Why your indoor FTP is lower (and what to do about it)
The Moveee team
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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.
Riders commonly report indoor power running roughly 5–10% below the equivalent outdoor effort. Treat that as a general finding, not a rule — the real figure varies enormously with your cooling setup, the room, your body size and how well you tolerate heat. Some well-cooled riders see almost no difference; some in a hot garage see considerably more than 10%. The only number that matters is your own, measured in both places.
Why it happens
Heat build-up
Main causeThe big one. Outdoors at 30 km/h you have a constant 30 km/h airflow stripping heat away. Indoors you produce the same heat with none of that. Core temperature rises, blood is diverted to the skin to cool you, heart rate drifts up and sustainable power falls.
No cooling airflow
Main causeThe direct consequence of the above — and the one thing you can genuinely fix with money. Most riders under-fan by a wide margin.
Static position
ContributingOn the road you shift about constantly — out of the saddle, hands moving, weight redistributing. Indoors the bike is fixed and so are you, so the same muscles take the same load for the whole hour.
No coasting, no micro-rests
ContributingOutdoor 'steady' riding contains hundreds of tiny recoveries — junctions, descents, easing off. Indoors an hour at threshold is genuinely a full hour at threshold.
Motivation and boredom
ContributingStaring at a wall while your legs burn is psychologically harder than riding a real climb. Effort perception rises even when physiology hasn't changed.
Power meter differences
Check thisYour trainer and your bike's power meter may simply disagree by a few percent. Worth ruling out before you conclude anything physiological.
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
Two fans, aimed at chest and face
Big gainThe highest-return fix by a distance. Most riders recover a meaningful chunk of the gap with cooling alone.
Drink more, and add electrolytes
Moderate gainIndoor sweat rates are far higher than most people assume. Two bottles an hour is normal, not excessive.
Open a window or door
Moderate gainRecirculating hot air in a sealed room has limits. Fresh air genuinely lowers the ceiling you're fighting.
Wear less
Moderate gainBib shorts and nothing else. A jersey indoors is just insulation you're paying for in watts.
Pre-cool before hard sessions
Small gainA cold drink or a cool room before you start buys you a few minutes of headroom before heat becomes limiting.
Get out of the saddle periodically
Small gainTen seconds every few minutes redistributes load and relieves pressure — closer to how you ride outside.
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.
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