Detraining calculator: how much fitness will I lose?

You lose less fitness than you fear in the first two weeks: trained athletes lose about 7 % of VO2max after three weeks of complete rest and about 16 % after eight weeks, then it levels off. Keeping some shorter, harder sessions in your week slows the loss a lot. Enter your break to see the estimate and the rebuild time.

Calculator

My fitness

CTL

Days off or reduced

days

During the break

Aerobic fitness lost

about 5 %

VO2max estimate for trained athletes

Fitness (CTL) after the break

from 60, −28 %

CTL drops faster than real fitness because it measures recent training load, not your engine.

Back to 95 % of your CTL in about 73 days of normal training.

VO2max: about −7 % after 21 days and −16 % after 56 days of rest, then stable (Coyle et al., 1984). CTL: 42-day exponential average of daily TSS.

How the detraining calculator works

Detraining is the fitness you lose when you stop or cut back training, after an injury, an illness, a holiday or a busy spell at work. The calculator gives you two estimates: how much aerobic fitness you lose, and how far your training load (CTL) drops. It is one of the free LeCoach cycling calculators.

Aerobic fitness is based on a classic study by Coyle and colleagues (1984). Trained endurance athletes lost about 7 % of their VO2max in the first 21 days of complete inactivity, and the loss levelled off at about 16 % below their trained value after about 56 days. The calculator draws a straight line between those points.
Fitness (CTL) is the 42-day exponentially weighted average of your daily training stress score (TSS), the fitness line most training platforms show.
Your starting point can be your CTL, your weekly TSS (divided by 7) or your weekly hours (at 50 TSS per hour, which assumes mostly endurance riding).

Worked example. Your CTL is 60 and you take 14 days of complete rest. The calculator estimates you lose about 5 % of your aerobic fitness. Your CTL falls to 43, which is 28 % lower. Back on your normal training, it takes about 73 days to return to 95 % of your old CTL. A weekly TSS of 420 also gives a CTL of 60; 8 hours a week gives about 57.

How much fitness you lose

Days off	Aerobic fitness lost, complete rest	Aerobic fitness lost, half of normal training	CTL after complete rest (from 60)	CTL after half training (from 60)
7	about 2 %	about 1 %	51	55
14	about 5 %	about 2 %	43	51
21	about 7 %	about 4 %	36	48
28	about 9 %	about 4 %	31	45
42	about 12 %	about 6 %	22	41
56	about 16 %	about 8 %	16	38

The complete-rest column follows the Coyle study. The reduced-training columns are a simplification by the calculator: it scales the loss by how much training you keep, so half of normal training halves the loss. Coyle's athletes stopped completely, so treat these as rough estimates. Keeping some shorter, harder sessions in a reduced week usually protects fitness better than easy riding alone.

Why CTL drops faster than real fitness

Look at the two halves of the table. After two weeks off, CTL is down 28 % but the aerobic fitness estimate only about 5 %. They measure different things.

CTL is a rolling average of your recent training load. Stop training and it falls straight away, because it describes what you have been doing, not what your body can still do. Your heart, blood volume and muscles lose their adaptations much more slowly. So a steep drop in your fitness chart after a holiday does not mean you have lost a quarter of your engine. Our TSS calculator explains how TSS and CTL are calculated.

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Getting it back

The rebuild estimate assumes you return straight to your old daily load. It is the time for your training load to climb back to 95 % of where it was, not necessarily the time for your fitness to return. In practice:

Start with easy riding. After more than a week or two off, give yourself a few days to a week of easy endurance before adding intensity.
Build volume before intensity. Get back to your normal hours first, then add threshold and VO2max work.
Retest FTP after two to three weeks of riding, not on your first day back.
Be patient after illness. Recovery after being ill is usually slower than after a planned break. Read training after illness or time off, and if you are still unwell, start with can I train with a cold?.

For a longer comeback, our guide to returning to cycling gives a step-by-step plan.

Common mistakes

Panicking at the CTL line. It drops fast and rises fast. It is not a measure of your engine.
Jumping back to old intensities. Your zones may be a little high for the first sessions. Ride by feel and heart rate.
Trying to make up missed TSS. Big catch-up weeks after a break are a common route to illness or injury.
Thinking a few easy days cost fitness. A week of rest costs only about 2 % in the estimate, and often leaves you fresher.

When the numbers change

Short breaks under a week behave more like a taper: you may feel fresher, not slower.
Reduced training with some intensity loses much less than complete rest.
Less trained riders may lose fitness at a different rate: the Coyle figures come from well-trained athletes.
Injury or illness adds its own recovery time on top of the detraining. Read fatigue management for cyclists for how to balance load and recovery on the way back.

After a break, ask LeCoach to adjust your plan: it works from your recent training and recovery data, not the fitness you had before.

Sources

Coyle EF et al. Time course of loss of adaptations after stopping prolonged intense endurance training. Journal of Applied Physiology, 1984;57(6):1857-1864.
Allen H, Coggan A, McGregor S. Training and Racing with a Power Meter, 3rd edition. VeloPress, 2019.

Frequently asked questions

How fast do you lose cycling fitness?

Slower than most riders fear. In trained endurance athletes, VO2max fell about 7 % after 21 days of complete rest and levelled off at about 16 % after about 56 days. The first week costs very little. Your CTL drops much faster, because it measures recent training load rather than the fitness you have built.

Will I lose fitness in one week off?

Very little. The calculator estimates about 2 % of VO2max after 7 days of complete rest for a trained athlete, and many riders feel fresher after a week off. Your CTL will fall, from 60 to 51 in the example, but that reflects the training you did not do, not a matching loss of fitness.

How long does it take to get fitness back?

It depends on how long you were off. After 14 days of complete rest, the calculator estimates about 73 days of normal training to bring CTL back to 95 % of where it was. That is the time for your training load to catch up. Because aerobic fitness drops much less than CTL, the CTL line is a cautious guide to the gap.

Why did my CTL drop so much after a break?

CTL is a 42-day exponentially weighted average of your daily TSS. When you stop training, your daily TSS is zero, so the average falls quickly: from 60 to 43 in two weeks. It describes your recent training load, not your engine, so a big CTL drop does not mean a big fitness loss.

Does reduced training stop fitness loss?

It slows it a lot. The calculator assumes that keeping half of your normal training halves the loss, so 14 days at half training costs about 2 % instead of about 5 %. Keeping some short, hard sessions, rather than only easy riding, is the most practical way to protect your fitness during a busy spell.

Should I retest FTP after a break?

Yes, but not straight away. Give yourself two to three weeks of normal riding first, so you test your fitness rather than your rustiness. Until then, ride by feel and heart rate, and expect your old zones to feel a little harder than usual for the first sessions.