Are your power numbers accurate? Take a quick test.

Are your power numbers accurate? Take a quick test.

For many cyclists, Zwift is their first experience of riding “with power”–that is, seeing a wattage reading for their effort. And they have no idea if what they’re seeing is way off or absolutely accurate, and if that number means they are super strong or very weak.

We feel your pain, and we’re here to help!

Today’s Accuracy Outlook

Smart trainers and power meters are improving in terms of accuracy and reliability as the technology matures, and I think the vast majority of Zwifters enjoy acceptably accurate power numbers today.

But we still see riders on “classic” (or “dumb”) trainers which use Zwift’s virtual power algorithms to calculate power numbers–and those numbers may be inaccurate due to simple mistakes like under-tightening the roller or having low tire pressure. Additionally, other riders may be using uncalibrated trainers or trainers with known accuracy issues.

How to Test

The only way to test the accuracy of your power numbers precisely is to compare them with a power meter you know is completely accurate. This article doesn’t cover this sort of testing.

Instead, let’s discuss two simple ways you can learn if your power numbers are at least in the neighborhood of being accurate.

Test #1: The Outdoor Speed Test

The easiest way to approximate your FTP is to look at your solo outdoor efforts on a flat stretch of paved road. Ideally, you would do a one-hour out and back effort on the flattest roads possible with as little wind as possible. Flat roads are essential for this because when the road is flat, your weight doesn’t affect your speed much once you’ve accelerated to “cruising speed.”

Sidenote: I actually enjoy doing these rides a few times each year, I call them my “one-hour challenge” rides, and it’s a fun test to see how much stronger I’ve become since my last attempt. Here’s one I did July 2018 targetting 24 miles in the hour, and I made it by just two seconds according to my trusty Wahoo Elemnt.

Even if you can’t do an all-out solo hour on flat roads, you should know by now what kind of speed you can hold by yourself outside on a flat road with no major winds for a longer stretch of time (20+ minutes).

Here are rough estimates of the power needed for a solo road cyclist to hold various speeds:

  • 25kph (15 mph) – 92 watts
  • 30kph (18-19 mph) – 143 watts
  • 35kph (21-22 mph) – 212 watts
  • 37.5kph (23-24 mph) – 254 watts
  • 40kph (24-25 mph) – 301 watts
  • 45kph (28 mph) – 415 watts

So if you’re unsure if your power numbers are accurate in Zwift, just find the speed above that you can hold outdoors on a flat ride going all-out for 20+ minutes. Then see if that wattage matches what you see in Zwift when you’re giving it everything you’ve got for that same amount of time.

Disclaimer on the Numbers

  • Of course, the numbers above assume you aren’t extremely lightweight or overweight, or extremely short (or tall). They will be most accurate for a person in the 60-80kg range, and of average height.
  • The numbers above are an average from several bike speed calculators to check for accuracy. Each calculator computes speed a bit differently, but if you want to calculate numbers for your own weight/height, we recommend giving kreuzotter.de a try.
  • Speeds can vary greatly if you are in a more or less aero position. The numbers above were done assuming riding in the drops on a standard road bike setup (not TT).

Test #2: The Sniff Test

The sniff test is much simpler than the outdoor speed test. But it’s also less scientific.

If you aren’t a trained athlete (let’s say you’re in your first year or two of serious cycling) and you are of average size, chances are very slim that your FTP is anywhere above 300 (for males). Chances are it’s close to 200, and maybe 250 if you have good genes or have been training for several months. Many riders will be significantly lower than 200 at this phase, especially if you are an older rider. And that’s OK too!

The important thing to understand is that you certainly won’t have an FTP above ~325 at this phase in your training, which is why any time a newbie rider posts about such an FTP increase in on Facebook, folks laugh it off.

It doesn’t pass the sniff test.

Pro male riders often have FTPs in the mid to high 300s, but these people have been training hard for hours per day, for years on end. If you’ve only been riding a year or two and you weren’t a serious endurance athlete beforehand, it will take you at least a few years to get into anything touching pro territory. (And even then, once you get there you’ll realize you’ve got a long way to go.)

Got Problems? Here are the solutions.

