Estimating your maximum heart rate
Heart rate zones are percentages of your maximum heart rate, so everything starts with an estimate of that maximum. The formula most people know is 220 minus age: at 35 that is 185 bpm. It is simple and it is what the American Heart Association uses in its guidance, but it was never derived from careful research and it tends to overestimate the maximum in young adults and underestimate it in older ones. The Tanaka formula, 208 - 0.7 x age, came from a 2001 meta-analysis of 351 studies and fits the population better; it gives 183.5 bpm at 35 and 173 at 50, versus 170 from the classic formula. Either way, the spread between individuals is about plus or minus 10-12 bpm, so if you have measured your true maximum in a hard workout or a lab test, enter it and the calculator will use it instead.
Percentage of maximum vs. the Karvonen method
The straightforward approach multiplies the maximum by a percentage: 70% of 185 is 130 bpm. The Karvonen method instead works with your heart rate reserve, the gap between resting and maximum heart rate, and adds the resting rate back: target = (max - resting) x percent + resting. For a 35-year-old with a resting heart rate of 60, the reserve is 125 bpm, and 70% is 125 x 0.7 + 60 = 148 bpm. Karvonen zones are higher at every percentage, and many coaches consider them a better match for perceived effort because they account for fitness (a lower resting heart rate means a bigger reserve). Measure your resting pulse first thing in the morning before getting up for the most useful number.
What each zone is for
Zone 1 (50-60%) is very light effort, for warm-ups, cool-downs and active recovery. Zone 2 (60-70%) is the conversational pace where the body burns the highest proportion of fat for fuel and builds aerobic base; endurance athletes spend most of their training here. Zone 3 (70-80%) is steady, moderately hard cardio that improves aerobic capacity. Zone 4 (80-90%) is tempo and threshold work: you can speak only a few words, and it raises the intensity you can hold for a long time. Zone 5 (90-100%) is the all-out effort of short intervals and sprints, sustainable for a minute or two at most. For a 35-year-old using 220 - age the five zones run 93-111, 111-130, 130-148, 148-167 and 167-185 bpm.
The AHA frames its recommendations more simply: moderate-intensity exercise at 50-70% of maximum (93-130 bpm at age 35) for at least 150 minutes a week, or vigorous exercise at 70-85% (130-157 bpm) for 75 minutes, or a mix. Beginners should start at the low end of the moderate range and work up over several weeks. Beta blockers, some blood pressure and thyroid medications, heat, dehydration, caffeine and illness all shift heart rate, and chest-strap monitors are considerably more accurate than wrist sensors during hard efforts. Anyone with a heart condition or who is starting exercise after a long break should get personal targets from a physician rather than a formula.