For most of your running, heart rate. It shows what a run costs you, while pace only shows how fast you went. Two runs at the same pace can cost very different amounts, because heat, a short night, hills and a hard week all raise the price. Pace still matters as the result: running faster at the same heart rate is how you know you got fitter.
When should I use which?
| Run | Go by | Why |
|---|---|---|
| Easy and long runs | Heart rate | Keeps easy running easy on hot, hilly or tired days |
| Steady runs and longer intervals | Heart rate | Keeps controlled work controlled. Give it the first minute to catch up |
| Short, fast repeats | Pace or feel | Heart rate is still rising when a short repeat ends |
| Races | Pace or feel | You run to a goal time. Heart rate is a check, not the guide |
| Hills | Heart rate or power | Pace says little on a climb |
Why not just run by pace?
Because the same pace is not the same run from one day to the next. On a hot afternoon, after a short night or in the middle of a hard week, your heart works harder at the same speed. Hold your usual pace and an easy run quietly becomes a medium one, which you then pay for in the runs that follow. Run by heart rate and the effort stays where it belongs while the pace adjusts. On a good day, the same heart rate gives you a faster pace for free.
Where does heart rate let me down?
- It lags. It needs 30 to 60 seconds to catch up with a change in effort, so for short repeats use pace, power or feel.
- It drifts. On long runs and warm days your heart rate creeps up even at a steady pace. This is called cardiac drift, and it is normal. Start low and let the pace come down.
- The sensor can be wrong. In the first minutes of a run the watch can pick up your step rate instead of your pulse. See Why is my Apple Watch heart rate too high at the start of a run?
- Zones are only as good as your maximum. If your maximum heart rate is set too low, every zone comes out too low and easy runs look hard. The heart rate zone calculator explains how to find a better number.
What about running by feel?
It works, and it is always available. The talk test is the simplest version: if you can speak in full sentences, you are running easy. Many experienced runners go by feel first and use heart rate as a check. Heart rate helps most while your sense of effort is still settling, and for anyone who tends to run every run a little too hard.
What about running power?
Apple Watch estimates running power during outdoor runs, and it reacts instantly where heart rate lags. That makes it useful on hills and in intervals. See What is running power on Apple Watch?
What does Kiiro show?
For every run, Kiiro shows pace and heart rate together, with your time in each zone, so you can see what the run really cost. On routes you run often, Route progress puts every attempt side by side, and that is where the two numbers meet: faster at the same heart rate, or the same pace at a lower heart rate, both mean you got fitter. See heart rate zones in Kiiro and route comparison in Kiiro.
Related questions
- How slow should my easy runs be?: Slow enough to talk in full sentences the whole way. For most runners that is zone 2, 60 to 70% of maximum heart rate, and it feels slower than expected. Here is how to find your easy effort and why it works.
- Why is my Apple Watch heart rate too high at the start of a run?: Usually it is the sensor, not your heart. In the first minutes of a run the optical sensor can pick up the rhythm of your steps instead of your pulse, especially with cold skin or a loose band.
- What is running power on Apple Watch, and what is a good number?: Running power is the work you are doing when you run, in watts. There is no universal good number, because it depends on your weight and speed. Its real use is pacing, since it reacts instantly where heart rate lags.
