About This Calculator (and Why It Insults You)

Right. So you typed in your weight, you typed in your power, and the calculator handed you a number and possibly called you a filthy liar. Fair enough that you'd want to know who's behind that.

Why this exists

It started, as most daft projects do, with me staring at a climb I couldn't finish. Alpe du Zwift is 12.2 km long, drags you up 1,036 metres of elevation at an average of 8.5% — kicking up to a thoroughly unpleasant 14% on the worst of its 21 hairpins — and I wanted to know, before I clipped in, roughly how long I'd be suffering for. Not a vague “ooh, about an hour”. An actual number I could fail to beat.

I couldn't find a calculator that just told me, so I built one. Then I made it slightly rude, because a tool that tells you a hard truth might as well have a bit of fun doing it.

Why it insults you

Here's the honest version. The calculator runs your numbers through a formula built from real ride data — give it your weight in kilograms and your power target, and it works out your watts-per-kilogram and spits back an estimated time. That's the useful half.

The other half is that it doesn't entirely trust you. Tell it you weigh more than is humanly plausible and it'll politely suggest you contact the Guinness Book of Records about your title of “The Heaviest Damn Person, Ever.” Claim a power figure that would make a Tour de France domestique weep and it'll invite you to apply for our up-and-coming pro cycling team — or, you know, adjust your number, you filthy liar. None of this is me being clever. It's me being suspicious, which is a different and more honest thing.

And if you keep clicking past your result — go on, I know you will — you'll find the bits I never quite got round to hiding. I'll be straight with you: I only sort of know what I'm doing when it comes to building websites. The original was an absolute bodge job. This rebuild is a marginally less embarrassing bodge job, which I'm choosing to call progress.

Is it accurate?

Accurate-ish. The estimate is built from an average of historic times for riders at your watts-per-kilogram, so it's a sensible middle-of-the-road prediction — which is exactly the problem with averages. Half the riders at your w/kg went faster than your predicted time. The other half went slower. You could be in either camp, and the calculator has no way of knowing which, because it has never met you and frankly doesn't want to.

So treat the number as a target to aim at, not a promise. If you want more certainty, push your power up a touch and the maths gets kinder. If you want total certainty, I'm afraid you'll have to actually ride the thing.

Who built it

Me. One person, a turbo trainer, and a stubborn refusal to accept that 1,036 metres of fake climbing should be that hard. There's no company, no team, no venture funding — just a calculator, a handful of guides I've since written so this site isn't quite so thin, and the 21 hairpins that haunt my dreams. It's free, it always will be, and it will continue to gently doubt your honesty for as long as it's online.

The disclaimer, one more time

Because it matters: this is a fan-made tool. It is not affiliated with or endorsed by Zwift Inc. All Zwift trademarks belong to Zwift Inc.; I'm just a fan who likes the climb, respects the brand, and wanted to build something useful around it. If anyone official ever reads this — hello, I'm a big fan, please don't sue me, the average gradient really is 8.5% and I have the leg burn to prove it.