Heart rate zones, and why 220 − age is wrong

How zones are actually calculated — and the small mistakes that put every one of them off.

Heart-rate zones are only as good as the number they're built on: your maximum heart rate. Get that wrong and all five zones shift with it — you end up training to the wrong targets all day. The most common way to estimate maximum heart rate, 220 − age, is also one of the least accurate.

Where 220 − age comes from

It isn't the result of a careful study. It traces back to a rough line drawn through limited data in the early 1970s, never intended as a precise formula, that stuck because it's easy to remember. Its error is large: individual maximums scatter around it by roughly 10–12 beats per minute in each direction, so a real maximum can easily sit 20 or more beats from the estimate. It also tilts with age — reading high for many younger people and low for older ones.

A better estimate: the Tanaka formula

208 − 0.7 × age. This comes from a 2001 meta-analysis of 18,712 subjects (Tanaka, Monahan and Seals) and holds up better across the age range — particularly past 40, where 220 − age tends to drift too low. It's still an estimate, but a far better-founded one, which is why In The Zone uses it by default.

An example: at 50 years old, 220 − age gives 170, while Tanaka gives 173. At 25 it's the other way — 195 versus 190. Small on paper, but every zone boundary moves with it.

The best number is a measured one

A formula is a stand-in for a measurement. If you've ever seen your true maximum during an all-out effort — the highest your heart rate has genuinely reached — that real figure beats any estimate, and you should enter it. A formal maximum-heart-rate test is genuinely demanding and best approached with care (and, if you have any doubt about your heart health, medical guidance). For most people, a good estimate is enough to train well.

Two ways to draw the zones — and they don't agree

Once you have a maximum, there are two common ways to set the boundaries, and they produce different heart rates at the same percentage:

• Percentage of maximum heart rate. The simplest: each zone is a straight percentage of your max. 70% of a 190 max is 133.
• Heart-rate reserve (the Karvonen method). This also uses your resting heart rate. Your reserve is your maximum minus your resting rate; a target is your resting rate plus a percentage of that reserve. It accounts for fitness — a well-trained heart with a low resting rate ends up with different zones than a formula on maximum alone would give.

The catch worth understanding: 70% of maximum and 70% of reserve are not the same heart rate. Same percentage, different number. An honest app tells you which method is in play so you're never comparing one against the other by accident — In The Zone always shows which is in use.

The zone boundaries themselves follow the American College of Sports Medicine's five-zone banding, a widely used standard.

If you use the Karvonen method, measure your resting heart rate

Take it first thing in the morning, before you get up, lying still — that's your true resting rate, not the slightly raised number you'll see once you're moving. An Apple Watch tracks resting heart rate automatically in Apple Health, which is the simplest way to keep it current.