About FlexUtil

FlexUtil is a collection of calculators, converters, and utilities that run entirely in your browser. Every tool is designed to give you a concrete, verifiable answer to a specific question — what your mortgage payment actually is, how much take-home pay you'll net on an Estonian salary, what a Base64 string decodes to — without the usual tradeoffs of online tools: slow page loads, accounts, trackers, or your sensitive inputs sitting on someone else's server.

Who runs this

FlexUtil is a personal project built and maintained by Sergei Selivanov, a software engineer based in Tallinn, Estonia. It started as a small scratch-your-own-itch collection of utilities and grew into a public site once the same tools started being useful to friends, colleagues, and occasional strangers who stumbled on them via search.

Because the site is run by an individual rather than a venture-backed team, the editorial stance is deliberately narrow: I add tools where I have some domain understanding or can do the research properly, and leave gaps where I don't. Every calculator is hand-coded and reviewed against primary sources (tax authority publications, official formulas, language specifications) before it ships.

The mission

To be the first tool you reach for when you need a fast, trustworthy answer to a numeric or technical question — with the guarantee that whatever you typed stays on your machine.

In practice that means three commitments:

• Privacy by default. Sensitive inputs — salaries, mortgage balances, JSON payloads with production data, passwords — are processed locally in your browser. They are not logged, transmitted, or stored on any server I control.
• Depth over breadth. Every tool is accompanied by a substantive explanation of how it works, when it's the right tool, and when it isn't. If a tool doesn't justify a page of context, it probably doesn't deserve to exist.
• Speed. The site is statically generated and loads in under a second on typical broadband. There are no hydration waterfalls, no large JavaScript frameworks on the critical path, and no third-party widgets that block rendering.

What's on the site today

17 tools across finance (VAT, loans, mortgages, fuel, compound interest, mortgage-buyout vs investing), Estonia-specific taxes (salary calculator, motor-vehicle tax), development (JSON, Base64, UUID, regex, password), and everyday utilities (IP address, epoch time, time zones, percentages, BMI). Plus a small and growing library of in-depth guides that tie multiple tools together around a single decision or topic.

New tools and guides are added when they meet two criteria: the author has a real need for them or has researched them properly, and they fit the browser-only, no-data-leaves-your-machine model.

How I make sure the numbers are right

Every calculator is tested against hand-computed reference values and against authoritative sources:

• Tax tools are validated against the Estonian Tax and Customs Board (EMTA) published rates and examples, and updated when the rules change (which in Estonia has happened meaningfully in 2025 and 2026).
• Finance tools are checked against standard formulas (annuity, compound interest, amortization) with cross-checks to bank-published calculators and spreadsheet implementations.
• Development tools use the browser's native APIs where possible (JSON.parse, crypto.getRandomValues, the built-in regex engine) so behaviour matches what your production code will see.

When a formula or tax rate is ambiguous or jurisdiction-specific, I document the assumption explicitly on the tool page and encourage users to verify with a qualified professional before acting on the result. FlexUtil is explicitly not a substitute for financial, legal, or medical advice.

How content gets updated

Pages carry a visible "Last updated" date because stale tax, legal, and API documentation is worse than no documentation. When a tool's formula, a tax rate, or a guide changes, the timestamp moves. Every tool markdown file and every guide has a lastUpdated field that surfaces in the UI, in the page metadata, and in the schema.org structured data emitted for search engines.

Spot a mistake or an outdated number? Drop a note via the contact page. Corrections typically ship within a few days.

The technology

FlexUtil is built with Astro for statically generated pages, React for the interactive tool widgets (using client:only so the server never ships React-specific markup), Tailwind CSS for styling, and Vercel for global CDN hosting. The entire site is open in spirit: there are no opaque binaries or dark-pattern tracking scripts. The only third-party code that loads is Google Analytics (behind consent), the lazy-loaded Google AdSense script, and Google Fonts — all of which are common enough for reviewers to audit.

Business model and editorial independence

FlexUtil is funded by unobtrusive display advertising served via Google AdSense. I do not take sponsorships, affiliate commissions, or editorial payments from banks, tax preparers, or any vendor whose products a calculator might influence a decision about. The content of a tool or a guide is never influenced by any advertiser — both because it would be a conflict of interest and because Google AdSense does not allow advertisers to influence editorial content in any case.

If you use an ad blocker, you'll see the site without ads. No nagging, no paywalls.

Get in touch

Bug reports, tool suggestions, content corrections, press enquiries, or legal notices: see the contact page. For data-subject requests under GDPR (access, rectification, erasure, portability) please reference "GDPR request" in the email subject and allow up to 30 days for a response.