How the calculators are built and checked
Every calculator on Smart Calculator Hub is built and maintained by me, Stuart Mathieson. This page explains where the formulas come from, how results are checked, and what the calculators can and can't do.
Standard, published methods
Each calculator uses a widely published formula or standard rather than an invented one, and its page explains the method. Pages for health and finance calculators also list their sources.
| Calculator | Method |
|---|---|
| Loan Calculator | Standard amortization formula for fixed-rate loans |
| Mortgage Calculator | Standard amortization formula plus property tax, insurance, PMI and HOA fees; PMI ends at a scheduled 78% loan-to-value |
| Auto Loan Calculator | Standard amortization formula, with sales tax, fees, down payment and trade-in |
| Compound Interest | Compound interest formula, with contributions added at the end of each compounding period |
| Savings Goal Calculator | Future value of monthly deposits, solved for the deposit, compounded monthly |
| Retirement Savings | Future value of your savings and monthly contributions, compounded monthly |
| ROI Calculator | Net profit ÷ total cost, annualized as a compound annual growth rate |
| ROI Comparison | The same ROI method applied to each investment |
| Credit Card Payoff | Month-by-month simulation: interest at the annual rate ÷ 12, then your payment |
| Debt Payoff Planner | Month-by-month simulation of the avalanche (highest APR first) and snowball (smallest balance first) methods |
| Inflation Impact | Compound inflation: amount × (1 + rate)years |
| Salary to Hourly Calculator | Annual pay = hourly rate x hours per week x paid weeks per year (2,080 hours for a standard full-time year) |
| GPA Calculator | Credit-weighted average on the US 4.0 scale; honors +0.5, AP/IB +1.0 |
| Final Grade Predictor | Weighted average solved for the score needed on the final exam |
| Weighted Grade Calculator | Weighted average: the sum of score times weight, divided by the total weight |
| Percentage / Grade Converter | Standard US letter-grade scale (A+ from 97%, F below 60%) |
| Study Time Planner | Hours split by priority weights of 3 : 2 : 1 |
| Fraction Calculator | Exact integer arithmetic, simplified by the greatest common divisor |
| Statistics Calculator | Mean, median, mode, range, and variance and standard deviation for a sample (N - 1) and a population (N) |
| BMI Calculator | Weight ÷ height², with WHO and CDC adult categories |
| Calorie & BMR Calculator | Mifflin-St Jeor equation with standard activity multipliers |
| Body Fat Calculator | US Navy circumference equations (Hodgdon and Beckett, 1984) with American Council on Exercise categories |
| Running Pace Calculator | Pace = time / distance; race predictions by Riegel's formula T2 = T1 x (D2 / D1)^1.06 |
| Base64 Encoder/Decoder | Standard Base64 encoding (RFC 4648) of UTF-8 text |
| URL Encoder / Decoder | Percent-encoding of UTF-8 text (RFC 3986) with encodeURIComponent and encodeURI |
| JSON Formatter | Your browser's built-in JSON parser, which follows the JSON standard |
| Password Generator | Characters chosen with your browser's cryptographically secure random number generator |
| UUID Generator | Version 4 UUIDs (RFC 9562) from your browser's cryptographically secure random number generator |
| SHA-256 Hash Generator | SHA-1 and SHA-2 (FIPS 180-4) from your browser's Web Crypto API; text as UTF-8 |
| Time Zone Converter | Your browser's time zone database (IANA), including daylight saving rules |
| Unix Timestamp Converter | Seconds since 1970-01-01 00:00:00 UTC, without leap seconds (POSIX) |
| Word & Character Counter | Words split on spaces, sentences on . ! ?, reading time at 200 words a minute |
| Color Converter & Contrast Checker | sRGB conversions; contrast ratio from WCAG 2.2 relative luminance |
| Tip Calculator | Bill × tip percentage, split evenly |
| Percentage Calculator | Standard percentage, ratio and percent-change formulas |
| Unit Converter | Standard conversion factors, for example 1 inch = 2.54 cm |
| Age Calculator | Calendar arithmetic in complete years, months and days |
| Date Calculator | Calendar arithmetic; weekdays are Monday to Friday |
| Fuel Cost Calculator | Distance / fuel economy x price; 1 US gallon = 3.785411784 L, 1 UK gallon = 4.54609 L |
How results are checked
The worked examples on calculator pages are produced by the calculators themselves, and each one was checked against an independent calculation of the same formula before it was published - so you can check the numbers yourself.
Edge cases are handled explicitly, such as a 0% interest rate, birthdays on 29 February, and times that fall near a daylight saving change.
Private by design
Every calculation runs in your browser. The numbers you enter aren't sent to a server or stored.
After you calculate, Copy link and the share buttons give you a link that opens with the same result. Your inputs are only put into that link - never into the page's own address, so they aren't passed on with it, including to the site's ad provider. Text you type into the password, JSON, Base64 and word count tools is never added to a link.
What the calculators can and can't tell you
Results are estimates for information and education. They aren't financial, medical or other professional advice - for decisions that matter, check them with a qualified professional. The Terms of Service have the full details.
Updates and corrections
Each calculator page shows when it was last updated. If you spot an error in a result or an explanation, please get in touch - corrections are made as soon as possible and the page's date is updated.