Calculator Guides
How the calculations behind the calculators work, with worked examples and the official sources they follow.
How to Calculate Percentages
The five percentage calculations, percentage change, discounts and the original price, and percentage points.
Read guide →How to Calculate VAT
Add VAT or take it out of a gross price, the VAT fraction, rounding on invoices and VAT rates in 16 countries.
Read guide →Loan Repayment Methods Compared
Equal payments, equal principal, interest-only and flat-rate loans compared, with the payment formula and APR.
Read guide →Compound Interest Explained
The compound interest formula, compounding frequency and APY, monthly contributions and the rule of 72.
Read guide →BMI Categories by Country
WHO and CDC classes, the lower NICE thresholds, and the standards of Korea, Japan, China, Indonesia and Thailand.
Read guide →BMR and Daily Calorie Needs
Mifflin-St Jeor, Harris-Benedict and FAO/WHO/UNU equations, activity levels, and Japan's and Korea's 2025 standards.
Read guide →How to Calculate Age Exactly
Exact age in years, months and days, 29 February birthdays and the legal age rules of Korea and Japan.
Read guide →Korean Age Explained
International age, year age and counting age, the 2023 change, 29 February birthdays and lunar birthdays.
Read guide →Use the Calculators
The guides explain the rules; the calculators do the arithmetic in your browser.
Frequently Asked Questions
Where do the rates and standards come from?
From official sources: tax authorities and legal texts for VAT, health authorities, international reports and published studies for BMI and calories, and laws for age rules. Each one shows its source and the date it was last checked.
Are the amounts I enter stored?
No. The calculators run in your browser, and what you enter is not sent to LeedyBox or saved.