Every Deresaw calculator uses a published, standard formula, and every assumption it makes can be changed. This page sets them all out in one place, so you can see exactly where a number comes from — and when a lender, employer or fund might get a different one.
Last reviewed: 1 October 2026
The short version
- Results are estimates for planning, not quotes. Real loans, accounts and payslips include fees, taxes and rules a calculator can’t know about.
- Each tool’s page states its own formula. This page explains the conventions they share and the ones where they deliberately differ.
- Everything is calculated in your browser at full precision and rounded only for display.
- We check the calculators against worked examples — the ones at the bottom of this page were re-run on the date above.
1. Loans and amortization
Loan Calculator, Loan Comparison, Mortgage Payoff, Car Loan Payoff, Debt Snowball vs Avalanche
These use the standard fixed-rate, fully amortizing payment formula that banks use for mortgages, car loans and personal loans:
Payment = P × r × (1 + r)ⁿ ÷ ((1 + r)ⁿ − 1)
where P is the amount borrowed, n the number of monthly payments, and r the monthly rate — the annual rate divided by 12. That is how lenders quote a loan’s APR, which is why these tools use it (see section 3 for why savings tools don’t).
- Payments are monthly. Each month, interest is charged on the remaining balance and the rest of the payment reduces it.
- Extra payments go straight to principal in the month they are made.
- Biweekly payments are modelled as 13 monthly payments a year instead of 12 — half a payment every two weeks — spread evenly across the months.
- The Loan Comparison’s effective APR folds fees into the rate. It is the rate at which the present value of all your payments equals the money you actually receive (the loan minus fees), found by numerical search rather than a formula.
- The Debt calculator pays every minimum, puts the extra budget on one target debt, and rolls a cleared debt’s payment into the next target. Avalanche targets the highest rate first; snowball the smallest balance.
Not included: property tax, home insurance, PMI or HOA fees on a mortgage; arrangement fees (except where the Loan Comparison asks for them); prepayment penalties; and lenders that charge interest daily rather than monthly. Lenders also round each payment to the cent and adjust the last one, so a real schedule can differ from ours by a few cents.
2. Savings, interest and investing
Compound Interest, Savings Goal, Investment Fee, Coast FIRE
These treat the annual rate you enter as an effective yearly return — what the balance actually grows by in a year — and convert it to the equivalent monthly rate: (1 + annual rate)^(1/12) − 1. Balances are then stepped forward month by month.
- Compound Interest lets you choose daily, monthly, quarterly or annual compounding. It converts the chosen frequency to its exact monthly equivalent, so a nominal 5% compounded monthly grows 10,000 to 16,470 in ten years, while 5% compounded annually reaches 16,289.
- Savings Goal solves the future-value equation
FV = P(1 + i)ⁿ + c × ((1 + i)ⁿ − 1) ÷ ieither for the number of months or for the monthly deposit. - Investment Fee grows the same money twice: once at the gross return, once at the return minus the expense ratio and any advisor fee. The difference is the fees plus all the growth they would have earned.
- Returns are assumed constant every year. Real markets are not: a bad decade early in retirement matters far more than the average suggests, and no calculator here models that sequence risk.
3. Why the same 6% gives two different monthly rates
This is the one place our calculators deliberately differ, and it trips up a lot of people comparing tools.
| Rate entered | Loan tools | Savings and investing tools |
|---|---|---|
| Treated as | Nominal APR | Effective annual return |
| Monthly rate used | 6% ÷ 12 = 0.5000% | 1.06^(1/12) − 1 = 0.4868% |
| What it really costs or earns in a year | 6.168% | 6.000% |
Both are correct for their purpose. Lenders quote loans as a nominal APR, so a loan calculator must divide by 12 to match your statement. Fund returns and savings APYs are effective yearly figures, so a savings calculator must not — otherwise a “6%” account would appear to earn 6.168%.
