Starting a cardio routine and not sure how hard you should actually be pushing? The number that matters is not your speed or the calories your watch claims you burned, it is your target heart rate zone. Here is how to calculate it in under a minute, and why the formula most fitness apps still use is not as accurate as people assume.
What a target heart rate zone actually means
Your heart rate is simply how many times your heart beats per minute (bpm). At rest, a healthy adult typically sits somewhere between 60 and 100 bpm, though some research suggests 50 to 90 bpm is a more realistic normal range. Trained athletes often rest much lower, between 40 and 60 bpm, which is a sign of good cardiovascular fitness rather than a problem.
A target heart rate zone is the range of bpm you should aim for during cardio exercise to train your heart effectively without overexerting yourself. The classic cardio training zone sits between 65 percent and 85 percent of your maximum heart rate (HRmax), the highest number of beats per minute your heart can sustain during intense effort.
The formula behind the number
The most widely used formula to estimate HRmax, popularized by Haskell and Fox back in 1970, is simple:
HRmax = 220 minus your age
Once you have HRmax, your target training zone is just two multiplications:
Low end of the zone = HRmax x 0.65
High end of the zone = HRmax x 0.85
A real example
Take someone who is 40 years old. Their estimated HRmax is 220 minus 40, which equals 180 bpm. Their cardio training zone is then 180 x 0.65 = 117 bpm on the low end, and 180 x 0.85 = 153 bpm on the high end. So during a cardio workout, this person should aim to keep their heart rate somewhere between roughly 117 and 153 beats per minute.
Why 220 minus your age is not quite right
The 220-minus-age formula is easy to remember, which is exactly why it stuck for decades, but it comes with a margin of error of about 12 to 15 bpm in either direction. That is large enough to push someone out of their real training zone without them realizing it. It also tends to overestimate HRmax in younger adults and underestimate it in older adults.
More recent research has produced tighter estimates. Tanaka, Monahan and Seals proposed HRmax = 208 minus (0.7 x age) in 2001, while Inbar and colleagues suggested 205.8 minus (0.685 x age) back in 1994. For the same 40-year-old, the Tanaka formula gives an HRmax of 208 minus 28, or 180 bpm, which happens to land close to the classic estimate here, though the gap grows wider at younger and older ages.
Common mistakes to avoid
Do not assume a low resting heart rate is automatically a red flag. For someone who trains regularly, 40 to 60 bpm at rest is normal and often a marker of good conditioning, not a symptom to worry about.
Do not treat any HRmax formula as an exact medical measurement. Every version, classic or newer, is a population-level estimate. If you want a precise number, a supervised cardiac stress test is the only way to measure your true maximum heart rate rather than estimate it.
Finally, do not chase the top of your zone by default. Training at the lower end (closer to 65 percent of HRmax) still builds cardiovascular fitness and is often the more sustainable choice for beginners easing into a new routine.
