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. I feel your pain, and I’m here to help!
Today’s Power 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 uncalibrated trainers or trainers with known accuracy issues. And we also have riders on “dumb trainer” setups which use Zwift’s virtual power algorithms to calculate power numbers–and those numbers may be wildly inaccurate due to simple mistakes like under-tightening the roller or having low tire pressure.
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, it covers two quick methods to help you figure out 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 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. The last one I did (July 2018) targetted 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 wattage required for you to hold particular speeds outdoors:
- 25 kmph (15 mph) – 92 watts
- 30 kmph (18-19 mph) – 143 watts
- 35 kmph (21-22 mph) – 212 watts
- 37.5 kmph (23-24 mph) – 254 watts
- 40 kmph (24-25 mph) – 301 watts
- 45 kmph (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!
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 were taken from bikecalculator.com, and compared with kreuzotter.de to check for accuracy.
- Obviously, speeds can vary greatly if you are in a more 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, 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.)
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.