A home connection and a phone hotspot don't bill the same way, and macOS shows you neither.
Design, development and release, end to end.
Exact Totals, Estimated Attribution
~1%: Of one CPU core
Scope, Constraints & Reality
Billing cycles differ per connection: a home line that resets on the 1st and a hotspot that resets on the 9th. A single global counter would have been wrong for anyone with both.
Per-app traffic comes from /usr/bin/nettop, which a sandboxed app can't run. That rules out the Mac App Store, so NetMeter ships as a direct download.
macOS only reveals the network name to apps with Location permission. Without it the app still counts everything but can't tell networks apart, so the welcome flow has to explain the ask.
Releases are ad-hoc signed and not notarized, so first launch needs a Control-click Open. The README says so before the download link, not after.
Design Decisions
Exact Totals, Estimated Attribution
Network totals come straight from interface byte counters every 2 seconds, so they are exact. The per-app view is a nettop snapshot every 5 seconds, reconciled to those totals. Which app a closing connection's last seconds belong to is estimated, and the documentation says so plainly instead of implying precision it doesn't have.
Internet And Local Are Different Numbers
An ISP meters internet download only. Traffic that stays on the Mac, such as Docker talking to rclone, or on the home network is counted separately. Switch the view between Internet, Local and All, so the number you compare against your plan is the number your ISP actually bills.
The Menu Bar Item Is The Product
Most people will never open the main window; they glance at the menu bar all day. So that one item is the most configurable part of the app: icon, text or both, full, compact or tiny numbers, optional two-line stacking, and any order of items.
Impact
Light enough to leave running all day. The per-app snapshot costs about 30 ms of CPU every 5 seconds.
Network totals refresh from the interface counters every 2 seconds and are exact. The per-app breakdown is reconciled to those totals.
Usage history lives in one local file on the Mac, about 13 months of it. Export to CSV, or erase it from Settings.