Segment Lab.
Your segment as a watt workout — computed on your CP, pushed to your calendar
The card in the Race Lab shows what comes out. This page explains how it gets there — and where the model stops.
Your route, your segment
Your segment. Your workout. Your PR.
Upload your favourite segment — it becomes a Zwift workout that rides exactly that segment.
Try it on a real climb: 3.6 mi · 1,686 ft of climbing — ridden in 41:35
Target time 37:30 · avg watts 201 W · ≈ 86 % of critical power (CP)
Watt target per stretch · colour = intensity · dashed = CP.
Real climb, real pipeline · demo rider CP 215 W · 159 lb.
Zwift is a trademark of Zwift, Inc. — independent project.
The deep dive: how it is calculated
The idea in one sentence
You know a segment you want to ride faster — the Segment Lab builds the session that rehearses exactly that segment, with a watt target for every stretch instead of one number for the whole thing.
How it is calculated
The recording is cut into stretches, each with its own length and gradient. Every stretch carries the full cycling equation: rolling resistance, air resistance, gravity, drivetrain losses. It links power to speed — and therefore power to time.
The target-time slider runs that backwards. Instead of asking what time a given effort produces, it asks what effort produces a given time, and closes in on it until the total lands on the second. That is why the graphic responds the moment you drag it — it is computing, not swapping pictures.
Effort is not spread evenly but weighted by terrain: above the mean on the steep ramps, below it on the shallow run-in. That is how a climb is ridden fast, and that is how it lands in the calendar as an ERG workout.
Why the demo is honest
The climb in the demo card is not an illustration. It comes from a real recording — 5.7 km, 515 metres of climbing, ridden in 41:35 — and runs through the same engine as the lab inside the app. The demo rider carries 215 W CP at 72 kg; logged in, those are your numbers, kept current from your races.
Where the model stops
An ERG workout has no descent. Where you would freewheel outdoors the trainer holds its target, so steep downhill sections are modelled as recovery rather than reproduced. Wind, changing surfaces and drafting are not in the equation either: the model describes a solo ride in still air.
And it stays sports analytics. The numbers say what would be physically required — not what is sensible for you today. That call is yours, with medical advice where it matters.
Where your data goes
The GPX or FIT file is read and processed in your browser. It is not uploaded, not cached and not analysed — your GPS track never leaves your device. The finished workout leaves it only if you push it to your calendar yourself.
Full detail: Privacy.
What exactly do I upload?
A GPX or FIT file of your ride — from Garmin, Wahoo, a Strava export or Zwift itself. What the model needs is the elevation trace, not your power: distance and gradient build the profile, your recorded time provides the calibration.
Where do the watt targets come from?
From a physics model, not a lookup table. Each stretch carries the full equation of rolling resistance, air resistance and gravity; given a power output that yields a speed, and therefore a time. The target-time slider runs it the other way round, searching for the intensity that lands on the time you want.
Why is this better than “ride 95% of FTP”?
Because a climb is rarely even. An 18% ramp demands more in that same second than the shallow section does — one flat percentage gives away time low down and blows up high up. The model distributes effort by terrain instead of by average.
What happens to my GPS track?
It stays on your device. The file is read and processed in the browser; it is never uploaded. The only thing that can leave your machine is the finished workout, if you push it to your calendar.
Do I need an intervals.icu account?
No. With a connection the workout lands in your calendar watt-exact and flows on into Zwift. Without one you download the same thing as a .zwo file and drop it into your Zwift workouts folder.
Zwift is a trademark of Zwift, Inc. — an independent project with no affiliation. Model after Martin/Bassett (cycling power equation) and Skiba (W′); sports analytics, not medical advice. All values are computed rule-based — no AI-generated numbers.