Guides & explainers
Original, practical articles that teach the math behind the free calculators. Written by Ricardo Rodrigues for students, shoppers, freelancers, and anyone who wants to understand a percentage rather than only receive one.
How to Calculate Percentages
The four everyday percentage formulas — percent of, reverse percent, apply a change, and change from A to B — with worked numbers and the base each one uses.
7 min read · Updated September 26, 2026
Percentage Increase vs Percentage Points
Why a rate moving from 8% to 10% is 2 percentage points and also a 25% relative increase, with polls, grades, and interest-rate examples.
7 min read · Updated September 26, 2026
How to Calculate a Tip
Tip, total, and per-person split with pre-tax versus post-tax bases, a 15–20% table, and what to do when service is already included.
6 min read · Updated September 26, 2026
How to Calculate Discounts
Sale price, amount saved, and successive percent-off deals, including why 20% then 10% is not 30% off and how coupons change the order.
6 min read · Updated September 26, 2026
How Loan EMI Works
How equated monthly installments split interest and principal, with a worked amortization, extra-payment examples, and the limits of a fixed-rate loan calculator.
9 min read · Updated September 26, 2026
How to Calculate VAT
Add VAT to a net price or remove it from a gross price, with European rate examples, a gross-versus-net table, and the mistakes that overstate tax.
7 min read · Updated September 26, 2026
Mental Math for Percentages
Build 5%, 15%, 20%, and awkward tips from a 10% anchor, with restaurant-bill walkthroughs and a clear line for when to stop estimating.
7 min read · Updated September 26, 2026
Common Percentage Mistakes
The base-value mixups, percentage-point confusions, stacked discounts, and reverse-percent errors that make a correct formula give the wrong decision.
7 min read · Updated September 26, 2026
Markup vs Margin Explained
Why a 40% markup is not a 40% margin, with the two formulas, a conversion table, and the pricing mistakes that erase a freelance profit.
8 min read · Updated September 26, 2026
Compound Interest Basics
How compounding frequency and regular contributions change a balance, with a simple-interest contrast and the assumptions a calculator quietly makes.
7 min read · Updated September 26, 2026
Reverse Percentages
Recover the original number when you only know the result after a percent was added or removed, including VAT-inclusive prices and “before the discount” tags.
5 min read · Updated September 26, 2026
Percentage Change from A to B
How to measure growth or decline between two numbers, why the base is the starting value, and how this differs from a gap in percentage points.
5 min read · Updated September 26, 2026
How Stacked Discounts Work
Successive percent-off promotions multiply; they do not add. Worked stacks, fixed coupons, and a check you can do on a receipt.
5 min read · Updated September 26, 2026
Simple vs Compound Interest
Simple interest stays on the original principal. Compound interest pays interest on interest. A side-by-side balance sheet shows when the gap is small and when it takes over.
5 min read · Updated September 26, 2026
Effective Annual Rate
Turn a nominal rate and a compounding frequency into the true one-year growth factor, and see why daily compounding is a small edit to the rate rather than a new product.
5 min read · Updated September 26, 2026
How to Calculate a Test Score Percentage
Turn marks into a percent, weight several assignments, and avoid the average-of-averages trap when one exam counts more than homework.
6 min read · Updated September 26, 2026
How to Calculate a Salary Increase Percentage
Compare a raise with the old salary, separate cost-of-living talk from the cash, and see why a percent raise on a bonus is not the same base.
6 min read · Updated September 26, 2026
Inflation and Percent Change
Separate a nominal price change from a real one, apply an index without pretending it is your personal cost of living, and avoid subtracting large rates carelessly.
5 min read · Updated September 26, 2026
Sales Tax versus VAT
The shared arithmetic of adding a tax rate, and the practical difference between a US-style add-on and a VAT-inclusive price tag.
5 min read · Updated September 26, 2026
APR versus Interest Rate
Why a loan’s nominal interest rate and its APR can differ, what fees do to the comparison, and what a fixed-rate EMI calculator still will not show.
6 min read · Updated September 26, 2026
Percent Error and Percent Difference
How science-lab percent error uses a theoretical value, how percent difference compares two measurements, and why the denominator changes the story.
5 min read · Updated September 26, 2026
Percent of a Percentage
What people mean by “50% of 20%,” when to multiply the decimals, and when they actually wanted percentage points instead.
6 min read · Updated September 26, 2026