Training zones from your max and resting heart rate.
Free to use, no sign-up required. Heart Rate Zones is one of 198 tools built into Xenition, alongside a chat that can use them for you.
You do not have to open Heart Rate Zones to use it. Ask Xenition in plain language and it runs the right tool, then carries the answer into whatever you are working on — a document, a spreadsheet, a deck.