Heart-rate zones

The five zones#

A zone ends one beat before the next one starts. Zone 5 has no upper end. A beat under the start of zone 1 is in no zone: standing, stops and the walk back are not counted as training, and the app never shows a "zone 0".

Four of the five starting points (30, 40, 60 and 90%) are the edges of the intensity categories of the American College of Sports Medicine position stand (Garber and colleagues, 2011): light, moderate, vigorous and near maximal. The fifth, 75%, splits their "vigorous" range where the second ventilatory threshold tends to sit (a study of 972 cardiac patients found a median of 78% of reserve, Milani and colleagues, 2023, so this is a pointer, not a measurement of you).

Three words are used for a session as a whole, and for a session without power data they follow the zones: zone 1 and 2 read "Low" ("Nízká"), zone 3 reads "Moderate" ("Střední"), zone 4 and 5 read "High" ("Vysoká"). Training minutes that count as moderate for the weekly guideline start earlier, at zone 2: see the Activities entry.

How each zone's limits are worked out#

For every zone the app takes your resting rate and adds the zone's share of your reserve:

1. Reserve = maximum minus resting rate.
2. Start of a zone = resting rate + share × reserve.
3. The first whole beat at or above that number is the first beat of the zone (we round up).

Why the reserve and not a share of the maximum#

A share of the maximum ignores where you start. Take two people with the same maximum of 180 bpm. One rests at 45, the other at 70. At 130 bpm the first is working at (130 - 45) / (180 - 45) = 63% of reserve, the second at (130 - 70) / (180 - 70) = 55%. Same pulse, different effort. The reserve method (Karvonen) puts the starting line where you actually start, and your resting rate is also the number that moves as you get fitter or when your body is run down. That is why the zones are cut from it.

Your resting rate#

The resting rate is the median of your nights' resting heart rate over the 14 days up to and including the day of the session, and it needs at least 5 of those nights. Readings outside 30 to 150 bpm are treated as bad data and ignored. With an even number of nights the median is the mean of the two middle ones.

If there are fewer than 5 nights in that window, or the reserve would be under 30 bpm, the app uses 57 bpm, the average sleeping heart rate of 10,865 adults in the Fenland study (56.9 bpm, Gonzales and colleagues, 2023). The zone sheet then says that your resting rate is estimated. In the app the line reads, for example: "Maximum 175 bpm (estimated from age), we estimate your resting rate at 57 bpm for now, as we do not have it from the nights yet." Once enough nights arrive, the number changes by itself.

The nights' value is the one your device files as the night's resting rate. We do not add a correction for posture.

Your maximum#

The maximum is chosen in this order:

1. Entered by you. In Settings, section "Profile", field "Maximum heart rate (optional)". A whole number from 100 to 230 bpm is accepted ("Enter a value from 100 to 230 bpm."). An empty field removes it.
2. Estimated from your age with the Tanaka line: 208 - 0.7 × age. Age is in whole years from your birth date. At 40 that is 208 - 28 = 180; at 50 it is 173. An age under 10 or over 120 is not used.
3. Raised by a recorded peak, if one is trustworthy. If a corroborated peak from your own sessions is higher than the age line, the maximum becomes that peak, but never more than 32.4 bpm above the line (three times 10.8 bpm, the typical scatter of individual maxima around the age line). A peak below the line never lowers it.
4. A recorded peak alone when there is no birth date: it counts only if it lies between 138 and 210 bpm.
5. Nothing. With no birth date, no entered maximum and no trustworthy peak, there is no maximum, so no zones. The Time in zones card then says: "Zones are worked out from your maximum heart rate. Add your date of birth or your maximum heart rate in Settings."

A "trustworthy" peak is strict, because a single spike from a loose strap or an optical sensor is not a maximum. A session's highest pulse counts only when all of this holds:

• the session is one of your last 365 days of sessions that the app counts at all,
• the peak is between 100 and 220 bpm,
• the session was an effort: its average pulse is at least 65% of its own peak (an easy hike that spikes to 200 does not qualify),
• and a second such session reached within 5 bpm of that peak, higher or lower.

The highest peak that has such a partner is taken.

