Heart Rate When Running: What Your Watch Really Tells You
Your watch shows your heart rate with every stride, but what do you do with it? It is not a goal in itself. It tells you what running costs your body, as long as you know when to trust it.
The KILYO team
The sports coach that reads your workouts
The short answer
Heart rate when running measures what the effort costs your body at a given pace. It helps you keep easy runs easy and track the HR-to-pace ratio from one workout to the next. A max HR formula can be off by about 5 bpm from the measured value, according to a study of 180 recreational marathon runners, so check heart rate, pace, and how you feel together.
Key takeaways
- Heart rate measures internal load, what the effort costs you, while pace measures what you produce.
- A max HR formula gives a rough estimate for a group, not your own value: a measured test is more reliable.
- On a long run, the HR-to-pace ratio shows cardiac drift, not heart rate alone.
- Easy run: use heart rate. Speed workout or race: use pace. In every case, your perceived effort makes the final call.
You run with a watch on your wrist, and a number keeps changing: 142, 155, 168. Many runners look at it without knowing what to do with it, or follow it to the letter without asking whether it's accurate.
Heart rate (HR) is the number of times your heart beats per minute. Here is what it is useful for, when it can mislead you, and how to choose between heart rate, pace, and how you feel.
What heart rate measures: what the effort costs you
Pace tells you what you produce. Heart rate tells you what it costs you to produce it.
A 6 min/km (9:39/mile) pace doesn't cost the same on a fresh day as on a tired one: the pace stays the same, but your heart rate changes. Heart rate is useful for three things:
- Control intensity on easy runs, so they stay easy.
- Tracking changes in your HR-to-pace ratio over the weeks.
- Monitoring recovery through resting HR and variability.
Your max HR: a formula gives a rough estimate
To place your heart rate, people often start with your max HR, the highest rate your heart can reach. A common formula is "220 minus your age." Another option, from a meta-analysis of 351 studies, 492 groups, and 18,712 subjects, is Tanaka's: 208 − 0.7 × age. At 40, it gives 180 beats per minute.
But these formulas describe group averages, not you. In 180 recreational marathon runners, about 43 years old and tested on a treadmill, the measured max HR differed from the formulas:
- in women, it was about 5 bpm lower than the value predicted by Tanaka;
- in men, it was 2.8 bpm higher than the value from Fox, the other formula compared.
Five beats is enough to shift all your zones. Coaches advise comparing the formula's result with the highest HR you've recorded in your hardest workouts, and using the measured value if the gap is clear.
Cardiac drift: HR rises while pace stays the same
On a long run at a steady pace, your heart rate creeps up over time. This is cardiac drift. To see it, use cardiac cost: your HR divided by your speed, a drift indicator used in research on marathon runners.
For example, you run at 10 km/h. At the start, your HR is 150: 150 ÷ 10 = 15. After an hour, it's 160 at the same speed: 160 ÷ 10 = 16. Your cardiac cost has risen by about 7%. This ratio is what matters: 160 bpm at 12 km/h does not mean the same thing as 160 bpm at 10 km/h.
Resting HR and variability: a modest signal
Your watch may also show heart rate variability (HRV): the small changes in time between heartbeats. A 2021 meta-analysis on HRV-guided training found improvements in HRV indicators, but not in resting HR. Gains in endurance performance were small and not significant (SMD = 0.20). A second meta-analysis, on data from wearable devices, found an average positive effect on submaximal physiological measures (g = 0.296).
Studies measure groups. Coaches read HRV and resting HR as a trend over several days. A single number one morning decides nothing.
Chest strap or wrist: where measurement can go wrong
An optical sensor on your wrist reads your heart rate through the skin. A chest strap reads closer to the heart, and it is more reliable when running. A 2025 validation study tested two Polar optical sensors at three positions (upper arm, forearm, wrist) in 16 healthy participants, across nine activities from rest to intense intervals.
In practice, coaches recommend:
- wearing the watch a bit tighter than usual, just above the wrist bone;
- using a chest strap for intervals, where heart rate changes quickly;
- being suspicious of a number that jumps suddenly, like 180 at minute 3 of an easy run.
What you can do at your next workout
Zones split your heart rate into intensity ranges. Their boundaries vary by watch brand and method, so treat them as reference points, not fixed rules.
Free calculatorHeart rate zonesOpen the calculator →| Workout | Main cue | Why |
|---|---|---|
| Easy run | Heart rate (low zone) | Avoid starting too fast |
| Long run | HR-to-pace ratio | Spot drift |
| Speed workout or intervals | Pace | Heart rate reacts with a delay |
| Race | Pace, then feel | Heart rate changes with stress and heat |
On an easy run, slow down as soon as you go above your low zone, even if it means walking up a hill. Let your sense of effort be the final judge.
This is the kind of reading KILYO does after your runs: it reads your workouts from your watch, compares pace, heart rate, volume, and consistency from one week to the next, then adjusts your plan. To build your week, read how to improve running with 3 workouts a week.
Frequently asked questions
Is "220 minus your age" reliable?
It's a rough estimate, valid for a group. In 180 recreational marathon runners, the measured max HR differed from Tanaka's formula by about 5 bpm in women. A measured test is more reliable.
Why does my heart rate rise during a long run at a steady pace?
That's cardiac drift. Divide your heart rate by your speed: if this cardiac cost climbs during the run, your heart rate is drifting. Track it from one week to the next rather than on a single day.
Should you run by heart rate or by pace?
Heart rate works well for easy runs, to keep them easy. Pace works better for fast workouts and races, because heart rate reacts with a delay. Your sense of effort is still your last check.
Is my wrist watch accurate enough?
It is often good enough for an easy run, if it's snug. For intervals, a chest strap is more dependable. If a reading seems impossible, ignore it and look at the trend across the whole workout.
Sources
- Tanaka H., Monahan K. D., Seals D. R. (2001). Age-predicted maximal heart rate revisited. Journal of the American College of Cardiology. DOI
- Cross-sectional study of 180 recreational marathon runners (2018). Age-Predicted Maximal Heart Rate in Recreational Marathon Runners, comparing the Fox and Tanaka equations. PubMed Central
- Manresa-Rocamora A. et al. (2021). Heart Rate Variability-Guided Training for Enhancing Cardiac-Vagal Modulation, Aerobic Fitness, and Endurance Performance. International Journal of Environmental Research and Public Health. DOI
- Düking P., Zinner C., Trabelsi K. et al. (2021). Monitoring and adapting endurance training on the basis of heart rate variability monitored by wearable technologies. Journal of Science and Medicine in Sport. JSAMS
- Do heart rates of elite marathon runners exhibit room for drift? (2025). PubMed Central
- Validation of heart rate measurement by two optical sensors worn on the wrist or arm (2025). JMIR Cardio. JMIR
Written with the help of AI.
