How long should I ride indoors to replace an outdoor ride?

It depends on which yardstick you match and the steady power you hold. To match the work, multiply the outdoor ride’s kilojoules by 1,000 and divide by your indoor power in watts to get seconds: 1,728 kJ at 185 W is about 9,340 seconds, or 2 h 36 min. To match the training load (TSS), with the same FTP scoring both rides, multiply the outdoor moving time by the ratio of the outdoor normalized power to your indoor power, squared. Holding the outdoor ride’s normalized power indoors, that is the full outdoor time. No fixed multiplier gives both.

Is an hour on the trainer worth 1.5 hours outside?

Only at one particular steady power, and it differs between yardsticks. To match the work, hold 1.5 times the outdoor ride’s average power with coasting included: 240 W for a ride averaging 160 W. To match the TSS, with the same FTP scoring both rides, hold about 1.225 times (the square root of 1.5) its normalized power: about 227 W for a normalized power of 185 W. The 1.2, 1.5 and halve-it conversions come from a coaching company, a coaching column and a cycling magazine, none of which gives a source for its figure.

Does an indoor ride score more TSS than the same time outside?

Not at the same normalized power and FTP. TSS is calculated from duration, normalized power and FTP wherever you ride, and an hour at FTP scores 100. A trainer ride scores more TSS per hour than an outdoor ride only if its normalized power, which for a steady ride is the power you hold, is higher than the outdoor ride’s, or if a lower FTP is used to score it.

Why is my heart rate higher on the trainer at the same power?

Heat is a likely reason: indoors there is no airflow from your own speed. In a laboratory study at 30 °C, 8 men cycling at a fixed 70% of peak oxygen uptake had higher heart rate, core temperature and perceived exertion with no airflow, and reached exhaustion after 41 minutes on average against 90 minutes with a 30 km/h airflow. A fan aimed at you is the simplest fix. If the trainer and the bike use different power meters, the same number of watts may not be the same effort either.

Do cyclists produce less power indoors at the same effort?

Not always. In a maximal 20-minute time trial, 20 competitive cyclists averaged almost the same power indoors and outdoors (280 W against 284 W). But when 12 cyclists were asked to ride 40 km at the same perceived effort, they averaged 163 W in a laboratory with no airflow and 208 W outdoors, with similar ratings of effort and warmer skin indoors. Riding to a power target, for example with the trainer in ERG mode, with a fan running, removes the guesswork.

Should I shorten my long ride if I train indoors for an IRONMAN?

Matching the TSS in less time means riding above the outdoor ride’s normalized power, and no study shows that a shorter, harder ride builds durability, the ability to hold power late in a long session, as the long ride does. When the long ride is the key session of the week, keep at least the trainer time that matches the outdoor ride’s work in kilojoules, and as much of the planned duration as you can. That is coaching judgment, not a trial result.

Indoor vs outdoor cycling calculator

Enter one outdoor ride from your power meter and the steady power you’ll hold on the trainer. See how long the indoor ride needs to be to match the same work, or the same training load, and what each rule of thumb implies.

The outdoor ride

From one power-meter ride file, ideally a ride like the one you are replacing.

Moving time

As h:mm or h:mm:ss, between 0:20 and 12:00. Stops at cafés and lights don’t count.

What your app shows

Work (kJ)Labelled Work, Total Work or Energy Output. It always includes coasting.Average power (W)Only if it includes the zeros from coasting. If you are not sure, use work.

Work (kJ)

Whole kilojoules, 50 to 12,000. Not calories.

Normalized power, NP (W) (optional)

From the same ride, 30 to 700 W. Leave blank to see the work match only.

On the trainer

Steady trainer power (W)

The power you will hold, such as an ERG target. Whole watts, 30 to 600.

FTP (W) (optional)

Adds TSS and Intensity Factor (IF) figures; the times don’t depend on it. 80 to 600 W.

