Home & Garden
How Much Sand Do I Need? Pavers, Sandboxes and More
How much sand do I need? Get the formula, the weight of dry and wet sand, and worked examples for paver bases, sandboxes, pool bases and fish tanks.
Tool: Sand Calculator
Guides
Written explanations of the problems our tools solve — why a PDF pastes with a line break on every line, what a JWT actually contains, how sales tax works backwards. Every guide ends at the tool that does the job for you.
8 guides, linked to the 70 free tools that run in your browser.
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Latest guides
Home & Garden
How Much Rock Do I Need? Formula, Charts and Examples
How much rock do I need? Square feet x inches / 324 gives cubic yards, then multiply by tons per yard. Charts, depths, coverage and worked examples.
Tool: Rock Calculator
Home & Garden
How Much Salt to Add to Pool Water: Formula and Charts
Work out how much salt to add to pool water with one formula. Charts by pool size, targets for Hayward, Jandy and Pentair, and how to fix too much.
Tool: Pool Salt Calculator
Home & Garden
How Much Dirt Do I Need? Formula, Chart and Examples
How much dirt do I need? Use square feet x inches / 324 for cubic yards, then add for settling. Charts, weights, bags and worked examples inside.
Tool: Dirt Calculator
Health
Fractional Excretion of Sodium (FENa): Formula and Meaning
The fractional excretion of sodium (FENa) helps tell prerenal kidney injury from tubular damage. Learn the formula, the cut-offs and when it misleads.
Tool: FENa Calculator
Astrology
What Is a Saturn Return? Meaning, Ages and What to Expect
A Saturn return is when Saturn comes back to where it was at your birth, at about 29, 58 and 88. See what it means, when yours is and how long it lasts.
Tool: Saturn Return Calculator
Converters & Units
How to Convert YouTube to MP3 Legally
The routes YouTube allows: your own uploads from YouTube Studio, the Audio Library, and permission from the creator. Then convert the file to MP3 without uploading it.
Tool: YouTube to MP3 Converter
Developer Tools
GPT-6 Astra for Developers: API, Pricing and Benchmarks
GPT-6 Astra has a 1.05M context window and costs $10/$50 per million tokens. It also breaks your integration in four specific ways, and loses to Claude on three coding benchmarks at the same price. What the release notes do not tell you.
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