Back to the plan Recomputes as you type
The same chest, twice

Your watch and your bike computer disagree about Zone 2 by — beats.

Reversed-out technical plate: a human heart drawn as one white contour line, sitting inside five concentric rings spaced evenly apart, the innermost ring picked out in orange, tick marks around the outside.

Both are doing the arithmetic correctly. They are doing different arithmetic. Two numbers below and you can read yours both ways.

You
Measured max, if you have one

Leave it blank and the page estimates it. Fill it in and every band on this page is rebuilt from your number instead.

Five bands, and every one of them is the same width

The zones are cut out of your reserve — the beats between sitting still and flat out — ten per cent at a time. Ten equal slices, five bands of two.

Where the two methods part company

One method takes a percentage of your maximum. The other takes a percentage of your reserve and adds your resting rate back on. Same five bands, same person, side by side.

Zone% of max% of reserveApart

The gap is your resting rate, wearing a hat

The distance between the two methods is not complicated and it does not depend on your maximum at all. It is your resting rate, shrunk by however hard you are going.

Both methods agree at exactly one place: flat out. At 100 per cent they are both your maximum, so the gap is zero. Every step below that, the reserve method keeps a fraction of your resting rate that the other one has already thrown away — which is why the two versions of Zone 2 can miss each other completely while the two versions of Zone 5 sit almost on top of each other.

The formula everyone knows is right on one birthday

220 minus your age is the one people can recite. Held against the regression that replaced it, the two cross once and then walk away from each other in opposite directions.

Neither line is a measurement of you. Both are averages drawn through crowds, and the spread around them is roughly ±10 beats either way — wider than the entire distance between the lines for most of a life. The point of the picture is not which line to trust. It is that the old one is not conservative and not generous; it is one, then the other, and it swaps over at forty.
220 − age
208 − 0.7 × age

A birthday costs seven tenths of a beat. One off your resting rate buys back a whole one.

Your reserve is squeezed from the top by a number you cannot argue with and propped up from the bottom by one you can. Both ends move at a fixed price, which makes them tradeable at a rate that never changes.

If this movesReserveBand widthZone 2 floor

How long today’s Zone 2 lasts

Hold your resting rate where it is and let nothing else happen. The band drifts down at a rate you can write on one line, and the day it clears the floor it is standing on today is a long way off.

How each figure is worked out

  1. Maximum. 208 − 0.7 × age, a regression fitted across 351 published studies, and the same line for everybody — it has no sex term in it. There is a women-specific alternative, 206 − 0.88 × age, which sits about six beats below it at twenty and thirteen below it at sixty; this page does not switch to it, because the moment two formulas are in play the honest answer is that both carry a spread of about ±10 beats and neither was measured on you. Type a measured maximum into the box and every number on the page is rebuilt from that instead — the formula is only standing in until you have one.
  2. Reserve is maximum − resting. Everything else here is a fraction of it.
  3. The two methods. Percentage of maximum is p × max. Percentage of reserve — the Karvonen method — is resting + p × reserve. The zone edges are the same in both: 50, 60, 70, 80, 90, 100 per cent.
  4. The gap between them is not typed into this page. Subtract one from the other and the maximum cancels: what is left is resting × (1 − p). The table prints the subtraction and the line plots the formula, so if they ever disagreed you would see it.
  5. Every band is the same width because each one is two of ten equal slices of the reserve: reserve ÷ 10. Printed to whole beats they come out one apart here and there; that is the rounding, not the bands.
  6. Why the old formula crosses at forty. Set 220 − a equal to 208 − 0.7a and you get a = 40. Away from that birthday the two answers separate by 0.3 beats a year in whichever direction you are walking.
  7. The drift is the same arithmetic run forwards. The top of Zone 2 is resting + 0.7 × reserve, and ageing takes 0.7 beats a year off the reserve, so that edge falls by 0.7 × 0.7 = 0.49 beats a year. Nothing in it knows whether you trained.
  8. None of this is measured. No test, no chest strap, no lab. A resting rate taken after coffee, after a bad night or standing up reads high, and one beat of error in that box moves your Zone 2 floor by four tenths of a beat. Take the numbers as a starting frame and correct them against what you can actually hold for an hour.

This is general information, not medical advice. Heart-rate training is not appropriate for everyone, and beta blockers, thyroid conditions, pregnancy, arrhythmias and a long list of medications change both your resting rate and your maximum — sometimes enough to make every band on this page meaningless. Chest pain, fainting, or a rate that will not settle are reasons to stop and see a doctor, not reasons to adjust a formula.

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