What is RFI?
A running-fitness index based on your speed and heart rate as a percentage of your maximum heart rate.
RFI explained
Are you getting more speed from the same effort?
RunFit turns pace and heart-rate effort into one simple number, then tracks it over time. The point is not one impressive run—it is seeing whether your running fitness is actually improving.
See your RFI
Compare RFI with other platforms
Higher RFI means more speed for the same relative heart-rate effort.
What does the score mean?
If you can run faster at the same relative heart-rate effort—or the same pace at a lower effort—your RFI rises.
How should I use it?
Follow your own RFI over weeks and months. Individual runs can be noisy; the direction of the trend is what matters.
The calculation
A deliberately transparent fitness metric
RFI asks one narrow question: how much running speed are you producing for your cardiovascular effort? There is no hidden readiness score or black-box prediction behind it.
The heart-rate input is expressed as a percentage of your configured maximum HR, which makes the score respond to relative effort rather than raw beats per minute.
RFI formula
400 × speed (km/h) ÷ HR (% of max)
The factor of 400 only puts the result on an easier-to-read scale.
12.0 km/h at 76% max HR → 63.2 RFI
Why combine pace and heart rate?
Pace alone cannot tell you how hard you worked for it
A five-minute kilometre can be easy one month and difficult the next. Heart rate adds context. RFI combines the two so that a faster pace only looks better when the cardiovascular cost makes sense too.
More speed, same effort
If speed increases while relative heart-rate effort stays similar, RFI rises.
Same speed, lower effort
If you hold the same pace at a lower percentage of max HR, RFI rises too.
Fitness can move both ways
Training, detraining, fatigue, weather and recovery can all show up as changes in the trend.
Designed for real training
Different workout types need different handling
Easy, steady and interval runs remain distinct. That keeps short recovery laps and heart-rate lag from distorting the RFI you use to follow fitness.
| Run type | How it is identified | How RFI is calculated |
|---|---|---|
| Easy | At or below your configured top-of-easy HR or pace breakpoint. | Uses the post-warm-up portion of the run. |
| Steady | Continuous running above the easy breakpoint without an interval work/recovery pattern. | Uses the post-warm-up portion of the run. |
| Intervals | Repeated faster work periods separated by slower recoveries. | Short-rep sessions use the whole post-warm-up workout. Longer reps use the work portions after the first 60 seconds to reduce HR-lag distortion. |
The interval classification and the RFI calculation are separate: a workout can remain classified as Intervals even when its RFI is calculated from the whole run.
What you can actually see
The useful part is what happens after the calculation
RunFit turns individual scores into views that make changes easier to understand.
RunFit Index over time
See every calculable run across recent months or your longer locally stored history, color-coded by workout type.
Heart rate vs speed
Look at the same runs in HR-versus-speed space, with recency shown visually and your race-goal RFI as a reference line.
Race-goal reference
Choose a race distance, target finish time and expected race HR. The app translates that into a required pace and RFI reference.
A few important details
Keep the metric grounded in your own data
Warm-up is excluded
The first 10 minutes are excluded by default so the opening part of a run does not dominate the score. The setting is editable.
Max HR matters
Because RFI uses heart rate as a percentage of maximum, your configured max HR directly affects the score.
Use it longitudinally
RFI is most useful for comparing you with you. Physiology, sensors, terrain and settings make direct comparisons between runners less meaningful.
Local-first analysis
RunFit analyzes compatible Strava activity data in your browser and does not request GPS streams for the RFI calculation. Read the privacy policy →
Common questions
RFI FAQ
Is a higher RFI better?
Yes. A higher score means more running speed for the same relative heart-rate effort.
Should I judge fitness from one RFI value?
No. Conditions and individual workouts vary. RFI is intended to be followed as a trend over time rather than treated as a verdict from one run.
Can I compare my RFI with another runner?
The metric is most useful for comparing your own training over time. Maximum-HR settings, physiology, terrain, weather and sensor accuracy can all make direct comparisons misleading.
Does reaching the race-goal RFI guarantee my target race time?
No. It is a training reference, not a race prediction. Course profile, endurance, weather, fueling, pacing and race-day conditions still matter.
How does RFI compare with Polar Running Index, COROS Running Fitness, Garmin Running Economy or Strava Fitness?
They are related but different metrics. See the comparison of running fitness metrics across Strava, Garmin, Polar and COROS.
Do I need to use kilometres?
No. The app can display pace and distance in kilometres or miles. The underlying RFI stays consistent regardless of your display-unit preference.
See what your own runs say
Turn your Strava history into an RFI trend.
Connect Strava, sync compatible runs and see how your running fitness has changed over time.