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Guides

Every calculator and converter on this site answers a question, but the answer is only useful if you understand it. These guides explain the math and the reasoning behind the tools — in plain English, with worked examples and the edge cases that actually matter. Read one before you reach for the tool, or after, when you want to know why the number came out the way it did.

They are grouped loosely by topic — health, finance, math, developer utilities and more — and each links back to the tool it explains.

⚖️Health

Understanding Your BMI: What the Number Really Means

BMI is a population screen, not a personal verdict. Here's how the number is built, how to read the categories, and where it quietly fails.

6 min read
🔥Health

BMR, TDEE, and How Many Calories You Actually Need

Two numbers drive every diet plan. Learn how BMR and TDEE are estimated, how to turn them into a goal, and why they're only a starting point.

7 min read
🏠Finance

How Mortgage Amortization Works

Why early payments are almost all interest, when principal finally takes over, and how a little extra each month can save you years.

7 min read
📈Finance

Simple vs Compound Interest, Explained

One clause separates a straight line from a curve that accelerates. See how compounding builds fortunes — and drains them on a credit card.

7 min read
💹Finance

How to Calculate ROI (and Read It Correctly)

The basic formula is easy. The traps — ignoring time and risk — are what make a great-looking return misleading. Here's how to use ROI honestly.

6 min read
📊Math

Working With Percentages: Increase, Decrease, and Reverse

Every percentage problem is the same equation rearranged. Master the three core questions — and the reversal trap that costs people money.

6 min read
🌈Developer

HEX, RGB, HSL and CMYK: Color Models Explained

Why one color has so many names, what each model is good at, and why a shade that glows on screen can look muddy in print.

6 min read
🔐Developer

What Base64 Is and When You Actually Need It

Base64 is encoding, not encryption — it offers zero security. Here's the real problem it solves, how the conversion works, and when to avoid it.

6 min read
🖼️Photography

Aspect Ratios for Photo, Video and Screens

Ratio is shape, not size. Learn how it's calculated, the ratios you'll actually meet, and how to crop or resize without distorting anything.

6 min read
🔑Security

How to Build Strong Passwords You Can Actually Use

The old 'add a symbol' advice is outdated. Length beats complexity — here's what really makes a password strong and a system that works.

6 min read
📐Construction

Measuring Area: Square Footage for Flooring, Paint and Concrete

Get the area right and add the correct waste allowance. The unit traps, awkward shapes, and why depth turns area into volume.

6 min read