Guides: How the Calculations Work
Each guide explains the method behind one or more of our tools, step by step, with worked examples you can check in the calculators. They are written for people making real decisions — and they say clearly where a formula stops being reliable.
Money
- How Mortgage Payments Are Calculated: Principal, Interest, Tax and Insurance
The amortisation formula behind every fixed-rate mortgage, a full worked example, why early payments are mostly interest, and what PMI and escrow add.
5 min read
- Fixed vs Adjustable-Rate Mortgages: How ARMs Work and When They Make Sense
How an ARM’s fixed period, index, margin and caps work, a side-by-side example against a fixed loan, and the questions to ask before choosing one.
5 min read
- How Much House Can I Afford? The 28/36 Rule With a Worked Example
Turn income, debts and a down payment into a realistic price range using the 28/36 rule, then sanity-check it against cash and lifestyle costs.
4 min read
- Compound Interest vs Simple Interest: Examples, APY and the Rule of 72
What compounding really adds, how frequency and APY compare, why the Rule of 72 works, and how the same maths makes credit-card debt so expensive.
4 min read
- US Sales Tax Explained: State and Local Rates, Exemptions and Online Purchases
Why your sales tax rate depends on your address, which states have none, how to back tax out of a total, and what changed for online shopping in 2018.
5 min read
- How Stacked Discounts Really Work: 30% + 20% Is Not 50%
Why successive discounts multiply, how coupon order changes the price, what “buy one get one” is really worth, and how to spot a discount that is not a deal.
4 min read
- Exchange Rates Explained: Mid-Market Rates, Card Fees and What You Actually Pay
Where reference exchange rates come from, why your bank’s rate is different, how dynamic currency conversion costs you, and how to compare transfers.
4 min read
Time & dates
- Daylight Saving Time Around the World: Who Changes Clocks, and When
The 2026 clock-change dates for the US, Europe, Australia and New Zealand, places that never change, and why time differences shift for a few weeks a year.
4 min read
- Scheduling Meetings Across Time Zones: Overlaps, Rotations and Invites That Work
How to find working-hour overlap between cities, what to do when there is none, and how to write invitations that nobody misreads.
4 min read
Developers
- JSON vs YAML vs TOML: Choosing a Config Format (and Converting Safely)
What each format is good at, the type-conversion surprises that bite during conversion, and a practical rule for which one to use where.
4 min read
- Cron Syntax Explained: 20 Schedules You Can Copy, and the Traps to Avoid
The five cron fields, copy-ready schedules for common jobs, and the time-zone, DST and day-of-week traps across Linux, GitHub Actions and Kubernetes.
4 min read
- Regex Cheat Sheet With Tested Examples (JavaScript Flavour)
Character classes, quantifiers, groups, lookarounds and flags, with patterns for dates, prices, colours and URLs — every example tested in a JavaScript engine.
4 min read
- Image Formats for the Web: JPEG vs PNG vs WebP vs AVIF vs SVG
Which format to use for photos, screenshots, logos and transparency, how to size images for high-density screens, and the HTML that loads them fast.
4 min read
Security
- Password Strength Explained: Entropy, Length and Passphrases
What “strong” means in bits, why length beats symbols, how long passwords hold up against offline cracking, and a practical setup with a password manager.
4 min read
Health
- BMI, TDEE and Macros Explained: What the Numbers Mean and Their Limits
How BMI, basal metabolic rate, total daily energy expenditure and macronutrient targets are calculated, with one worked example and the limits of each number.
4 min read