About ZwiftCalculator.com

Every rider who turns onto the bottom of Alpe du Zwift is asking the same question: how long is this going to take me? The climb is 12.2 km long, it gains 1,036 m, it averages roughly 8.5%, and it makes you sit through 21 hairpins to get to the top. That is a long time to spend finding out the hard way that you set off two tenths of a watt per kilo too hot.

This site answers that one question before you start. You give the Alpe du Zwift calculator your weight and the power you think you can hold, and it gives you back a finishing time you can plan a ride around: whether to chase the sub-60 badge today or wait another training block, whether you have time for the descent before dinner, what pace to hold in the first three hairpins so the last three do not end you.

That is the whole product. Everything else here grew out of the follow-up questions people asked after they got their first number: what does Ven-Top cost by comparison, what is my FTP really, which category does that put me in, and how much faster would I be if I lost three kilos.

How it started

I am Christian Lassen Dam. I am 25, I live in Aalborg in northern Denmark, I am studying for an MSc in Mechanical Engineering at Aalborg University, and I have been riding on Zwift long enough to have ground out the ascents that unlock the Tron bike. Right now most of my training is pointed at Ironman Copenhagen.

The first version of this calculator was a spreadsheet, and it was entirely selfish. I wanted to know what the Alpe would cost me at a power I could actually hold, and I did not trust my own guesswork. So I did the obvious engineering-student thing: collected finishing times, plotted them against watts per kilogram, fitted a curve, and typed my own numbers into the cell. Then a couple of riding friends asked me to run their numbers too. Sending screenshots of a spreadsheet gets old fast, so I put the formula on a web page. The web page turned into this site.

There is no secret model here and nothing to buy. The procedure is the same for every climb calculator on the site, even where the fitted shape is not, and it is deliberately boring:

1. Collect finishing times. Real ascents, each one paired with the rider's weight and the average power they held, converted into watts per kilogram.
2. Fit a curve. The simplest shape that survives contact with the data, chosen per climb rather than imposed on all of them. Alpe du Zwift takes a second-order polynomial. Ven-Top takes an inverse, where the predicted time falls as one over watts per kilogram.
3. Check it against landmarks. The fit has to reproduce times riders already recognise. If it puts a 4.0 W/kg rider well away from where 4.0 W/kg riders actually finish, the fit is wrong, not the riders.
4. Publish the equation. The coefficients are printed on the calculator page, so anyone can rebuild the result in a spreadsheet and check my work.

Why watts per kilogram decides a steep climb

On the flat, the thing fighting you is the air, and air resistance barely cares how much you weigh. On a slope averaging 8.5%, nearly everything you produce goes into lifting you and your bike against gravity instead. The power needed to gain height at a given rate scales with total mass, so if you double a rider's mass and double their power, they climb at exactly the same speed. What survives that cancellation is power divided by mass. A 60 kg rider at 240 W and an 85 kg rider at 340 W are both at 4.0 W/kg, so the calculator hands them the same finishing time even though one is producing 100 W more, which is why raw watts are close to useless as a predictor on a climb this steep. The same quantity is what sorts riders into Zwift racing categories, and I have written the longer version of the physics in the power-to-weight article.

Why the relationship bends

Time is not proportional to power. It behaves much more like one divided by power, and that bends. Half a watt per kilo is worth a lot when you are slow and much less when you are already fast. Run it through the equation: going from 2.5 to 3.0 W/kg takes about 9 minutes 28 seconds off your Alpe time, while going from 4.5 to 5.0 W/kg takes off about 4 minutes 31 seconds. Same half a watt per kilo, less than half the payoff.

A quadratic is not a law of nature. It is a shape that fits the range riders actually ride in, roughly 2 to 6 W/kg, and it is honest about being a fit rather than a simulation. Push it far outside that range and it stops behaving: the parabola turns around at 1,954.08 divided by twice 148.60, which is about 6.57 W/kg, and beyond that point it starts claiming that more power makes you slower. That is exactly why the coefficients sit in the open instead of behind a button, so you can see where the model stops being useful. Ven-Top gets its own fit for the same reason: at about 20.9 km and 1,534 m of gain it is a different climb rather than a scaled Alpe, and it fits an inverse instead of a parabola, time_minutes = 3.21 + 253.38 ÷ (W/kg). The side-by-side comparison shows how differently the two punish the same rider. If you want the method written out properly, it is in the regression methodology article.

The site is free to use, and the running costs of the domain, the hosting and the time are covered by display advertising. That is the whole business model, and you should know it before you weigh anything I write.

Advertising never influences the content. Advertisers get ad slots. They do not get advance sight of articles, a say in what gets covered, or a way to buy a mention or a link. If an advertiser objected to something published here, the article would stay and the ad would go. What advertising does mean for you is that you will see ads and that your visit is measured in aggregate. What gets collected and why is set out in the privacy policy, and what you may do with the numbers you get here is covered by the terms of service.