Wondering whether your watts are real? Power meter accuracy is usually claimed at ±1% to ±2%, but lab testing found some individual units off by more than 5%.
This guide sits under our wider overview of cycling power meters.
| Item | Value |
|---|---|
| Claimed accuracy, Favero Assioma pedals | ±1% |
| Power meters in the 2017 lab study | 54 units, 23 models, nine manufacturers |
| Units that deviated by more than 5% | Six |
| Change needed to trust a difference with the worst meter | At least 2.8% |
| Assioma pedals versus a KICKR 5 trainer, one reviewer | 1.5-2.5% higher |
Figures come from Cycling Weekly’s report on the study and a Zwift Insider review. Specifications and test results can change, so check the source before you rely on them.
What does a power meter’s accuracy claim mean?
A claim such as ±1% is the manufacturer’s stated tolerance. It says the reading should sit within one percent of the true force-and-speed product under defined conditions. The tighter the claim, the smaller that band.
Claims also depend on conditions. A meter that is zeroed, installed correctly and used within its design range should land near its stated figure. A meter that is knocked, badly torqued or left unzeroed may not.
Zwift Insider lists the Favero Assioma pedals at “+-1%” accuracy in its spec summary. The same review gives the Wahoo KICKR 5 trainer as 1% too.
The claim is not a guarantee for every unit you can buy. It describes a design target, and what matters in practice is how far your own unit sits from the truth.
What did independent testing find?
The best-known check is a Swiss laboratory study by Thomas Maier and colleagues, published in the International Journal of Sports Medicine. Cycling Weekly described it as an “Accuracy of cycling power meters against a mathematical model of treadmill cycling” in its May 2017 report.
The researchers tested 54 power meters on a downward-sloping laboratory treadmill, because road or track riding makes reliable measurement difficult. The units covered 23 models from nine manufacturers, including SRM, PowerTap, Quarq, Stages, Verve, power2max, Garmin, Polar and Rotor.
Key findings, as reported by Cycling Weekly:
- Individual power meters deviated a lot, even when units came from the same manufacturer.
- Six units deviated by more than five per cent, including products from Stages, Quarq and power2max.
- Models from long-established makers SRM and PowerTap were among the best.
- A rider using the worst meter would have to change their own performance by at least 2.8 per cent to have reasonable confidence the change was real.
That study is from 2017, so current hardware may behave differently. It is still useful because it shows the spread between units, not just the brand claim.
The report quotes the finding that “Precision is generally high but differs between manufacturers.” In plain terms, a meter usually repeats itself well, yet its average can sit away from the truth.
That distinction between precision and accuracy matters for training. A meter that is consistently a little off is still useful for tracking trends. It is much less useful if you compare files from two different devices or want to match published power targets.
Why do two power meters disagree?
Even if both are within spec, a pair of meters can differ. If the first reads at the top of its tolerance and the second at the bottom, they sit at opposite ends of the band while both stay inside the claim.
That is why a gap between devices is not automatically a fault. It becomes a worry when the gap falls outside the range expected after accounting for both devices’ tolerances and the direction and size of drivetrain losses, or when it changes from ride to ride.
Zwift Insider’s reviewer saw the Assioma pedals average 1.5-2.5% higher than the KICKR 5 trainer. The review argued that was within spec because drivetrain losses can easily amount to 2%.
Common reasons for a gap between devices:
- Measurement location: pedals, cranks, hubs and trainers sit at different points in the drivetrain, so they see different losses.
- Zero offset: an incorrect zero offset can bias readings, so follow your meter’s instructions for manual or automatic zeroing. Our guide to power meter calibration covers what zeroing does.
- Installation: pedals and crank bolts that are not torqued correctly can affect readings.
- Left-only estimates: a one-sided meter doubles one leg, as explained in single sided vs dual sided power meter.
How does temperature affect accuracy?
Most meters use strain gauges, which measure tiny flexes in metal. Metal also expands and contracts with heat, so temperature can disturb the signal.
The maker 4iiii says that when a crank changes temperature it is actually changing length. For aluminium, it states a 0.0023% increase in length for every 1°C, and the bigger the temperature change, the bigger the change in length.
Good meters compensate for this. 4iiii says every unit from its factory is tested under a wide range of temperatures. This is a company statement about its own product, not an independent test.
Sudden weather changes make the issue easier to see. A long ride that starts warm and ends cold exposes the meter to a swing in temperature, which is exactly when compensation is tested. If readings jump after such a change, zero again and compare.
A practical habit helps: zero the meter once it has settled to the temperature you will ride in, not in a warm kitchen before a cold start.
How much accuracy do you actually need?
For most riders, consistency matters more than absolute truth. Training zones, trends and pacing all work well if the same meter reads the same way each ride.
Accuracy matters more when you compare numbers between devices, share targets with a coach, or chase small gains. Cycling Weekly’s report notes that many elite cyclists are happy with an improvement of just one per cent. A meter with a few percent of error can hide that.
Think about what you use the numbers for. A weekly average power trend tolerates a steady offset. A threshold test used to set zones also tolerates it, provided you repeat the test on the same meter.
Problems appear when you switch equipment. Moving from an indoor trainer to outdoor pedals, or from one brand to another, can shift your numbers without any change in fitness. Expect to re-test your threshold after a switch rather than carrying old zones across.
To keep your own data trustworthy:
- Stick to a single meter for all your key tests and training.
- Zero it before each ride, as the maker recommends.
- Re-check after crash, battery change or any hardware service.
- Judge gaps using both devices’ accuracy specifications, measurement locations and test conditions.
Frequently asked questions
Is a ±1% power meter really that accurate?
Show answer
The figure is the manufacturer’s claim. Independent lab work in 2017 found large differences between individual units, so a claim describes the design target rather than every unit.
Why does my trainer read lower than my pedals?
Show answer
They measure at different points. Zwift Insider noted pedals reading 1.5-2.5% higher than a trainer and linked it to drivetrain losses that can reach 2%.
Does cold weather change power readings?
Show answer
It can. Metal changes length with temperature, and 4iiii gives 0.0023% per 1°C for aluminium. Good meters compensate, and zeroing at riding temperature helps.
How big a change is real?
Show answer
Cycling Weekly’s report on the 2017 study said a rider with the worst meter needed a change of at least 2.8 per cent to be reasonably confident it was real.
