Guides that explain the maths
Each guide answers one question people often get wrong, works through real numbers, cites official sources and links to the calculator that does the sums for you.
- Interest rates · 10 min readAPR, AER, APY and nominal rates: what interest rates really meanWhy two “5%” rates can cost or pay different amounts, and how to convert between them.
Calculators: Compound interest, Loan, Mortgage, Savings goal
- Borrowing · 10 min readDebt avalanche vs debt snowball: how each method works and what it costsHighest rate first or smallest balance first, worked through month by month.
Calculators: Loan, Percentage, Compound interest
- Mortgages · 11 min readOverpay the mortgage or save the money? How to compare the twoInterest saved versus interest earned, after tax, with the catches that matter.
Calculators: Mortgage, Compound interest, Savings goal
- Inflation · 11 min readHow inflation is measured: consumer price indexes explainedThe basket, the weights, the index and the headline rate, country by country.
Calculators: Inflation, Currency, Compound interest
- Pay & tax · 12 min readGross vs net pay: what comes out of your salaryIncome tax, social contributions and pre-tax savings, country by country, with real numbers.
Calculators: Salary ↔ hourly, Tax brackets, Savings goal
- Pay & tax · 11 min readMarginal vs average tax rate: what a tax bracket really meansWhy moving up a bracket only taxes the extra, and where the real marginal rate jumps.
Calculators: Tax brackets, Percentage, Salary ↔ hourly
- Pay & tax · 12 min readYour US paycheck explained: federal tax, FICA and state withholdingEvery line on a US pay stub, and why withholding and the tax you owe can differ.
Calculators: Salary ↔ hourly, Tax brackets, Percentage
- Pay & tax · 10 min readConverting a salary to an hourly rate (and back again)Hours, paid leave, overtime and tax: what makes two pay offers comparable.
Calculators: Salary ↔ hourly, Percentage, Tax brackets
- Everyday maths · 11 min readHow VAT and sales tax work: adding and removing tax correctlyThe formulas for adding and removing VAT, GST or sales tax, and the common mistakes.
Calculators: VAT / GST, Percentage, Currency
- Everyday maths · 10 min readPercentages that trip people up: change, points, margin and markupPercentage change, percentage points, reversing a discount, and margin versus markup.
Calculators: Percentage, VAT / GST, Tax brackets
How we write guides
Guides are written by the MoneyOtter team and checked against official publications: central banks, financial regulators, statistics offices and tax authorities. Every worked example is recalculated with the same engine the calculators use, and the sources are listed at the end of each guide. Guides explain how things work; they don’t tell you what to do with your money. If something looks out of date, the contact page explains how to tell us, and corrections are dated in the changelog.