Calculator Guides
Our guides explain the formula, assumptions, and worked examples behind each calculator — so you understand where a number comes from, which mistakes to avoid, and when a result is a planning estimate rather than the final word. Each one links to the calculator it explains.
Created and maintained by Jay Sudha, finance educator · Last reviewed 2 July 2026
Finance Guides
How to Calculate Monthly Loan Payments Before BorrowingThe amortization formula behind a fixed monthly loan payment — worked through a real $15,000 example step by step, including what the number leaves out.How Amortization Works: Principal, Interest, and Loan Balance ExplainedWhy early loan payments barely touch the balance: how a fixed payment splits between interest and principal, and how extra payments cut total interest.Mortgage Payment vs Total Loan Cost: What Borrowers Often MissThe monthly payment is only part of the story. See how principal, interest, and term shape the total cost of a mortgage, with a worked 30-year example.
Investing Guides
How Compound Interest Works With Regular ContributionsCompound interest with monthly deposits: a worked 10-year example, the math behind it, and where the estimates break down.SIP vs Lump Sum Investment Calculation: What the Numbers Actually ShowSIP or lump sum? How each is calculated, why the same money invested differently grows differently, and how to model both with a compounding calculator.How to Estimate Retirement Withdrawals Without Overtrusting One NumberWhy the “years your money lasts” figure shifts so much: how retirement withdrawal estimates are built, plus three levers for stress-testing a plan.
Tax Guides
How to Estimate Take-Home Pay Without Confusing Gross and Net IncomeGross versus net income, how deductions stack up, and a step-by-step worked example for estimating your own take-home pay.VAT, GST, and Sales Tax Calculations Explained With ExamplesHow VAT, GST, and sales tax are added to and removed from a price — worked examples covering net, gross, tax amount, reverse rates, and mixed-rate invoices.Why Tax Calculators Are Estimates and When to Check Official SourcesA tax calculator gives a fast estimate, not your filed tax. Learn what the math captures, what it leaves out, and when to check official IRS guidance.
Budget & Credit Guides
What Is a Good Debt-to-Income Ratio and How to Calculate ItWhat counts as a good debt-to-income ratio, how front-end and back-end DTI are calculated, and a worked example with real monthly numbers you can copy.How to Build a Monthly Budget Using the 50/30/20 RuleThe 50/30/20 rule in practice: splitting take-home pay into needs, wants, and savings, with a worked $5,000 example and exact weekly-to-annual targets.Debt Snowball vs Debt Avalanche: How to Compare Payoff MethodsDebt snowball or avalanche? A worked two-card example showing months to payoff, total interest, and the real trade-off between the two methods.
Business & Ecommerce Guides
How to Calculate Profit Margin, Markup, and Break-Even CorrectlyMargin, markup, and break-even are not the same thing — worked examples show why a 40% markup is only a 28.6% margin, not 40%.Ecommerce Profit: Fees, Shipping, Ads, Returns, and Real MarginWhat an online order really earns once platform fees, shipping, ad spend, and returns are subtracted — with a full worked example per order.Break-Even ROAS Explained for Small Business AdvertisingBreak-even ROAS explained: why it depends on your margin, fees, shipping, and returns, and how to read the number before you scale ad spend.
Math & Statistics Guides
Why a Percentage Loss and an Equal Percentage Gain Don't Cancel OutThe reverse-percentage trap explained: why a 25% drop then a 25% rise doesn't return to the start, worked with real numbers, plus loss recovery and CAGR.Adding, Multiplying and Dividing Polynomials — the Four OperationsHow the four operations on polynomials work, where each goes wrong, and why FOIL stops at two binomials — with the counting checks that catch a dropped term.
Economics Guides
Health Guides
Time & Date Guides
Home & Energy Guides
Guides are educational and explain how each calculator works. Results are planning estimates, not financial, tax, legal, investment, or professional advice. See our methodology and how error reports are handled for how they are built and reviewed.