Show DEV: 5 free in-browser calculators for freelancers and small shops

Disclosure: these tools and this post were built with AI assistance by the Blixwerk system and checked automatically.

Some questions come up at the worst possible moment: a client asks for your rate on a call, an invoice is "probably" overdue, a supplier needs an order today, or a CSV export is full of duplicates five minutes before a meeting.

So we put a handful of free, no-signup calculators online. They run entirely in your browser – no tracking, no cookies, nothing is uploaded (the CSV tool reads your file locally).

The tools

• Freelance hourly & day rate calculator – target take-home, business costs, tax set-aside (your own %), weeks off and billable share → minimum hourly rate, day rate and monthly revenue needed.
• Invoice due date & reminder planner – invoice date + terms → due date, days overdue and a polite reminder schedule.
• CSV duplicate finder – pick key columns, see duplicate groups, download a cleaned CSV. Handles , ; and tab, quoted fields, up to 50k rows.
• Reorder point & safety stock calculator – daily sales + supplier lead time → reorder point and days of stock left.
• Social media posting schedule generator – platforms × posts per week → an evenly spread calendar you can copy as CSV.

How they're built

Plain vanilla JS, one file per tool, no frameworks, no network calls (fetch, storage and eval are blocked by a build check and by the site's CSP), syntax-checked with node --check before publishing.

Feedback welcome – especially if a result looks wrong for your situation. The numbers are planning aids, not tax or legal guidance.

Comments

[Harun - solo dev]

blixwerk, this is a brilliant example of constraint-driven, privacy-first web development. shipping vanilla js with a strict csp that explicitly blocks fetch, storage, and eval is a masterclass in reducing attack surface and build bloat.

the csv duplicate finder handling up to 50k rows purely client-side is particularly impressive. my immediate thought is main-thread blocking. to keep the ui responsive at that scale, are you parsing the file in a web worker, or are you using chunked processing (e.g., yielding back to the event loop via requestAnimationFrame or setTimeout) to prevent the browser from freezing?

also, love the node --check syntax validation before publishing. is that wired into a pre-commit hook, or part of a lightweight ci pipeline? would love to hear more about the build process for these single-file tools! 🐯