Replacing a 3 h 00 min outdoor ride at a steady 185 W

185 W equals the outdoor ride’s normalized power: by that measure, the same intensity as the ride it replaces.

Same work

2 h 36 min

24 min shorter than the outdoor ride

1,728 kJ, the outdoor ride’s work
142 TSS, 22 fewer than outside

Same training load (TSS)

3 h 00 min

The same time as the outdoor ride

1,998 kJ, 270 kJ more than outside
164 TSS, the same as outside

Outside: 164 TSS at an IF of 0.74. On the trainer: IF 0.74.

Trainer time at any steady power (time against watts)

Same work (solid)Same training load (dashed)Outdoor ride, 3 h 00 min (dotted)Rules of thumb, left to right: ÷ 1.2, ÷ 1.5, ÷ 2Both match

The lines cross at about 214 W, the one steady power that matches both the work and the training load, in about 2 h 15 min at an IF of 0.86.

What each rule of thumb implies

Each rule divides the outdoor time by a fixed number. The power columns show the lowest steady power, in whole watts, that would do the outdoor ride’s 1,728 kJ, or reach its TSS, within that time.

Outdoors is 20% longer (÷ 1.2)
Trainer time
2 h 30 min
For the same work
192 W IF 0.77
For the same TSS
203 W IF 0.81
An hour inside equals 90 minutes outside (÷ 1.5)
Trainer time
2 h 00 min
For the same work
240 W IF 0.96
For the same TSS
227 W IF 0.91
Halve it (÷ 2)
Trainer time
1 h 30 min
For the same work
320 W IF 1.28
For the same TSS
262 W IF 1.05

Trainer time each rule of thumb gives for this ride, and the steady power it would take to do the outdoor ride’s work, or reach its TSS, in that time
Rule	Trainer time	Power for the same work	Power for the same TSS
Outdoors is 20% longer (÷ 1.2)	2 h 30 min	192 W IF 0.77	203 W IF 0.81
An hour inside equals 90 minutes outside (÷ 1.5)	2 h 00 min	240 W IF 0.96	227 W IF 0.91
Halve it (÷ 2)	1 h 30 min	320 W IF 1.28	262 W IF 1.05

The rules come from a coaching company, a coaching column and a cycling magazine, none of which gives a source for its figure. Sources are listed below the calculator.

What this assumes

A steady trainer ride, as in ERG mode, so its normalized power equals the power you hold. Intervals indoors raise TSS per hour above these figures.
TSS follows TrainingPeaks’ formula from the moving time and NP you enter, with the same FTP scoring both rides, so it can differ by a few points from the figure your platform printed.
Both rides measured by the same power meter, with a fan indoors. Without a fan, the same watts tend to feel harder and push heart rate higher, more so in a warm room.
Equal work or equal TSS is not equal training. Neither captures the late-ride fatigue of a long ride, fuelling practice or bike handling.

How the calculator works

It compares the trainer ride with the outdoor ride on two yardsticks. The first is work: the kilojoules your power meter adds up from every pedal stroke. Coasting adds time to a ride but no work, so a steady trainer ride at a given power does the same work in the outdoor ride’s kilojoules times 1,000 divided by that power, in seconds.

The second is training load, measured as TrainingPeaks’ Training Stress Score (TSS). TSS scores a ride from its duration and its normalized power (NP), relative to your functional threshold power (FTP); an hour at FTP scores 100. NP is TrainingPeaks’ estimate of the steady power that would have had the same physiological cost as the ride’s varying power. On a steady trainer ride NP is simply the power you hold, so matching the outdoor TSS takes the outdoor moving time multiplied by the ratio of the outdoor NP to the indoor power, squared. With the same FTP scoring both rides, FTP cancels out, which is why the calculator needs it only to print the TSS figures themselves.

You need a power meter on the outdoor ride: without one there is no work or NP to match. The trainer ride is assumed to be steady, as an endurance ride in ERG mode is. Use an outdoor ride like the one you are replacing, because the answer comes from that ride’s own mix of pedalling, coasting and surges.

