📊What heart rate zones actually mean

A zone is a percentage of an estimate

Every heart rate zone chart is built on one number: your maximum heart rate. Unless you have completed a supervised maximal exercise test, that number was not measured. It was predicted from your age by an equation fitted on a population, and the zones are percentages of that prediction.

So there are two independent choices sitting between you and the band on the chart. Which equation produced the ceiling, and where somebody decided to draw the lines beneath it. Neither choice is usually shown, and both move the answer by more than most readers expect.

The five-zone convention has no paper behind it

The familiar five bands - 50 to 60 percent, 60 to 70, 70 to 80, 80 to 90, 90 to 100 - are a training-industry convention. No paper derives those particular edges. This site renders them because they are the chart on your watch, and labels them a convention in its formula pack because that is what they are.

The published classification in the same territory is ACSM's, from the 2011 position stand, and its boundaries are different: relative intensity as a percentage of maximum heart rate is very light below 57 percent, light from 57 to 63, moderate from 64 to 76, vigorous from 77 to 95, and near-maximal to maximal at 96 and above.

Lay one over the other and the mismatch is immediate. Zone 2, at 60 to 70 percent, starts inside ACSM's light band and ends inside its moderate one. ACSM's moderate band straddles the convention's zones 2 and 3. There is no way to map one scheme onto the other, which is why this site prints both tables next to each other rather than implying the five zones are ACSM's.

Percent of maximum versus heart-rate reserve

The second method in common use is Karvonen's, published in 1957: target heart rate equals maximum minus resting, times the intensity, plus resting. The quantity it scales is heart-rate reserve - the span between the two ends of your range - rather than the maximum alone.

The two give materially different numbers at the same nominal intensity. At a maximum of 190 with a resting rate of 60, 70 percent by Karvonen is 151 bpm; 70 percent of maximum is 133. Eighteen beats apart, from identical inputs, because they are percentages of different things.

Carried into a zone chart the effect persists. At age 50, with a Tanaka maximum of 173 bpm, zone 2 as 60 to 70 percent of maximum is 104 to 121 bpm; through Karvonen with a resting rate of 60 it is 128 to 139. Karvonen bands always sit higher at the same nominal percentage, and a chart that does not name its method cannot be reconciled with one that used the other.

And which ceiling you started from

Underneath both methods is the equation that produced the maximum. At age 60 the three published options give 166 bpm from Tanaka, 160 from Fox, and 153 from Gulati, which was derived in 5,437 asymptomatic women. Zone 2 becomes 100 to 116, 96 to 112, or 92 to 107 bpm respectively - a 9 bpm shift in the upper edge from nothing but the choice of equation.

The equations also cross. Fox's 220 minus age and Tanaka's 208 minus 0.7 times age give the identical answer at exactly age 40; below that Fox reads higher and above it lower, by 9 bpm at 70. Any statement that 220 minus age is too high or too low needs an age attached to it before it can be true.

Why your watch disagrees, and why it might still be right

Devices anchor zones to different quantities, and several of the options are defensible. Some scale percent of maximum. Some scale heart-rate reserve. Some anchor to a lactate threshold or a threshold heart rate established in a test, which is what most endurance coaches mean by a zone and which moves the boundaries again. A device that has watched you train for a year may also be estimating your maximum from observed data rather than from your birthday.

Two watches can therefore place the same run in different zones and both be following a chart they can cite. The lesson is not that one is broken; it is that a zone label is a property of a chart, not a property of the run.

A maximum heart rate is not a fitness score

It is worth stating plainly, because the number invites the opposite reading. Look at what goes into the three equations: Fox takes age, Tanaka takes age, Gulati takes age. Not one of them contains a term for fitness, training history, weight or anything else about the person. A higher predicted maximum at the same age is not a report on you - it is a report on which equation the calculator ran.

The equations also all slope downwards, so a maximum falls with age by construction. That is a property of the regression lines, and reading a lower predicted number as a decline in fitness is reading the calendar, not the heart.

The measurements that do respond to training sit elsewhere. Resting heart rate is one of the ten metrics this site tracks in its Numbers Log precisely because it is directly measurable rather than predicted: you count it, you do not estimate it from a birthday. When a training number is worth watching over months, prefer the one you can measure.

The precision that is not there

Robergs and Landwehr report a standard error of estimate of 7 to 11 beats per minute across two decades of maximum heart rate prediction research, and Gulati 2010 reports about 9 bpm for its own equation. This site renders every predicted maximum inside a 10 bpm band for that reason.

That band propagates straight into the zones, because a zone edge is a percentage of the maximum. Ten beats of uncertainty at the ceiling becomes six beats at the 60 percent edge and seven at the 70 percent edge - which is most of the width of zone 2 itself in the older age rows. Nothing is gained by carrying the arithmetic to a decimal place.

Ten beats at a predicted 170 bpm is a 12 percent span, which is wider than a whole zone in some rows of the table. A boundary printed to the beat implies a resolution the underlying estimate does not have. Read each band as a broad region, treat a reading a few beats outside one as noise, and cross-check against effort: whether you can hold a conversation is a more reliable instrument here than the third digit on a watch.

What no zone chart tells you

There is no published fat-burning zone. Fat supplies a larger share of energy at lower intensities and higher intensities use more energy overall; both statements are true, and neither singles out a band. This site does not label one, because the label promises more than the physiology delivers.

A zone also says nothing about how much time to spend in it, which is a training question rather than an arithmetic one. What the calculator can do is show you exactly which chart it used, which equation produced the ceiling, and how far apart the alternatives would have been. That is the part that is knowable, and it is the part this site publishes.

Nor does a zone chart describe effort in the way the word suggests. Heart rate at a given workload drifts upward over a long session, rises in heat, and responds to caffeine, illness, altitude and how badly you slept - so the same run can sit in two different zones on two days without the training having changed. ACSM's classification is of relative intensity, not of how hard something felt.

The practical consequence is a shift in what the chart is for. Read across sessions rather than within one: the distribution of time across zones over a month is a more stable quantity than which band a particular ten minutes landed in, and it is far less sensitive to the three arbitrary choices this guide has spent its length pulling apart.