs, with monthly and yearly compounding, must match the standard future-value formula, and every year's paid-in amount plus interest must equal its balance.
- No quoted market rates. Ratewit does not publish current interest rates or offers. Enter the figures from your own offer or contract.
Every calculator page shows who wrote it and when its formula, self-tests and text were last reviewed: currently 25 September 2026.
Corrections
Spotted a mistake, a confusing explanation or a number that looks wrong? Email hello@ratewit.com with the page and the numbers you entered. Confirmed errors are fixed and, where possible, added to the self-test so they cannot come back.
Not financial advice
Results are estimates for planning and comparison, not financial advice or a loan offer. Real interest rates, fees, insurance and taxes differ between lenders and change over time, so your own figures may differ. Before you sign anything, ask your bank, lender or a qualified adviser. See the terms of use.