Two notes, thirty seconds. Sing your lowest comfortable note on "ahh" (not a growl — a note), then your highest (falsetto counts, say so-ish). Quiet room, mic 30 cm away.
The same detection engine as the pitch detector (±1 cent on a held tone) watches for a pitch you can hold — a dozen confident readings inside a narrow band. A squeak on the way up doesn't count; a note you can sit on does. That's also the honest reason your result may be a semitone shy of your party trick: the range that counts is the range you can hold.
Warmed up ≠ cold: most people gain 2–4 semitones after ten minutes of singing. Test cold for your floor, warmed for your ceiling, and try the pitch match trainer to tighten what's in between.