Why your indoor FTP is lower (and what to do about it)

You test your FTP outdoors on a favourite climb and get a number you're pleased with. A week later you run the same test on the trainer and it comes back 20 watts lower. Nothing about your fitness changed in seven days — so what happened?

This is one of the most common and most demoralising experiences in indoor training, and it catches out beginners and experienced riders alike. The short answer: you are not less fit indoors, you are hotter. Here's the full picture, and what you can actually do about it.

Why it happens

The heat problem, properly explained

It's worth understanding why heat costs you watts, because it makes the fixes obvious. Cycling is roughly 20–25% efficient — for every 200 watts you put through the pedals, something like 600–800 watts is being dumped as heat inside you. Outdoors, a constant airflow carries most of that away by evaporating sweat off your skin.

Remove the airflow and that heat has nowhere to go. Your body responds by pushing more blood to the skin to shed it — but that blood is no longer available to deliver oxygen to your legs. Your heart rate climbs to compensate (this is cardiac drift), and eventually your sustainable power simply falls. It isn't a fitness limitation. It's a thermodynamics one.

What actually fixes it

If you do exactly one thing, make it the fans. Cooling is the cheapest performance upgrade in cycling — our pain cave setup guide goes into detail on placement and airflow.

So which FTP should you train to?

This is the practical question, and the answer is refreshingly simple: use the FTP you measured in the environment you're training in.

• Training indoors? Test indoors, and set your indoor zones from that number. Otherwise every session will feel disproportionately brutal and you'll fail workouts that aren't actually too hard.
• Racing or riding outdoors? Your outdoor number is the one that describes what you can do on the road.
• Doing both? Keep both numbers. Many riders simply set their indoor targets a few percent below their outdoor FTP and get on with it.

What you should not do is treat the lower indoor number as evidence you've lost fitness, or grind out failed sessions trying to hit outdoor targets in a hot room. That's how riders end up over-reaching and demoralised in January. Our full guide to testing and raising your FTP covers the testing protocols.

One useful upside

Training in a hot environment is a genuine stimulus of its own — heat acclimation is a real and well-studied adaptation. Riders who train indoors through summer often cope noticeably better with hot race days. So the trainer isn't only costing you watts; it may be quietly building a form of resilience you can't get on a cool spring morning.

Track it, don't guess

The gap is personal, so measure it rather than adopting someone else's number. Do a solid indoor test and a solid outdoor test within a couple of weeks of each other, then compare like for like on similar-duration efforts.

Moveee reads your power profile straight from your ride history and tracks it across every ride, indoor and out, so you can see the difference in your own data rather than in a forum thread. Trainer sessions ridden in Moveee Indoor save with full power streams — NP, IF, TSS and best efforts — exactly like a Strava import, so they sit alongside your outdoor rides in the same profile rather than in a silo. Which makes the comparison honest.

Point a fan at your chest, drink more than feels necessary, and stop comparing your January garage watts to your August climb. They were never the same test.