Calculators that publish their formula, source, and where they break
Every formula, a worked example, and where the answer stops being right.
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Mortgage Calculator (Monthly Payment with Taxes, Insurance and HOA)
Principal & interest = P × r × (1 + r)^n / ((1 + r)^n − 1) · Monthly total = P&I + property tax ÷ 12 + insurance ÷ 12 + HOA
Worked example
A $350,000 mortgage at 6.5% over 30 years (360 months): the monthly rate is 6.5 ÷ 12 = 0.5417%, (1 + r)^360 = 6.99, and the principal-and-interest payment comes to about $2,212. Over the full term that is roughly $796,406 in payments, of which $446,406 — 56% — is interest. That interest exceeds the amount borrowed, which is why the term and extra principal payments matter as much as the rate.
Where this can give the wrong answer
A 0% rate is accepted and falls back to principal ÷ term, the straight-line split — useful for 0% promotional financing, not a division-by-zero bug.
Formula last checked against OpenStax, Contemporary Mathematics §6.8 'The Basics of Loans' and §6.12 'Renting and Homeownership' (level-payment formula); Consumer Financial Protection Bureau, Loan Estimate explainer (consumerfinance.gov) — 'Projected payments' made up of principal and interest, mortgage insurance and estimated escrow for property taxes and homeowner's insurance on 2 October 2026.
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- Mortgage Calculator (Monthly Payment with Taxes, Insurance and HOA)
Monthly mortgage payment — principal and interest, plus property tax, homeowners insurance and HOA — with total interest, the schedule and what extra payments save.
- BMI Calculator: Body Mass Index and Healthy Weight Range
Body Mass Index from height and weight, with the CDC/WHO adult categories, the Asia-Pacific cut-offs and the healthy weight range for your height.
- Percentage Calculator: X% of Y, What Percent, Percent Change
Find X% of a number, what percent one number is of another, the whole from a part, or the percent change — with the working.
- Compound Interest Calculator (Daily, Monthly, Quarterly or Annual)
Future value of a lump sum with compounding annually, quarterly, monthly, daily or continuously — with the APY, doubling time and a year-by-year table.
- Age Calculator: Exact Age in Years, Months and Days
Exact age in years, months and days on any date, with total months, weeks and days and the next birthday.
- Car Loan Calculator (Auto Loan Payment with Down Payment and Trade-In)
Monthly car payment from price, down payment, trade-in, rate and term, with total interest, the full cost of the car and a year-by-year schedule.
- Personal Loan Calculator (Payment, Interest and APR with Fees)
Monthly payment and total interest on a personal loan, and the true APR once an origination fee is taken out of the money you receive.
- EMI Calculator (Loan EMI, Interest and Schedule)
Work out the monthly EMI on any loan, with the total interest, the working behind the formula and a year-by-year repayment schedule.
- Retirement Calculator (How Much Do I Need to Retire?)
How much you need saved at retirement for inflation-rising spending over your retirement years, and the monthly saving that gets you there.
- Credit Card Payoff Calculator (Months to Pay Off or Payment Needed)
How long a credit card balance takes to clear at a fixed payment, or the payment that clears it by a date — with total interest and the 36-month payment.
- Tip Calculator: Tip on the Pre-Tax Bill, Split and Tip Table
Tip, total and each person's share, tipped on the pre-tax bill if you like, with a table from 10% to 25%.
- Sales Tax Calculator (Add Tax or Find the Price Before Tax)
Add sales tax to a price, or work out the price before tax and the tax included in a total, at any state and local rate.
- BMR Calculator (Mifflin-St Jeor and Harris-Benedict)
Basal metabolic rate from weight, height, age and sex, by the two standard formulas, working shown.
- TDEE Calculator (Total Daily Energy Expenditure from BMR)
Total daily energy expenditure from your BMR and activity level, with the working, all five levels side by side and the FAO/WHO activity bands.
- Date Calculator: Days Between Dates, Business Days, Countdown, Week Number
Days between two dates, business days, days until a date, and the ISO and US week number.
- Unit Converter: Length, Weight, Temperature, Area, Speed and More
Length, weight, temperature, area, speed, pressure, power, time and fuel economy, with the factor shown.
- Simple Interest Calculator (Years, Months or Days)
Simple interest and the total on a principal for any number of years, months or days, with the interest per year, month and day.
- ROI Calculator (Return on Investment, with Annualised Rate)
Return on investment as a percentage of what you put in, with the gain, the multiple and the annualised rate if you give the years.
- Simplify Fractions Calculator: Lowest Terms, Mixed Number, Decimal
A fraction in lowest terms with the common factor shown, plus its mixed number, decimal and percentage.
- Body Fat Calculator (US Navy Tape Method)
Body fat percentage from a tape measure by the US Navy equations, with the working in inches, the ACE category, and fat and lean mass.
- Pregnancy Due Date Calculator (LMP, Conception or IVF)
Due date from your last period, conception date or IVF transfer, with how far along you are, the trimesters and the term dates week by week.
- Salary to Hourly Calculator (and Hourly to Salary)
Annual salary to hourly pay, or the other way round.
- CAGR Calculator (Compound Annual Growth Rate, with Steps)
The compound annual growth rate between a starting and ending value, with total growth, the simple average for contrast and the doubling time.
- Home Loan Calculator with Offset Account and Extra Payments
How much interest and time an offset balance and extra monthly payments cut from a home loan, month by month.
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Every calculator is checked against a published source, then spot-checked by hand. Source and date shown on each page.
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