Calculators that show their work.
Free, fast calculators for people who sell and build — Etsy and Amazon fees, roof pitch, deck materials. Every answer comes with the arithmetic behind it, so you can check it, trust it and explain it.
- No sign-up
- Runs in your browser
- Fee schedules dated & sourced
- Free to embed
For online sellers
For contractors and DIY builders
Roof Pitch Calculator
Pitch from rise and run (or angle) plus degrees, slope, rafter length, squares and shingle bundles.
atan(6 ÷ 12) = 26.57°
Deck Material Calculator
Boards, joists, hangers and screws for a rectangular deck — cut-aware, with the least-waste board length.
⌈144″ ÷ 5.6875″⌉ = 26 rows
For developers and AI agents
Time Zone Converter
Convert one moment into up to 20 zones: DST gaps and repeats flagged, next clock changes, working hours, UTC math.
UTC = local − offset
Cron Expression Generator & Explainer
Paste a cron expression for plain English, a field-by-field breakdown and next runs in any time zone, or build one.
*/15 9-17 * * 1-5
JSON Schema Validator
Check JSON against a JSON Schema (2020-12, 2019-09, 07, 04): every error as a JSON Pointer, most specific first.
valid(instance, schema)
JSON Schema Generator
Paste sample JSON and get a draft 2020-12 schema with types, required keys and formats, checked against your sample.
{ "a": 1 } → "integer"
YAML to JSON Converter
Convert YAML to JSON and back in your browser, with line-numbered warnings where YAML 1.1 and 1.2 disagree.
country: NO → false
Robots.txt Tester
Paste a robots.txt and test any URL: verdict, deciding rule and line, plus ten search and AI crawlers at once.
Allow: /a/b > Disallow: /a
Guides: how the numbers work
How Etsy Fees Work in 2026
Listing, transaction, processing, Offsite Ads, Share & Save and the fees that only apply outside the US — what each one is charged on, and three orders worked through to the cent.
How to Price Etsy Products for Profit
Start from what an item really costs you, decide what you need to keep, and work back to a list price that survives Etsy's fees — including orders that arrive through Offsite Ads.
Amazon FBA Fees Explained (2026)
Size tier, shipping weight, price band, season and surcharge: the five things that set an FBA fulfillment fee, plus referral and storage fees, worked through on real boxes.
How to Calculate Amazon FBA Profit Margin and ROI
Turn Amazon's fees into the numbers that decide whether a product is worth stocking — profit per unit, margin and return on investment — and see what moves them.
How to Calculate Roof Pitch, Rafter Length and Shingles
From a level and a tape measure to a shingle order: how pitch, the pitch multiplier, rafter line length and roofing squares fit together, with three roofs worked through.
How to Estimate Deck Materials
A materials takeoff you can check by hand: rows of decking, cut-aware board counts, joists and hangers, fasteners, and the code span limits that decide joist size.
Why 17x7
The math is on the page
Open "Show the math" under any result and you get each step with your numbers substituted. No black boxes, no "estimate may vary" shrug.
Calculations stay in your browser
Calculations run locally in JavaScript. There are no accounts, no forms to fill in before you see a result, and no tracking cookies.
Sources with dates
Fee tables, code limits and rules of thumb cite where they come from and when we last checked. When a marketplace changes its fees, the page says so.