Calculator Guides & Explainers

The reasoning behind the numbers — written to be understood, not to rank. Each guide links to the calculators that do the arithmetic.

Linux
Linux Is Out of Disk Space: A Systematic Way to Find Out Why
Why df and du disagree, how deleted-but-open files consume space invisibly, and the order to check things in.
7 min readRead
Cloud
How to Size Cloud Instances Without Overpaying
A practical method for right-sizing compute: measure before you buy, understand the pricing models, and know which metrics actually matter.
8 min readRead
Math
How to Read Percentage Change Without Getting Fooled
Percent versus percentage point, why a 50% fall needs a 100% rise to recover, and how to reverse a percentage correctly.
6 min readRead
Financial
Compound Interest Explained: Why Starting Early Beats Saving More
How compounding actually works, why frequency matters less than you think, and the arithmetic behind starting ten years earlier.
8 min readRead
Fitness & Health
BMI Limitations: What It Misses and What to Measure Instead
Why BMI misclassifies athletes and older adults, how ethnicity shifts the thresholds, and which body measurements predict health risk better.
7 min readRead
Networking
What Is CIDR Notation, and Why Did It Replace Network Classes?
CIDR explained from first principles: what the slash number means, why classes A/B/C were abandoned, and how to read any prefix at a glance.
7 min readRead
Financial
Understanding Mortgage Amortisation: Where Your Payment Actually Goes
Why early mortgage payments are almost all interest, how overpaying works, and how to read an amortisation schedule properly.
9 min readRead
Networking
How to Calculate a Subnet Mask (By Hand and Quickly)
Work out subnet masks, network and broadcast addresses and host counts by hand — with the binary shortcuts network engineers actually use.
8 min readRead