4. Inflation and “today’s money”
- The real return is
(1 + return) ÷ (1 + inflation) − 1, not return minus inflation. At 7% and 3% that is 3.883%, not 4%. Over 30 years the difference changes a Coast FIRE target by more than 10,000 on a 1,000,000 goal. - Coast FIRE works entirely in today’s money. Your FIRE number is annual spending ÷ safe withdrawal rate, and the Coast number is that figure discounted back at the real return. The common 4% withdrawal rate is a rule of thumb from historical US market data, not a guarantee.
- Compound Interest can show a balance’s real value: the future amount divided by
(1 + inflation)^years. - Pay Raise reports a real raise with the same exact formula, so a 4% raise during 3% inflation is a 0.97% real gain.
5. Salary and hourly pay
Hourly to Salary, Pay Raise, the salary-to-hourly pages
Annual pay = hourly rate × hours per week × paid weeks per year
- The default is 40 hours × 52 weeks = 2,080 hours, the common US convention and the figure US job ads and salary surveys use. It is not universal: full-time is 38 hours a week in Australia and often 37.5 in the UK. Every calculator lets you change both numbers.
- Overtime defaults to 1.5× where it is paid at all — rules vary by country, state and contract, and the multiplier is editable.
- All pay figures are gross, before income tax, social security or pension deductions.
6. Business calculations
- Markup is profit as a share of cost:
(price − cost) ÷ cost. Margin is profit as a share of price:(price − cost) ÷ price. A 40% markup is a 28.6% margin; a 40% margin on a cost of 60 needs a price of 100. - Invoice Generator totals are: subtotal of quantity × rate, minus the discount (a fixed amount, never more than the subtotal), plus tax charged on the discounted amount.
7. Exchange rates
The Currency Converter uses mid-market reference rates, downloaded once when the page opens:
- Primary source: Frankfurter, which republishes the European Central Bank’s reference rates. The ECB updates them once each working day, around 16:00 CET — there are no new rates at weekends or on ECB holidays.
- Fallback, if that is unavailable: ExchangeRate-API, updated daily.
- The page shows when the rates were last updated.
Banks, card issuers and exchange bureaus add a margin to the mid-market rate and sometimes a fixed fee, so the amount you actually receive will be lower. The amount you type is never sent anywhere — see the privacy policy.
8. Rounding and precision
Calculations run at full double precision and are rounded only when displayed. Some results are shown to the cent, others to the whole unit where cents would be false precision — a 30-year projection is not accurate to the penny. A schedule’s displayed rows can therefore differ from its total by a cent or two.
9. How we test
Each calculator is checked against independently worked examples. These were re-run against the calculators on 1 October 2026 and matched exactly:
| Calculator | Inputs | Result |
|---|---|---|
| Loan Calculator | 200,000 at 6% over 30 years | 1,199.10 a month; 231,676.38 total interest |
| Compound Interest | 10,000 at 5% for 10 years, compounded monthly | 16,470 (16,289 compounded annually) |
| Savings Goal | Reach 10,000 in 24 months from zero at 4% | 401 a month |
| Coast FIRE | 40,000 a year spending, 4% withdrawal, 7% return, 3% inflation, 30 years | FIRE number 1,000,000; Coast number 318,862; real return 3.88% |
| Markup vs Margin | Cost 60, target margin 40%; then a 40% markup instead | Price 100; with the markup, 84 (a 28.6% margin) |
| Hourly to Salary | 25 an hour, 40 hours, 52 weeks | 52,000 a year |
10. What these calculators are not
They are estimates for informational purposes, not financial, tax, legal or investment advice. Actual rates, fees, taxes and regulations vary by lender, employer and country. For a decision that matters — a mortgage, a retirement date, a job offer — confirm the figures with your lender, employer or a qualified adviser.
Found a mistake?
If a result looks wrong, tell us the calculator, the numbers you entered and what you expected. We check every report.