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Why is my FTP lower indoors than outdoors?

Updated 2026-10-23 · Why indoor riding feels different

Short answer

Almost everyone tests lower indoors, commonly by five to ten per cent. The main cause is heat: without airflow you cannot shed heat as fast, so you fatigue sooner at the same power. Less coasting, a fixed position and the mental grind of a fixed test all add to it. It is not lost fitness.

The gap is normal and it is mostly heat

If you test at 280 outdoors and 255 indoors, nothing has gone wrong. That kind of gap is extremely common and the main driver is thermal.

At 200 watts on the pedals your body is spending about 950 and shedding three quarters of it as heat. Outdoors the air you are moving through removes it continuously. Indoors there is nowhere for it to go, which is the whole reason a fan matters more than any other piece of indoor kit.

Hard efforts generate a lot of heat. Outdoors, moving air carries it away. Indoors it does not, so your core temperature climbs faster, your body diverts blood to the skin to cool you, and the effort you can hold for twenty minutes drops. The better your cooling, the smaller your gap tends to be: people with serious fans often find the difference shrinks to very little.

The other contributors

No coasting. An outdoor test usually has some variation in it. An indoor test is an unbroken block of pedalling at one number.
A fixed position. You cannot move around, shift your weight or stand up for a few seconds to relieve a contact point.
The mental side. Staring at a number and holding it for twenty minutes is its own kind of hard. Outdoors there is a road, a hill, a segment, something to chase.
ERG mode. If you test in ERG, the trainer holds the power whether you are coping or not, which is unforgiving in a way the road never is. See what ERG mode is and when to use it.

Which number should you train with?

Train with the number from the environment you are training in.

If your sessions are indoors, set your zones from an indoor test. If you set indoor sessions off an outdoor FTP, every session will be slightly too hard, you will fail intervals you ought to complete, and you will conclude you are going backwards when the only problem is an inflated number.

Keep both if you ride in both. There is no rule saying you get one FTP. An indoor number for indoor sessions and an outdoor number for outdoor sessions is more useful than one compromise number that is wrong in both places.

Making the gap smaller

Cooling, then cooling, then cooling. A strong fan at your head and chest. Most people are astonished at the difference.
Open a window. Even a cold draught helps.
Start cool. Do not do your warm-up in a jacket in a closed room.
Test the same way every time. Same fan, same room, same time of day, same warm-up. A repeatable number is worth more than a flattering one.
Drink more than you would outdoors, because you will be sweating more. See why you sweat so much more indoors.

Common questions

How much lower is normal?

Five to ten per cent is typical. Larger gaps usually point at the room rather than at you: a hot space with no fan can produce a much bigger difference.

Does a lower indoor FTP mean I have lost fitness?

No. It means you tested in a harder environment. If your indoor number is climbing over time, your fitness is climbing, regardless of how it compares to an outdoor figure.

Should I retest after I get a fan?

Yes, and expect it to go up. That rise is not fitness, it is the removal of a handicap, so do not read it as a sudden training breakthrough.

Can my indoor FTP ever be higher than outdoors?

Occasionally, usually for people whose outdoor riding is on busy roads with constant interruptions, or who find a steady indoor effort mentally easier than a road test.

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