What Is a URL Slug? A Practical Guide to Clean Page URLs
Learn what a slug is, how to make one readable, and why changing a published URL requires care.
Read guideHelpful guides
The Probably Useful Guides explain the calculations, formats, web tasks, and everyday decisions behind our tools. They are written to help you understand why a result works—not just copy an answer—using practical examples, formulas, common mistakes, and limitations where they matter.
Browse the latest guides below, then jump to the related calculator, generator, or converter when you are ready to apply what you learned. Topics currently include business pricing, percentages, Canadian sales tax, campaign tracking, email authentication, redirects, URLs, JSON, encoding, passwords, and other practical web tasks.
How to Read, Format and Fix JSON
Learn the structure of JSON, the syntax errors that break parsing, and a practical process for debugging malformed data.
Read guidePayment Processing Fees: Percentage + Fixed Fee
Understand percentage-plus-fixed transaction fees, effective fee rates, and how to work backwards from a target payout.
Read guideGST, HST, PST & QST in Canada: Sales Tax Guide
Understand Canada’s main sales taxes, how combined rates differ by province, and how to estimate tax before or after it is included.
Read guideWhat Is a UUID and When Should You Use One?
Learn why UUIDs are useful for distributed identifiers, the trade-offs versus sequential IDs, and why a UUID is not an access-control mechanism.
Read guideWhat Is a Unix Timestamp?
Learn how Unix time represents an instant, why seconds and milliseconds get confused, and how time zones affect display.
Read guideURL Encoding Explained: Percent-Encoding Without the Jargon
Learn why URL characters become %20 and similar sequences, what should be encoded, and how double encoding happens.
Read guideHow to Calculate a Percentage Discount
Calculate the discount amount, final price, stacked discounts, and original price from a known sale price.
Read guideHow to Calculate Markup and Set a Selling Price
Use markup to turn cost into a selling price, then check the resulting dollar profit and margin.
Read guideProfit Margin vs Markup: The Difference That Trips People Up
See the formulas for margin and markup, compare them with the same numbers, and learn how to convert a target margin into a selling price.
Read guideHow Word Counts and Reading Time Estimates Work
Learn why word and character counts can differ between tools and how reading-time estimates are calculated.
Read guide301 vs 302 Redirects: What to Use and Why
Learn when a URL move should be permanent or temporary, how redirect chains happen, and what to test during a site migration.
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