The zone sheet names the source in brackets next to the maximum: "entered by you", "estimated from age", "from age and measured peaks" or "from measured peaks". When only the last one applies (no birth date), the Time in zones card adds that a session may read a zone harder than it was, and points to Settings.

The age line is a population average. Individual maxima scatter around it by about 10.8 bpm (Nes and colleagues, 2013), so if you know yours from a test, enter it.

The rounding#

The zones are cut from the exact numbers (a maximum of 176.5 at age 45 is kept as 176.5), and only the result is rounded up to a whole beat. The zone sheet prints the maximum and the resting rate as whole numbers, so you may see 177 on screen while the zones were cut from 176.5. The difference is at most one beat at one edge.

What changes your zones, and what does not#

Changing the maximum, birth date or sex#

When you change your birth date, sex or entered maximum in Settings, every stored day is marked for recalculation. The recalculation runs at the next sync or in the nightly catch-up, and the help text under the field says: "A change recalculates past sessions too." The Activities tab cuts zones when you open it, from the stored heart-rate data of each session, so the zones there follow your new maximum straight away. Sex does not move the zones; it is used by the training load (see Training load).

The maximum is not dated: one maximum is used for all sessions. It changes when you enter another one, when a new trustworthy peak appears or an old one is older than 365 days, and each birthday (the age line falls by 0.7 bpm per year).

Why a session keeps its own day's zones#

The resting rate is dated per session, using the 14 nights before that session. A session therefore keeps the zones of the body that ran it, and your history does not shift every morning as your resting rate moves. The zone sheet's ranges say "for today": they use the resting rate as of today. The same session may show different ranges on its own sheet if your resting rate was different that day.

The zones of an old session can still change when the nights behind it change, for example when a late night arrives for a day inside its 14-day window.

Where you see the zones#

The Activities tab is part of Premium. Zones appear in:

• Time in zones: the card on the Activities tab. Each row is a zone with its name and range, minutes and share. A tap opens the zone sheet with the sports that fill it, the five sessions with most minutes in it, and a block "Your zones" with your five ranges, the basis (maximum, resting rate) and the cut.
• A session's own sheet: the panel "Heart rate over the session" with a strip of zones over time and "Time in zones" for that session.
• The route map of a session, colored by the zone of the pulse (see Activities).
• The session list, where a pill such as "Moderate Z3" shows the zone of the session's average pulse.

In energy pacing mode (Energy pacing) the zones card is hidden.

Minutes in zones and the zone of a session#

Two different things are called "zone":

• Minutes in zones are counted beat by beat from the session's stored heart-rate record, so forty minutes of intervals land in the zones they were really run in. A beat under zone 1 is in no zone and is reported separately as "Below zone 1 (under 87 bpm): 12 min".
• The zone of a session (the pill in the list) comes from the session's average pulse, rounded into the zones. An average under the start of zone 1 still reads zone 1, because the question there is "which zone is this session", not "how many minutes".

A session without a heart-rate record (some devices send only an average) has no minutes in zones and is counted under the bars as "Without a heart-rate trace: N sessions". If no session in the chosen period has a record, the card places each session whole into the zone of its average and says so: "These sessions have no heart-rate trace, so each sits whole in the zone of its average heart rate."

The "% of max" on a session sheet is a share of your maximum, not of the reserve: it will not match the zone share.

The color strip under the pulse chart and the route colors read the pulse smoothed over half a minute, and a stretch shorter than 20 seconds takes the zone of its neighbour. The minutes in zones are not smoothed. A strip and the minutes can therefore differ by a little.

The day's light, moderate and vigorous minutes#

The day's light, moderate and vigorous minutes (the "Intensity minutes" card, the movement overview, the day's active minutes) are cut differently: as a share of the maximum, not of the reserve. Light starts at 60% of the maximum, moderate at 70%, vigorous at 80%. The phone's own heart rate and the heart-rate records of your sessions are cut in these same three bands, so that a day carries one kind of number whichever source wrote it. They answer a different question (how active was the day) from the training zones (how hard was this session). We have not moved the day's minutes onto the reserve scale.