If you believe your power numbers are inaccurate, there are several ways to get them fixed:

  • If you are on a classic trainer, make sure you’re following the manufacturer’s setup instructions regarding how tight the roller should be against your wheel. This is essential, since Zwift’s calculated power numbers assume you are following these instructions.
  • If you are on a smart trainer, make sure your trainer is properly calibrated. Using the manufacturer’s suggested calibration procedure (usually via a simple phone app) is the way to go.
  • Consider an upgrade! Today’s direct-drive smart trainers bring a high-quality Zwift experience within reach of more riders than ever before. See our 2019 Smart Trainer Recommendations Guide >

Conclusion

That’s it–two ways to quickly see if your numbers are at least somewhat accurate. I hope this helps some folks out!

Comment below if you have questions, or if this post has helped you figure out your power situation.

About The Author

Eric Schlange

Eric runs Zwift Insider in his spare time when he isn't on the bike or managing various business interests. He lives in Northern California with his beautiful wife, two kids and dog. Follow on Strava

Subscribe
Notify of
guest
24 Comments
oldest
newest most voted
Inline Feedbacks
View all comments
Mike Kauspedas
Trusted Member
Mike Kauspedas (@mike)
1 year ago

Something to consider. Roller on wheel trainers give you a mighty advantage. If you want closer to a real world outdoor experience, get a direct drive. It feels more like riding outside, especially if you crank the trainer difficulty up in Zwift.

Noroot
Noroot
1 year ago

Regarding the pro-FTP comparison: one has also to take into account how very lightweight pros usually are – we have to be talking W per kg… even if you reach absolute numbers in the 300eds, depending on your weight that can still be far far off…

Niels
Trusted Member
Niels (@kaasineenvaas)
1 year ago

I have some problems with this article. Here’s what I found: Pros have FTPs in excess of 400w varying by weight and height. Tom Dumoulin has stated in a podcast(live slow ride fast dating to the 1st of January 2020) that his 20min power output is 465W. Apply the usual formula of FTP=20min*0,95 and you get an FTP of 441,75w. So 442W if we round it up. This is WAY higher than what is stated in this article(mid 300s). It would be ignorant to call Tom Dumoulin an average pro. But the sheer difference with the number in this article… Read more »

George Homer
George Homer (@pass_a_bud_a_bud)
1 year ago
Reply to  Niels

I totally agree…pros reach way over 300watts I’ve seen pros on Zwift doing into the 500s not only mid 400s….Zwift is way off of being accurate…I’m.pushing 423watts of http://Ftp…I‘m.a bike tourist use to moving close to 300 pounds of cargo and bike….300 watts on a light road bike is nothing to me….this article as you stated in your comment is way off! LoL

Jan Večeřa
Jan Večeřa
1 year ago
Reply to  George Homer

Halo, i agree with you, but…. I weight 94 kg… I am amateur cyclist 41yo, riding for 3 years. My FTP Is 420watts(different methods approved). The point Is, that what matters most in Speed above 30km/Hours Is Wind resistence… And the difference in someone 70kg in aero position And 95kg 1.9m height on hoods Is massive when Cda may go from 0,2 to 0,45 which changes everything. The power
necessary to rise the Speed Is related to the 3rd exp of the Speed. Physics can’t be ruled out And beaten…. Wish you good luck And endorphines. Jan

Niels
Niels (@kaasineenvaas)
1 year ago
Reply to  Eric Schlange

On your first point: I am sure I mentioned that in my original comment and explained what the levels of those riders usually are and what I have found with riders in my area and punt them into context well enough. Hence why I stated that they are my findings and so yours may differ. The definition of pro varies from country to country and from person to person. But I see UCI continental as pro and baseline elite as semi-pro. All of which is explained in my original comment. World tour pros exceed the benchmark set in the article,… Read more »

Jan Večeřa
Jan Večeřa
1 year ago
Reply to  Eric Schlange

Hi Eric, i am that athlet you ask for…. See my response no. 30875. Zwift Is an amazing Tool, i Wish you And your team all the best And thank you. I understood the importance of power measurement in cycling immediatelly , the simplicity And versatility And validity of watts Is game changing even for hobbies. Best Regards Jan Večeřa , Czech republic

