What Is Intensity Factor (IF) in Cycling and Running

Intensity Factor (IF) is workout intensity relative to threshold: Normalized Power ÷ FTP. It answers “how hard was this session?” so a 150 W ride is easy for one athlete and all-out for another.

It answers “How hard was this session?”—and makes training intensity comparable across athletes with different ability levels.

Calculation Formula

IF is the ratio of Normalized Power (NP) to Functional Threshold Power (FTP):

$$IF = \frac{NP}{FTP}$$

Example:

• Suppose your FTP is 200 W.
• You complete a ride with NP = 150 W.
• Then $IF = 150 / 200 = 0.75$.

Note: the same logic applies to running (using rFTP and NGP). Heart rate is generally less precise than power due to lag and drift.

IF calculation in running

For running, we usually use rFTP (running threshold power) or CP (Critical Power) as the baseline:

$$IF_{run} = \frac{RunPower}{CP}$$

Or, if using pace:

$$IF_{pace} = \frac{NGP}{ThresholdPace}$$

Note: NGP (Normalized Graded Pace) is a grade-adjusted normalized pace.

Why focus on IF?

1. Cross-athlete comparison

• Absolute power: a pro can ride 200 W easily; a beginner may ride 200 W all-out. Watts alone don’t reflect effort.
• Relative intensity (IF): if both athletes have IF = 0.75, they’re doing the same relative aerobic endurance work.

2. Identify training zones

IF is the fastest way to see which intensity domain a session sits in:

Typical application scenarios

1. Validate FTP settings

If you hold IF ≈ 1.1 for a full solo 1-hour time trial (TT) (i.e., NP is 1.1× FTP), your FTP may be set too low.

• Why: FTP is commonly treated as the highest power you can sustain near steady state for about an hour. If IF stays well above 1.0 for a full hour, your threshold estimate likely needs review.

2. Race pacing strategy

In long endurance events, controlling IF helps protect late-race pacing and nutrition:

3. A core building block of Training Stress Score (TSS)

IF is a multiplier in TSS—it sets how fast TSS accumulates:

$$TSS = \frac{t \times NP \times IF}{FTP \times 3600} \times 100$$

• A small increase in IF raises TSS a lot (the formula effectively involves an $IF^2$ relationship).

Trainingload.ai recommendations

Trainingload.ai uses IF to interpret the character of a workout, not just its total load:

1. Match IF to the session goal: aerobic base sessions should usually stay controlled; threshold or VO2max sessions are intentionally higher.
2. Be cautious with very short sessions: for workouts shorter than roughly 20 minutes, IF can look unusually high and should not be interpreted the same way as a long steady session.
3. Review threshold settings: if easy sessions frequently show IF above 0.8, either the workout is not as easy as intended or your threshold estimate may need recalibration.

Limitations

• Bias in short, intense sessions: for very short workouts (e.g., ~10 minutes), NP behavior can make IF look unusually high.
• Depends on a reliable FTP: if FTP is outdated or wrong, IF loses meaning.

FAQ

What is Intensity Factor?

Intensity Factor is NP ÷ FTP (or NGP ÷ threshold pace for running). IF 0.75 is a controlled aerobic session for most athletes; IF near 1.0 is about an hour at threshold.

How is IF used in cycling training?

Use the IF range to check whether a ride stayed in the intended zone, to pace long events, and to spot an FTP that is too low—holding IF ≈ 1.1 for a full hour usually means the threshold is understated.

Does IF work for running?

Yes, with running threshold power/CP or with NGP versus threshold pace. Heart-rate IF is less precise because HR lags and drifts.

How does IF change TSS?

TSS scales with IF². A small rise in IF adds more load than the same rise in duration, which is why an incorrect FTP distorts both IF and TSS.

Next steps

• See how to calculate Training Stress Score
• Estimate FTP from an activity file
• Calculate power training load zones
• Manage training load with CTL, ATL, and TSB in your plan

Normalized Power (NP), Intensity Factor (IF), and Training Stress Score (TSS) are registered trademarks of Peaksware, LLC.