Estimate intensity using the talk test.
The talk test uses speech as a free proxy for ventilatory threshold. While you can hold a full conversation, ventilation is still coupled to metabolic demand and the effort is aerobic. The point where speech breaks into short sentences corresponds closely to the first ventilatory threshold in laboratory testing, usually near seventy to seventy-five percent of maximum heart rate, and losing speech altogether marks the second threshold.
Speech to intensity mapping
Full conversation about 50%, full sentences 62%, short sentences 74%, few words 85%, no speech 94% of maximum heart rate
Matching heart rate band
Heart rate band = (220 - age) x (intensity percent +/- 5) / 100
Educational estimate only. Not medical advice — discuss results with a qualified clinician before changing treatment, medication or exercise.
It needs no equipment, is unaffected by heat, caffeine or cardiac drift, and tracks ventilatory threshold directly rather than through an age formula that can be ten beats off.
Common endurance practice keeps the large majority of volume in the range where full sentences are still possible, reserving the short-sentence and no-speech zones for a small share of weekly time.