Why the rules of thumb disagree

Coaches and forums trade fixed conversions. An email from Carmichael Training Systems quoted on the Slowtwitch forum says an indoor session equates to an outdoor ride 20% longer. Road Bike Rider’s coaching column says an hour inside equals 90 minutes on the road. A 2022 We Love Cycling article says to cut your usual training time roughly in half. None of them gives a source for its figure, and they cannot all be right, because each is one number for a ratio that changes from ride to ride.

For work, the ratio is your indoor power divided by the outdoor ride’s average power, coasting included. Dividing the outdoor time by 1.2 only matches the work if you hold 1.2 times the outdoor average on the trainer, and halving it needs twice the average. Training load sets a different requirement: dividing the time by a number matches the TSS at the outdoor NP times that number’s square root, about 1.095, 1.225 and 1.414 times NP for the three rules. How hard the work route is depends on how much of the outdoor ride was coasting or soft-pedalling, which a steady trainer ride removes; the TSS route depends only on the outdoor NP. If 10 to 15% of a ride is coasting, the share We Love Cycling’s article calls normal, then riding indoors at your average power while pedalling outside matches the work in roughly the outdoor time divided by 1.1 to 1.2, not half of it.

A study comparing the two shows how much more variable outdoor riding is. Jeffries and colleagues (2019) had 20 competitive cyclists ride a maximal 20-minute time trial twice, on a laboratory trainer and on a traffic-free racing circuit, with the same bike and power meter. Average power was almost identical (280 W indoors, 284 W outdoors), so for that effort the work ratio was close to 1, but power varied about twice as much from moment to moment outside: a within-rider standard deviation of 69 W against 33 W. An ordinary road ride adds descents, junctions and traffic, which is where coasting comes from.

A worked example

The calculator opens on a 3 h 00 min rolling ride: 1,728 kJ of work, an average of 160 W with the coasting included, and an NP of 185 W. Ridden on the trainer at a steady 185 W, the outdoor NP, the same work takes 2 h 36 min, 24 min less than outside. Training load gives a different answer. With an FTP of 250 W the outdoor ride scored 164 TSS, and at 185 W it takes the full 3 h 00 min to reach that, by which time you have done 270 kJ more work than outside. The shorter ride matches the work and scores 142 TSS.

The three rules give 2 h 30 min, 2 h 00 min and 1 h 30 min. To finish 1,728 kJ in those times you would hold 192 W, 240 W and 320 W: Intensity Factors (IF, power divided by FTP) of 0.77, 0.96 and 1.28. The second is close to threshold for two hours, a hard race effort rather than an endurance ride; the third is far above threshold for an hour and a half, which is not a sustainable ride. Matching the outdoor TSS in those times instead takes 203 W, 227 W and 262 W, IFs of 0.81, 0.91 and 1.05; the last is still above threshold for an hour and a half.

On the chart the two lines cross at about 214 W. That is the one steady power that matches both the work and the TSS, and it does so in about 2 h 15 min at an IF of 0.86: a much harder session than the endurance ride it replaces. To do the same work in less time, raise the power; to keep the TSS in less time, raise it above the outdoor NP.

For a plain endurance ride, a reasonable default is to hold about the outdoor NP and ride at least the work match, 2 h 36 min here, and the full outdoor time if you want the TSS as well. That is coaching judgment, not a trial result.

If you ride indoors by feel

Riding to a power target is what makes those figures come true. Without one, the evidence is mixed. In the Jeffries time trial, riders going as hard as they could produced almost the same average power indoors. When Mieras and colleagues (2014) instead asked 12 recreationally trained cyclists to ride 40 km at the same perceived effort in a laboratory and outdoors, they averaged 163 W in the laboratory and 208 W outside, with heart rates of 143 and 152 beats per minute, while their ratings of effort did not differ. The laboratory had no wind (0.0 against 2.5 m/s outdoors), and skin temperature was higher there (33.0 against 31.4 °C).

