Auto Convert

The idea

Recipes in Fahrenheit, product specs in inches and webinar times in EST all mean a trip to a converter. Auto Convert removes that step: it reads the text of every page you visit and adds the equivalent value in your preferred system, currency and time zone right where the original appears. I started it in July 2025 as a vanilla JavaScript extension and rebuilt it in 2026 on WXT, Svelte 5 and TypeScript, adding a Pro tier licensed through my own platform, ScriptIQ.

Implementation

• Manifest V3 extension built with WXT and TypeScript; the settings popup is a Svelte 5 app with light and dark themes.
• A custom detection engine: a single-pass sticky-regex tokenizer feeds an 11-pass parser that handles compound (5 ft 10 in), fractional, hyphenated and dimensional (4.5 × 2.5 × 1.5 in) values, times, time zones and currencies.
• A context analyser scores nearby keywords to tell weight 'pounds' from currency and temperature 'degrees' from angles, and leaves unclear cases unconverted.
• The content script walks text nodes with a TreeWalker, follows dynamic pages with a MutationObserver and works in 12 ms idle-time slices; every conversion is a reversible span.
• DST-aware time-zone conversion through IANA zones and the Intl API, with day-change markers.
• A background service worker owns exchange rates (24-hour cache with an alarm refresh) and license checks, and settings reach every tab through storage change events.
• Integrates the ScriptIQ Connect SDK: browser approval with a polling handshake, and ES256 license tokens verified offline with WebCrypto.

Challenges

• Separating real measurements from ordinary English such as '3 in a row', '1st', '5G', '12 pt' or 'John 3:16'.
• Resolving ambiguous words like 'pounds' and 'degrees' from their context instead of guessing.
• Staying fast on large, dynamic pages without touching inputs or editable content.
• Keeping a five-minute sign-in poll alive within MV3 service-worker lifetimes, and preserving Pro settings when a license lapses.

Result

Version 2.0.0 converts seven measurement categories plus 12/24-hour time for free, and a ScriptIQ license unlocks conversion into 163 currencies, DST-aware time zones and per-site exclusions. A test page with more than 300 cases covers both conversion directions and known ambiguity traps. The extension builds to a production Chrome package but hasn't been published to the Chrome Web Store yet.

Gallery

Light themeThe popup in the light theme, over a forecast with temperatures and wind speeds converted in place.

Live conversionsForecast highs and lows and wind speeds converted inline on forecast.weather.gov.

Products, recipes and travelDimensions, weights, volumes, temperatures, distances and times converted on the extension's test page.

Settings popupPreferred system, time format, display mode and highlight colour, plus the Pro features a ScriptIQ connection unlocks; shown in the dark theme.

Hover modeOnly the converted value is shown, and hovering reveals the original.

Metric to imperialThe same engine converts the other way when imperial is the preferred system.

Version history

v2.0.0Live now

v1.0 (Auto Unit Converter)Jul 2025

The original vanilla JavaScript extension with regex-based detection, released under the name Auto Unit Converter. A premium login against a custom backend followed in August 2025 and was later shelved in favour of ScriptIQ.

• Metric and imperial conversion in seven categories
• 12/24-hour time and three ways to show the original value
• Site whitelist and currency conversion through an exchange-rate API

Stack at the time

Languages

JavaScriptHTMLCSS

Supported Conversions

• Length, area and volume (US definitions)
• Weight and temperature
• Speed and fuel economy
• 12- and 24-hour time
• 163 currencies (Pro)
• Time zones: 51 abbreviations plus UTC/GMT offsets (Pro)

Free and Pro

The free version covers unit and time-format conversion, three ways to show the original value, a highlight colour and dark mode. A ScriptIQ license adds currency and time-zone conversion and excluded sites.