Mark E
Mark E
1 year ago

I have a kickr snap. I had done the short ftp test in November and got an ftp of 183 which was a little disappointing given the riding I had been doing all summer and fall, but I was pretty tired when I did it. Come December and the tour of London events, I was treating those like races, going all out and I saw an ftp increase to 274. I took my bike off to lube the chain at the beginning of the month and saw could barely maintain 150 Watts for any period of time. I had done… Read more »

George Homer
George Homer (@pass_a_bud_a_bud)
1 year ago
Reply to  Eric Schlange

Nothing is accurate on Zwift since no braking is needed…in real life we would all be dead riding as we do on Zwift lol

George Homer
George Homer (@pass_a_bud_a_bud)
1 year ago

Because of the fact there is no braking on Zwift at all no measurements will even come close to the real world….this is nothing more then a video game for athletes so we dont get bored while training alone at home! Stop trying to make people believe something Zwift can never be! Zwift will never be an accurate measurement of power or speed ! LoL Keep dreaming! LoL

North Krimsly
North Krimsly
1 year ago

I wish there was a high-accuracy power meter we could rent and use concurrently with whatever power meter we use for daily rides. Then we could compare meter-to-meter, and check accuracy. Next would be a power meter standard such that our daily driver power meter could be re-calibrated against the reference meter so we would have accuracy from then on.

Tony
Tony
1 year ago

Thanks for the article, interesting read. As a newbie to zwift, where can I find graphs like the ones in the banner (power over time especially?)
Thanks

Lim
Lim
9 months ago

My virtual power on indoor dump trainer is 60 watts higher than my crank-based power meter on road. My indoor trainer is calibrated for an average rider weighting 74kg riding on a 1% gradient. I am 65kg. To me, it is like riding on a 2% gradient using the indoor trainer. My indoor trainer speed hardly goes above 32km/h when I am putting out 250w. How to explain the big difference in wattage between indoor and outdoor riding?

stuart
stuart
9 months ago

Okay, 270 inside is a struggle, 300 outside is easy. Done the calibration etc….. Just put down to zwift. I can’t get to max power inside, but can outside. Okay, speed… 90km outside is do able, 80km inside is big wattage. On a Tacx flux2 with all updates done.

Keith Ainsworth
Keith Ainsworth (@keitha99)
9 months ago
Reply to  stuart

Interesting Stuart – my FTP using a Stages outdoor is 340….indoor using Zwift on my basic rollers it is 330 approx (acceptable) – however using my Stages as the power source on my spanking new Neo 2T with same bike it’s more like 300w….do I have a problem with the “Smart” trainer? P*ssing me off, as of course it is screwing up my downloaded Training Peaks workouts to Zwift that my coach sets…I’ve done two TT “races” now…one on the Neo and one using my rollers….of course my performance was far better on the rollers but using the same power… Read more »

kevin blades
kevin blades
6 months ago

your power meters are probably single sided units so will be well off. @ keith your neo should be more accurate than your pm, i suspect stages will be off by 10% as that is a single sided unit. why does everyone always assume the lower figure cant possibly be correct?? some neo 2T have know to be off tho so could be a combination of the two. only way to know for sure is to source a set of dual reading pedals and do some comparisons to those. for clarity my single sided inpower crank gave me 25 to… Read more »

Mark Evans
Mark Evans
7 months ago

I have just upgraded my trainer from a wheel on Elite Qubo to a Kickr core……my last ftp test was 178w and was quite hard work, yet now riding on the kickr core 178w seems a doddle……was my previous trainer just wholly inaccurate and now I am experiencing a true power reading?

Roland Maatouk
Roland Maatouk
6 months ago

I got a reality check when upgrading from wheel on JetBlack Z1 Pro to a Kickr 18. Now I have to accept the fact my power output is about 20% less (from ~210 down to ~170) than wheel on. Doing more analysis on outdoor rides and “Strava Power”, however its calculated, appears to be more in line with what I’m seeing on my Kickr. Not looking forward to my next FTP 🙂

24
0
Would love your thoughts, please comment.x
()
x