Take two one-hour rides. One is a flat, steady endurance spin at 180 watts. The other is a criterium-style session that swings between 90 watts of soft-pedalling and repeated 500-watt surges, and it happens to also average 180 watts. Their average power is identical. They are nowhere near equally hard. Average power hides everything that makes the second ride brutal, which is why three other numbers exist.
Normalized Power: what the ride felt like
Normalized Power, or NP, is an estimate of the power you could have held for the same physiological cost if you had ridden perfectly steadily. It weights the hard bits far more than the easy bits, because the body pays a disproportionate price for surges.
The calculation is worth knowing because it explains the behaviour:
- Take a 30-second rolling average of your power for the whole ride. This smooths out one-second spikes.
- Raise each of those averaged values to the fourth power.
- Average all of those.
- Take the fourth root of that average.
The fourth power is the key step. It means a 500-watt surge counts for vastly more than the soft-pedalling that follows it, so a stochastic ride reads much higher than its flat average. In the criterium example, the average might be 180 watts but the NP could be 240 — and 240 is much closer to how the ride actually felt.
For a genuinely steady ride, like an ERG-mode endurance block, NP and average power sit almost on top of each other. The gap between the two numbers is itself information: a wide gap means a surgy, variable ride; a narrow gap means a metronomic one.
Intensity Factor: how hard, relative to you
Normalized Power is an absolute number, so 240 watts means one thing to a pro and another to someone starting out. Intensity Factor, or IF, fixes that by dividing NP by your FTP.
IF is just a ratio:
- An easy recovery spin sits around 0.5 to 0.6.
- Endurance riding runs about 0.65 to 0.75.
- A sweet-spot or tempo session lands near 0.85 to 0.90.
- A ride right at your threshold for the hour is 1.0 by definition.
- Short, very hard interval sessions can push the whole-ride IF above 1.0 because the surges drag NP up.
Because IF is measured against your own FTP, it means the same thing for everyone. An IF of 0.85 is a solid, sustained effort whether you are a 400-watt engine or a 150-watt beginner. This is also why a wrong FTP quietly corrupts everything downstream: set FTP too low and every ride’s IF reads inflated, so easy rides look like hard ones.
Training Stress Score: how much the ride cost
TSS combines intensity and duration into one number for the total load of a ride. The anchor point is simple and worth memorising: one hour at exactly your FTP is 100 TSS.
From there it scales with both how hard and how long:
- A two-hour endurance ride at IF 0.65 is roughly 85 TSS — long but easy.
- A one-hour threshold session at IF 0.95 is around 90 TSS — short but hard.
- A four-hour steady ride can rack up 250 TSS from duration alone.
The formula multiplies the ride’s duration, its NP, and its IF together and normalises so that the FTP-hour equals 100. You do not need to compute it by hand; the point is to read it. A rough weekly guide: under 300 TSS is light, 300 to 500 is a solid training week for most amateurs, and sustained weeks north of 700 are serious volume that need matching recovery.
Reading them together on a real ride
Say you finish a 60-minute session and the summary reads: average power 205, NP 248, IF 0.89, TSS 79.
Here is what that tells you at a glance. The 43-watt gap between average and NP says the ride was surgy — intervals, not a steady state. IF 0.89 puts it firmly in sweet-spot-to-threshold territory, a properly hard session. And TSS 79 for an hour means it cost you close to what a flat threshold hour would, which fits a hard interval workout. You now know, without replaying the ride, that it was a demanding session you should not stack another hard day on top of tomorrow.
That is the payoff. Three numbers, read in a couple of seconds, describe both what the ride was and what it took out of you.
Where CrankPilot computes them
Every ride you finish in CrankPilot gets NP, IF and TSS calculated on the device the moment you stop, alongside your calories, zone time, and a power curve from one second to two hours. Nothing uploads to a server to make it happen. And because TSS is the input to a performance management chart, those per-ride numbers are also what let the app track your fitness and form over a training block — but that is a longer story, told on the training plans page.