https://redgridtactical.github.io/RedGridMGRS/tools.html

Came across a few online MGRS, lat/lon, UTM, and DMS converters while doing some research for my app. They were covered in ads and none of them supported batch conversions or bearing calculations. I put one together as a single self-contained HTML file.

It auto-detects whatever format you paste in and converts to everything else simultaneously. Also has a bearing/distance calculator, dead reckoning, and batch mode for converting lists. Includes FixPhrase (the open-source, patent-free What3Words alternative) that turns any coordinate into a 4-word phrase.

The engine uses DMA TM 8358.1 (truncation, not rounding. The popular npm mgrs package gets this wrong). Everything runs client-side, no tracking, works offline after first load. ~88KB total. Link is to the repo if anyone wants to poke around.