Ridden by feel in that study, each indoor minute did about a fifth less work than an outdoor one, the reverse of every rule of thumb. The authors suggest riding at a higher perceived effort indoors. A power target held by the trainer in ERG mode, with a fan running, does the same job without the guesswork.

Heat is the hidden cost indoors

On the road, your own speed makes a breeze. In Otani and colleagues’ (2018) study in a 30 °C chamber, 8 men cycled at 70% of peak oxygen uptake until they could not continue. With no airflow they lasted 41 minutes on average, against 58, 73 and 90 minutes with air at 10, 20 and 30 km/h, and their core temperature, heart rate and perceived exertion were higher without it.

Most rooms are cooler than that chamber, so the effect should be smaller, but the study tested only 30 °C. A fan aimed at you helps you hold the power target the calculator assumes. If your heart rate runs higher on the trainer at the same power, warmth is the first thing to check, then whether the trainer and the bike’s power meter agree.

When the clock matters more than the numbers

Neither yardstick sees what happens late in a long ride. Maunder and colleagues (2021) argue that durability, how much and how soon an athlete’s physiological profile shifts during prolonged exercise, should be measured as an attribute in its own right. A shorter, harder trainer ride can match the kilojoules or the TSS of a long ride, but no study shows that it trains durability the way the long ride does.

For a 70.3 or IRONMAN build, the long ride is also where you practise fuelling and time in the aero position. That is coaching judgment rather than trial evidence, but it points one way: when the long ride is the key session of the week, keep at least the trainer time that matches the outdoor ride’s work, and as much of the planned duration as you can. Shortening makes most sense for endurance rides whose job is aerobic volume and nothing more.

Getting numbers you can compare

Use moving time. Café stops and red lights are not training, and an elapsed time lowers the average power and inflates both the outdoor TSS and the load match. Use the ride’s work in kilojoules where your app shows it, labelled Work, Total Work or Energy Output depending on the app; it already includes the coasting. Calories are an estimate, not a reading from the power meter.

If you enter average power instead, it has to include the zeros from coasting. Some devices can be set to leave zeros out of the average, which makes it higher than the work divided by the moving time; if you are not sure how yours is set, use the work figure. Take normalized power from the same ride. A figure with a similar name, such as a weighted average power, may be calculated differently and will move the load match.

Finally, compare power from the same meter. A smart trainer’s own power reading and the crank or pedal power meter on your bike do not always agree. If your trainer app can follow the bike’s power meter, use that for both rides.

Indoor rides in Transition

Every bike session Transition plans can be ridden indoors. With Indoor Bike Workouts switched on in the app’s settings, the app turns the session into power targets set as a percentage of your FTP and controls a smart trainer over Bluetooth. Riding a session indoors from the app keeps its planned steps and duration. With Zwift connected, you can also send a session to Zwift from its workout page when it starts within the next 24 hours. If you want it shorter on a busy evening, tell the coach how long you have and it can rework the session to fit.

