Guides
Practical answers, with a tool inside
Each guide answers one question people actually search for, shows the formula, cites its sources and includes a working tool that runs in your browser. Nothing to sign up for.
Reading
Reading goals, reading speed and audiobook length, with the arithmetic shown.
How many books can you read in a year?
Read 30 minutes a day and you finish about 29 books a year. Use the formula and live calculator to find your number, then set a reading goal you can keep.
Read the guide →How many pages is an audiobook?
A 10-hour audiobook is roughly 310 pages. Convert audiobook hours to pages and back, adjust for narrator speed and page size, and compare with reading time.
Read the guide →Planning
Power and sizing questions for vans, home servers and guitar rigs, with the current worked out.
Percentage change vs difference vs percentage points
Choose the correct percentage formula, work through signed change and symmetric difference, and distinguish relative changes from percentage points.
Read the guide →What size inverter for a camper van?
Size a camper van inverter from the appliances you run together. See what 1000, 2000 and 3000 W can run, the battery current each pulls at 12 V, and cable sizes.
Read the guide →What size UPS for a NAS or home server?
Size a UPS from measured watts, not the VA on the box. Get the VA vs watts rule, 25% headroom, runtime estimates and why a NAS needs minutes of battery, not hours.
Read the guide →How many mA does a pedalboard need?
Add up each pedal's mA by voltage, keep 25% headroom and give digital pedals their own isolated output. A pedalboard power supply sizing guide with a calculator.
Read the guide →Design
Check accessible color contrast for real text and backgrounds
Calculate WCAG text contrast, apply normal and large-text thresholds correctly and test actual backgrounds, focus and interaction states without rounding up.
Read the guide →Resize an image without stretching: aspect ratios explained
Simplify aspect ratios, calculate a missing dimension and choose between fitting, cropping and stretching with practical image and video examples.
Read the guide →Pixels, PPI and print size: calculate before printing
Convert image pixels to inches and centimeters, calculate pixels for a print and distinguish PPI from printer DPI with crop and bleed considerations.
Read the guide →Developer tools
Convert CSV to JSON without losing IDs, quotes or rows
Convert CSV and JSON carefully: preserve leading zeroes, handle quoted commas and multiline fields, validate headers and protect spreadsheet exports.
Read the guide →Unix timestamps: seconds, milliseconds and timezone mistakes
Convert Unix timestamps with explicit units, compare UTC with local time and avoid ambiguous dates, invalid calendar values and thousandfold errors.
Read the guide →URL encoding vs Base64: choose the right representation
Understand percent encoding, UTF-8, plus signs and Base64url with practical examples. Decode once at the right boundary and avoid treating encoding as secrecy.
Read the guide →UUID v4 explained: random IDs, collisions and permissions
Understand UUID v4 structure, its 122 random bits, practical collision reasoning and why a random identifier never replaces authorization checks.
Read the guide →Privacy & security
Check a file’s SHA-256 checksum without uploading it
Hash a local file, compare a trusted SHA-256 digest and understand mismatches, exact bytes, authenticity limits and browser memory constraints.
Read the guide →Generate strong passwords without sending them to a server
Create unique random passwords locally, choose a practical length and alphabet, understand unbiased sampling and store the result in a password manager.
Read the guide →Writing
How to count words and estimate reading time
Count words consistently, handle Unicode and punctuation, and turn a draft length into reading or speaking time with clear assumptions and worked examples.
Read the guide →Preview Markdown safely and export a clean draft
Structure a Markdown draft with headings, lists, links and code. Understand dialect differences, escaped HTML and what to check before exporting a document.
Read the guide →How these guides are written
- The formula is on the page. Every number comes from a stated formula you can redo with a pencil.
- Inputs are cited or labelled. Figures come from a named source (linked at the bottom of each guide), from the documented rules of the free tools on this site, or are marked as typical ranges to replace with your own.
- Calculators stay on your device. They run in your browser. Planning calculators can store non-sensitive settings in a share link; new text and security tools keep input out of the address bar.
- Estimates say they are estimates. Where reality drifts from the arithmetic, the guide says by how much and why.
Looking for something longer? The headless agentic automation guide covers running coding agents without a human at the keyboard. New guides are announced in the RSS feed.