Guides: the maths behind the calculators

A calculator gives you a number; these guides explain where it comes from, how to check it, and the mistakes that most often lead people astray. Each one includes worked examples you can reproduce by hand.

Finance · 7 min read Compound Interest and the Rule of 72, Checked Against the Math Simple versus compound interest, the formula, compounding frequency, how accurate the Rule of 72 is, and why starting early can beat saving more later. Pet · 7 min read The Dog Years Myth, and How Much Your Dog Should Eat Why one dog year does not equal seven human years, how size and life stage change a dog's needs, how to read feeding guides, and which foods are dangerous. Construction · 8 min read How Much Paint, Flooring and Concrete Do You Actually Need? Estimate paint, flooring, drywall, concrete, gravel and mulch with simple area and volume formulas, waste allowances, and worked room and slab examples. Finance · 7 min read How Big Should Your Emergency Fund Be? A Practical Way to Size It Work out an emergency fund target from your real essential costs, adjust for income stability and dependants, and build it in stages. General information. Finance · 7 min read How EMI Is Calculated, With a Full Worked Loan Example See exactly how a loan EMI is calculated, with the formula, a step-by-step example, tenure and rate comparisons, flat vs reducing rate, and prepayment. Tax · 7 min read Marginal vs Effective Tax Rate: Why a Raise Never Costs You How progressive tax brackets really work, with a step-by-step illustrative example, marginal versus effective rates, and deductions versus credits. Business · 7 min read Percentage Traps: Markup vs Margin, Stacked Discounts and More Why +50% then -50% loses money, how markup and margin differ, why two 20% discounts are not 40% off, and how to handle sales tax in or out of a price. Health · 7 min read What BMI Tells You, and What It Quietly Misses BMI is a quick screening number, not a diagnosis. Learn the formula, the WHO categories, a worked example, where BMI misleads, and what to measure next.