Common questions

How long should I ride indoors to replace an outdoor ride?: It depends on which yardstick you match and the steady power you hold. To match the work, multiply the outdoor ride’s kilojoules by 1,000 and divide by your indoor power in watts to get seconds: 1,728 kJ at 185 W is about 9,340 seconds, or 2 h 36 min. To match the training load (TSS), with the same FTP scoring both rides, multiply the outdoor moving time by the ratio of the outdoor normalized power to your indoor power, squared. Holding the outdoor ride’s normalized power indoors, that is the full outdoor time. No fixed multiplier gives both.
Is an hour on the trainer worth 1.5 hours outside?: Only at one particular steady power, and it differs between yardsticks. To match the work, hold 1.5 times the outdoor ride’s average power with coasting included: 240 W for a ride averaging 160 W. To match the TSS, with the same FTP scoring both rides, hold about 1.225 times (the square root of 1.5) its normalized power: about 227 W for a normalized power of 185 W. The 1.2, 1.5 and halve-it conversions come from a coaching company, a coaching column and a cycling magazine, none of which gives a source for its figure.
Does an indoor ride score more TSS than the same time outside?: Not at the same normalized power and FTP. TSS is calculated from duration, normalized power and FTP wherever you ride, and an hour at FTP scores 100. A trainer ride scores more TSS per hour than an outdoor ride only if its normalized power, which for a steady ride is the power you hold, is higher than the outdoor ride’s, or if a lower FTP is used to score it.
Why is my heart rate higher on the trainer at the same power?: Heat is a likely reason: indoors there is no airflow from your own speed. In a laboratory study at 30 °C, 8 men cycling at a fixed 70% of peak oxygen uptake had higher heart rate, core temperature and perceived exertion with no airflow, and reached exhaustion after 41 minutes on average against 90 minutes with a 30 km/h airflow. A fan aimed at you is the simplest fix. If the trainer and the bike use different power meters, the same number of watts may not be the same effort either.
Do cyclists produce less power indoors at the same effort?: Not always. In a maximal 20-minute time trial, 20 competitive cyclists averaged almost the same power indoors and outdoors (280 W against 284 W). But when 12 cyclists were asked to ride 40 km at the same perceived effort, they averaged 163 W in a laboratory with no airflow and 208 W outdoors, with similar ratings of effort and warmer skin indoors. Riding to a power target, for example with the trainer in ERG mode, with a fan running, removes the guesswork.
Should I shorten my long ride if I train indoors for an IRONMAN?: Matching the TSS in less time means riding above the outdoor ride’s normalized power, and no study shows that a shorter, harder ride builds durability, the ability to hold power late in a long session, as the long ride does. When the long ride is the key session of the week, keep at least the trainer time that matches the outdoor ride’s work in kilojoules, and as much of the planned duration as you can. That is coaching judgment, not a trial result.

Sources

Jeffries O, Waldron M, Patterson SD, Galna B. An analysis of variability in power output during indoor and outdoor cycling time trials. Int J Sports Physiol Perform 2019;14(9):1273-1279.20 competitive cyclists, maximal 20-minute time trials on a laboratory trainer and a closed circuit with the same power meter: mean power 280 W against 284 W, within-rider SD 33 W against 69 W.
Mieras ME, Heesch MW, Slivka DR. Physiological and psychological responses to outdoor vs. laboratory cycling. J Strength Cond Res 2014;28(8):2324-2329.12 recreationally trained cyclists, 40 km at the same perceived effort: 163 W in a laboratory with no wind against 208 W outdoors.
Otani H, Kaya M, Tamaki A, Watson P, Maughan RJ. Air velocity influences thermoregulation and endurance exercise capacity in the heat. Appl Physiol Nutr Metab 2018;43(2):131-138.8 men at 30 °C and 70% of peak oxygen uptake: time to exhaustion fell from 90 minutes with a 30 km/h airflow to 41 minutes with none.
Maunder E, Seiler S, Mildenhall MJ, Kilding AE, Plews DJ. The importance of “durability” in the physiological profiling of endurance athletes. Sports Med 2021;51(8):1619-1628.Review arguing that physiological profiles shift during prolonged exercise and that this durability should be measured.
Coggan A. Normalized Power, Intensity Factor and Training Stress Score. TrainingPeaks.Definitions of NP, IF and TSS, and the caveat that TSS ranges are an approximate guide.
Wahoo Fitness. By the numbers: what is TSS?The TSS formula, (seconds × NP × IF) / (FTP × 3600) × 100, and 100 TSS for an hour at FTP.
Slowtwitch forum. Trainer time vs road time (2006).Quotes a Carmichael Training Systems rule of thumb: an indoor session equates to an outdoor ride 20% longer.
Road Bike Rider. How do I equate indoor training time with riding outside?Coaching column: an hour inside equals 90 minutes on the road.
We Love Cycling. The differences between indoor vs outdoor cycling summarised (2022).Advises cutting usual training time roughly in half indoors, without a source for that figure, and calls 10–15% of outdoor riding time spent coasting normal.