When there is no heart rate, no maximum, or power#

• Power. For the word "Low / Moderate / High" a session with an average power is compared with your power anchor: 0.95 × the strongest average power of any session in the last 30 days (e-bike rides are left out, the motor lifts their watts; an anchor under 50 W is not used). Under 55% of the anchor is Low, 55 to 85% Moderate, 85% and above High. This only decides the word. Zones and minutes in zones always come from heart rate.
• No maximum. When there is no birth date, no entered maximum and no trustworthy peak, no zones are shown and the Activities list shows no intensity word. The scores that need a Low, Moderate or High judgment of a session (training balance, recovery) fall back on fixed numbers for its average pulse: under 120 bpm Low, 120 to 154 Moderate, 155 and above High.
• No heart rate. A session without a pulse has no zone and no intensity word.

The website calculator#

The free calculator at heart-rate-zones calculator has a switch "Zone bands" with two choices, "Classic" and "As in MyBodyAI". Classic uses 50, 60, 70, 80 and 90% of the reserve (the common textbook bands), "As in MyBodyAI" uses the 30, 40, 60, 75 and 90% above. Karvonen (1957) defined the reserve method, not the bands.

Other differences between the calculator and the app: the calculator takes the resting rate you move a slider to (35 to 100) while the app takes the median of your nights; it estimates the maximum from the age slider (18 to 85) and rounds it to a whole beat, while the app keeps decimals; and it accepts a known maximum from 140 to 220, the app from 100 to 230. The result can therefore differ by a beat at some edges.

The calculator is a general tool for adults and does not replace advice if you take medicine that lowers heart rate, have a heart condition, a pacemaker or atrial fibrillation, or are pregnant.

Frequently asked questions#

Why do my zones differ from my watch or another app?#

Because the starting lines differ. Watches and apps that use a percentage of the maximum heart rate show other numbers than a reserve-based scale, and our own zones were of that kind too until 1 November 2026 (50, 60, 70, 80 and 90% of the maximum). For the person in the example above (maximum 175, resting rate 48) those old lines started zone 1 at 88 bpm and zone 3 at about 123, and the zones now start at 87 and 125. With a resting rate of 70 the same comparison gives 102 against 88 for zone 1. Neither is "wrong"; the reserve scale has the advantage of following your resting rate.

My zones changed at the start of November. Why?#

Until 1 November 2026 the zones were 50, 60, 70, 80 and 90% of the maximum, with other names. They are now 30, 40, 60, 75 and 90% of the reserve, with our own names (Easy, Base, Tempo, Threshold, Peak). Sessions from before are shown in the new zones too, because zones are cut when you open the tab.

My resting rate fell. Why did the same pulse move up a zone?#

Because the resting rate is dated per session, and a lower resting rate means a bigger reserve, so the same pulse is a larger share of it. With a maximum of 175 and a resting rate of 56 a month ago, zone 3 started at 56 + 0.60 × 119 = 127.4, so at 128 bpm. With a resting rate of 48 today it starts at 125 bpm. A pulse of 126 was zone 2 then and is zone 3 now. The older session keeps the zones of its own day.

Why does the sheet say my resting rate is estimated?#

Fewer than 5 nights with a resting rate were found in the 14 days before the session, or your device files none. Until it does, we use 57 bpm. Wearing the device at night, or connecting a device that records the night, fixes it.

Can I set the limits of the zones myself?#

No, there is no setting for the edges. You can enter your maximum in Settings, section "Profile". That moves all five zones. The resting rate comes from your nights and cannot be typed in.

I take a medicine that lowers my heart rate. What should I do?#

Enter in Settings the maximum from a test taken while on the medicine. The zone sheet reminds you of this: "On heart-rate medication? Enter the maximum from a test taken on it in Settings." If you have a heart condition, a pacemaker or atrial fibrillation, ask your doctor for targets; zones from any formula may not apply.

Do my old sessions change when I enter a new maximum?#

Yes. The Activities tab is cut when it is opened, so zones and minutes in zones follow the new maximum at once. The stored daily scores that depend on it are recalculated at the next sync or the nightly catch-up.