If you've ever used a treadmill with a heart rate display, or a trainer at a gym once told you your "target zone," you've run into the 220-minus-age rule. It's everywhere -- printed on cardio machine panels, repeated in fitness articles, baked into default settings on fitness apps. It's also not the formula exercise physiologists consider most accurate anymore.
Where 220 Minus Age Actually Came From
The formula didn't originate from a large, carefully controlled study of maximum heart rate across the population. It traces back to a 1970s review that pooled results from a handful of older studies and rounded the relationship into a clean, memorable number: 220 minus your age in years. It stuck because it's dead simple to calculate in your head, not because it was ever the best-fitting line through the data.
The problem is that actual maximum heart rate -- the true ceiling your heart can reach under maximal effort -- varies a lot between two people of the same age. A 40-year-old's real max might be 165 or it might be 190. The formula gives you one number regardless, and for a meaningful share of people that number is off by a wide margin.
The Formula That Fits Better
In 2001, researchers Tanaka, Monahan, and Seals published a meta-analysis pulling together data from over 18,000 subjects across dozens of studies and proposed a different equation: 208 minus (0.7 times your age). It produces a noticeably different number than the classic formula, especially the older you are -- for a 50-year-old, 220-minus-age gives 170, while Tanaka's formula gives 173. For a 25-year-old the two are nearly identical. The gap grows with age because the classic formula assumes a steeper, more linear decline than the data actually supports.
Tanaka's version isn't perfect either -- no population-wide formula can be, because individual variation is simply too large. But it's the one most current exercise science literature treats as the better estimate, which is why some heart rate calculators now default to it instead of the older rule.
Resting Heart Rate Changes the Picture Too
Both formulas above only estimate your maximum. To turn that into actual training zones, there are two common approaches. The simpler one just takes straight percentages of your estimated max -- 60% of max, 70% of max, and so on. The more personalized one is the Karvonen method, which factors in your resting heart rate: it calculates your "heart rate reserve" (max minus resting) and adds a percentage of that reserve back onto your resting rate.
The Karvonen method tends to produce higher, more individualized zone numbers for people with a low resting heart rate, which is common in people who've been training consistently. Two 45-year-olds with the same estimated max heart rate can end up with meaningfully different zone 2 ranges if one has a resting heart rate of 55 and the other has 75.
What the Five Zones Are Actually For
Most zone systems split training into five bands: very light (active recovery), light, moderate, hard, and maximum effort. Zone 2 -- the light end -- has gotten a lot of attention recently because it's the range associated with building aerobic base fitness without accumulating the fatigue that harder sessions do, which is why endurance athletes spend so much of their training volume there. Zone 5, at the opposite end, is short, maximal efforts you can only sustain briefly.
The exact bpm boundaries of each zone shift depending on which max heart rate formula and which zone method (straight percentage vs. Karvonen) you use. That's precisely why the same person can get different "zone 2" numbers from different apps -- they're not disagreeing on what zone 2 means, they're starting from different estimates of max heart rate.
Why None of This Is a Diagnosis
Every formula here is a population average applied to an individual, and real max heart rate commonly differs from the estimate by 10 beats per minute or more. If you want your actual max, that typically requires a supervised maximal exercise test, not a formula. These calculators are a reasonable starting point for structuring a workout, not a medical result -- talk to a doctor before pushing into hard training zones, particularly if you have a heart condition or take medication that affects heart rate.
To see your own five zones calculated both ways -- with the classic or Tanaka formula, and with or without your resting heart rate factored in -- try the Heart Rate Zone Calculator.