Variability Index Calculator

How it is calculated

All inputs are watts except duration, which is in minutes. VI = NP / AP, where NP is Normalized Power and AP is average power. Intensity Factor is IF = NP / FTP. Training Stress Score is TSS = duration in seconds × NP × IF / (FTP × 3600) × 100, so one hour at FTP gives 100. Work is kJ = AP × seconds / 1000. The pacing label depends on VI: below 1.05 very steady, 1.05 to under 1.10 mostly steady, 1.10 to under 1.20 variable and 1.20 or more highly variable, each with a short verdict, and the label uses the unrounded VI. The tool does not compute NP itself; you take NP and AP from your head unit or analysis software. NP cannot be below AP, NP is limited to 2000 W, FTP to 800 W, duration to 4320 minutes (72 hours) and IF to 2.5; inputs outside these limits give no result.

Common mistakes

• Entering the average power from the first part of a ride or a lap instead of the whole ride. NP and AP must cover the same stretch, or the ratio means nothing.
• Swapping NP and AP. NP is never lower than AP, so the tool shows no result when AP is the larger number, which is a sign the values are in the wrong boxes.
• Typing duration in hours. The field is minutes, and 1.5 is accepted as 90 seconds, which turns this ride's TSS of 127 into 2.
• Entering a current FTP that is outdated or from a different sport. IF and TSS scale with it, while VI does not; an FTP so low that IF passes 2.5 (90 W against 230 W NP) gives no result.
• Reading the rounded 1.10 as the 'variable' band. The label uses the unrounded value, so 1.095 is still Mostly steady.