What is the difference between %MHR and %HRR?

%MHR uses only your max heart rate. %HRR uses your max HR minus your resting HR, then adds the resting HR back after scaling. For the same percentage, %HRR gives a higher target because it accounts for the range you actually have to work with.

Why do the zone boundaries differ between the two methods?

Because %HRR accounts for resting HR. Two people with the same max HR but different resting HRs will have different intensities at the same %MHR target. The %HRR method is more individualized and is equivalent to %VO2R per the ACSM guidelines.

Which max HR formula should I use?

No formula is universally accurate. The Tanaka formula (208-0.7 × age) is the most accurate general formula for adults over 40. The Gulati formula (206-0.88 × age) is derived from women. The 220, age formula has a standard deviation of ~12 bpm.

Can I use heart-rate zones if I take beta-blockers?

No. Beta-blockers suppress heart rate response to exercise, making all HR-based targets invalid. Use the Borg RPE scale (6-20) instead.

Is the fat burn zone the same as Zone 2?

No. The fat burn zone is a marketing term for low-intensity exercise. Zone 2 is a specific training intensity (60-70% of HRR) that improves mitochondrial function. Weight loss depends on total calorie deficit, not the fuel source during exercise.

How do I measure my resting heart rate accurately?

Measure it first thing in the morning before getting up, after a full night's sleep, and before consuming caffeine. Do not use a single measurement as your baseline.

What is the failure case for someone using the Karvonen method without a true max HR?

Substituting an age-predicted max HR introduces the same error as the %MHR method. For a 60-year-old woman with a true max HR of 140 bpm, using the 220, age prediction of 160 bpm would set her 70% HRR zone approximately 14 bpm too high.

Heart-Rate Zones as Percentages of Max or Reserve Explained

How Zone Boundaries Are Scaled

You enter your max heart rate and your resting heart rate, and the calculator draws lines at percentages. Those percentages mean different intensities depending on which method you use. Heart-rate zones are defined as percentages of max or reserve, showing exactly how each boundary is scaled.

The two methods are percent of maximum heart rate (%MHR) and percent of heart rate reserve (%HRR, also called the Karvonen formula). They give different numbers for the same percentage because %HRR accounts for your resting heart rate. The ACSM Guidelines, 11th edition, reports that %HRR is equivalent to %VO2R, making it more physiologically accurate than %MHR for exercise prescription. Both methods, however, depend entirely on the accuracy of your max heart rate.

Heart-Rate Zones as Percentages of Max or Reserve Calculation

For the %MHR method the calculation is straightforward: Zone boundary = Max HR × (percentage/100). For a max HR of 180 bpm, the 70% boundary is 180 × 0.70 = 126 bpm. This ignores resting HR entirely.

For the %HRR method the calculation adds resting HR back in after scaling the reserve. Zone boundary = ((Max HR, Resting HR) × (percentage/100)) + Resting HR. Using the same max HR of 180 bpm and a resting HR of 60 bpm, the 70% boundary becomes ((180-60) × 0.70) + 60 = 144 bpm. That is 18 bpm higher than the %MHR value for the same percentage.

This difference has a concrete effect. Moderate intensity per the ACSM is 64-76% of MHR or 40-59% of HRR. Vigorous intensity is 77-93% of MHR or 60-84% of HRR. A person targeting 70% of MHR may think they are in moderate territory, but the same 70% expressed as HRR moves them into a different zone. The research data confirms that %MHR zones ignore resting HR, so two people with the same max HR but different resting HRs will have different actual intensities at the same %MHR.

A Worked Example

Work through a concrete example using the inputs supported by the research. Take a person with a max HR of 180 bpm and a resting HR of 80 bpm. Compute the 70% boundary for both methods.

%MHR method: 180 × 0.70 = 126 bpm.

%HRR method: (180-80 = 100 bpm reserve) × 0.70 = 70 bpm, then add back resting HR: 70 + 80 = 150 bpm.

The gap is 24 bpm. The %MHR target of 126 bpm, when expressed as a percentage of heart rate reserve, is only 46% HRR. That is a lower intensity than the intended 70%. The calculation confirms that a person using %MHR who has an elevated resting HR will be working at a lower relative intensity than someone with a low resting HR who uses the same %MHR target. At this point you need a proportion calculator to convert between the two methods, because the same percentage means different things.

The failure case is direct. A person with max HR 180 bpm and resting HR 80 bpm who uses the 70% MHR target of 126 bpm is actually exercising at only 46% HRR. They intended moderate intensity but are working below the ACSM moderate threshold of 40-59% HRR. If they had a max HR of 180 and a resting HR of 60, the same 126 bpm target would be 46% HRR as well, but their HRR is larger, so the absolute intensity is lower still. The method itself does not fail, but the assumption that the same percentage produces the same effort fails for anyone whose resting HR is not average.

Calculation Steps

The real failure comes when the max HR itself is estimated rather than measured. The 220, age formula has a standard deviation of approximately 12 bpm. For a 60-year-old woman, the formula predicts a max HR of 160 bpm. If her true max HR is 140 bpm, the 70% MHR target using the predicted value is 160 × 0.70 = 112 bpm. The correct target using her true max HR is 140 × 0.70 = 98 bpm. That is a 14 bpm error. The same error propagates into the Karvonen method if the estimated max HR is used there as well.

The Tanaka formula (208-0.7 × age) is more accurate for adults over 40. For the same 60-year-old, Tanaka predicts 208-42 = 166 bpm. The Gulati formula (206-0.88 × age) is derived from women and predicts 206-52.8 = 153.2 bpm. These are still estimates. The only way to know your max HR is a maximal exertion test.

Limits and Failure Cases

Both methods share the same dependence on an accurate max HR. The 220, age formula is the most common and the most misleading. It was never derived from a representative sample and is particularly inaccurate for women and older adults. The Robergs and Landwehr 2002 study reported a standard deviation of ~12 bpm, meaning one in three people will have a true max HR more than 12 bpm away from the prediction. For a person whose max HR is actually 20 bpm lower than the formula, every zone boundary is set too high.

Beta-blockers suppress heart rate response to exercise. A person taking metoprolol or atenolol cannot reach the target HR the calculator produces, because their max HR is pharmacologically lowered. The Rating of Perceived Exertion (Borg RPE scale, 6-20) is the recommended alternative. The ACOG Committee Opinion No. 804 and the 2019 Canadian Guideline for Physical Activity throughout Pregnancy recommend RPE rather than HR targets during pregnancy as well.

Resting heart rate is not a fixed number. The AHA states the normal resting HR range is 60-100 bpm. A single measurement may be off by 10+ bpm due to hydration, sleep, caffeine, or illness. Using a single measurement as the baseline for the Karvonen formula introduces error before the percentage is even applied.

How to Interpret the Result

This method suits recreational exercisers and beginners who want a starting point for moderate-to-vigorous cardio, provided they understand the error margins. It also suits runners and cyclists who know that HR-based zones lag 30-90 seconds behind effort and do not align perfectly with lactate-threshold-based zones like Coggan or Friel. It does not suit anyone with a known cardiac arrhythmia that makes pulse counting unreliable, anyone taking rate-altering medications, or anyone who wants a precise calorie-burn figure from heart rate, that requires a metabolic testing lab.

The single thing that most often goes wrong is a person using 220, age to estimate max HR, then setting their zone boundaries based on that number, and then training at an intensity that is either too high or too low by 10-20 bpm without knowing it.