What is Marginal Watts?
Marginal Watts answers the question every cyclist eventually asks in the wrong order. Instead of "how much faster are these wheels", it asks "of everything I could change on this ride, which change buys the most time per pound spent" — and then it ranks them. Free changes like tyre pressure and riding position sit in the same list as a £2,000 wheelset, which is the point: the list frequently says the free thing first.
Underneath it solves the Martin et al. (1998) road cycling power model for your speed and time, then re-solves the same ride once for every compatible change and sorts the results. You describe the ride — power, distance, gradient, or pick a climb like Ventoux or Hardknott — and the setup, down to helmet, socks, frame, wheels, tyres, measured tyre width and pressure. It shows where your watts are actually going: pushing air, rolling resistance, bearings, drivetrain.
What makes it more than a toy is the sourcing. The figures come from a corpus of 1,383 published measurements drawn from 230 sources — drum tests, wind tunnel sessions, peer-reviewed papers — and each figure in the calculator carries its own source and grade. Two-thirds are peer-reviewed or independent lab work. 209 measurements are flagged for conflict of interest, meaning the publisher now sells the thing it measured. Anything not derived from a test is labelled an Estimate rather than dressed up as a measurement.
Key Features
Ranked by cost per second, not by watts
The output is a sorted list of changes with the time each one saves and what it costs to get. Free adjustments compete directly against expensive ones.
A power breakdown for your actual ride
Shows how your watts divide between aerodynamic drag, rolling resistance, bearings and drivetrain losses, for the setup you described rather than a generic rider.
Every number is traceable
Open any figure in the breakdown panel and you get its source, the count of comparable tests, and the date range behind it. 58 papers in the bibliography, each DOI checked against CrossRef.
Your measurements beat theirs
If you know a figure better — your bike on a scale, a drum-tested Crr, a measured drag area — you enter it and the model uses yours instead of the class figure.
Climbs and GPX built in
Preloaded profiles for Alpe d'Huez, Ventoux, Stelvio, Zoncolan, Angliru and more, or upload a GPX of your own route.
Documented ways to be fooled
The dataset flags 17 methodology traps — documented ways a published cycling number can be misread at face value. That's an unusual thing for a calculator to volunteer.
Who It's For
- Riders about to spend money who want to know whether the wheels or the skinsuit is the better buy, before buying either
- Time triallists and climbers chasing a specific course time
- The evidence-minded, who want to see the study behind a claimed watt saving rather than a brand's number
- Anyone with a floor pump, since pressure and position keep winning the ranking for free
