October 20, 2026 · 5 min read

Power Meter Zero Offset (Calibration) — When and How to Do It

A power meter costs a lot of money, and it can still read a few percent high or low if you skip one step before the ride: the zero offset. Apps and head units usually call it "calibration", but it does something narrower. Here is what actually happens, when it's worth doing and how to read the result.

What a zero offset is

A power meter measures how much the part you push on deforms: the pedal spindle, the crank arm or the spider. Strain gauges do the measuring — sensors whose electrical resistance changes as the metal flexes very slightly. From that deformation the power meter works out torque, and from torque and cadence, power.

The catch is that a strain gauge with no load on it still reads something. That resting value drifts with temperature, after the pedals are tightened, after transport. A zero offset measures that value while nobody is pushing on the pedals. The power meter stores it as its new "zero" and measures power from there.

A zero offset doesn't change the scale of the measurement. How many watts a given deformation stands for is set by the manufacturer at the factory. The zero offset only makes sure the measurement starts from the right point.

Why it matters

A zero error acts like a constant push that isn't there. If the power meter thinks something is already acting on it at rest, it adds that to every pedal stroke. At low power the error is biggest in percentage terms: a few watts out at 120 W in a recovery ride is a noticeable shift. And because your zones, TSS and FTP all come from the same data, the error carries through the whole analysis of the ride.

When to do a zero offset

• Before every ride that counts: an FTP test, a race, an interval session you'll draw conclusions from.
• After a temperature change. A bike taken from a warm flat into the frost, or from a cold garage into the room with your trainer, needs 10–15 minutes for the power meter to reach the surrounding temperature. A zero done straight after the change will keep drifting for the first few kilometres.
• After fitting or moving pedals to another bike, and after any change in tightening torque.
• After transport, a crash or a pedal strike.
• When the data looks odd: a sudden change in left–right balance, power higher or lower than usual at the same heart rate.

Many power meters also have an automatic zero that runs during the ride, for example while you coast without pedalling. It helps, but it doesn't replace a manual zero before the start — especially on a trainer, where you rarely stop pedalling.

How to do it

1. Wake the power meter with a few turns of the cranks and wait for it to reach the surrounding temperature.
2. Unclip and keep your feet off the cranks. The bike can stand on the trainer or lean against a wall.
3. Position the cranks as the manufacturer's manual says. Some systems want the crank arms vertical, others have no such requirement.
4. Run the zero offset in the manufacturer's app or on your head unit. On Garmin Edge head units it's under the power sensor's settings as "Calibrate".
5. Note the result. You'll want it for comparison next time.

WattLog.pro reads power from your meter over Bluetooth but doesn't send the calibration command. Do the zero in the manufacturer's app or on your head unit, then connect the power meter to the app you train with.

How to read the result

After zeroing, the power meter shows a number: the offset value. Its unit and typical range depend on the manufacturer, so the number on its own tells you little. What counts is how it changes between rides:

• A small change from day to day is normal. That's temperature and small differences in fitting.
• A big jump after fitting usually means the pedal isn't tightened to the right torque or something has worked loose.
• A big jump for no reason can mean damage from an impact. That's the time to contact the manufacturer's service.

If your system shows a range of valid values, follow the manual. If it doesn't, note the result after every zero for a week and you'll see your normal range.

A zero offset is not a spindown

Smart trainers have their own calibration, the spindown: you spin the flywheel up, stop pedalling, and the trainer measures how fast it slows down. That compensates for mechanical drag and, on wheel-on trainers, for tyre pressure against the roller. It's a different procedure for a different device, covered in trainer calibration — the spindown. If you use a trainer and power meter pedals together, calibrate both. The difference between them and which one to trust is in power measurement: trainer or pedals.

For the curious: the static test

A zero offset checks the zero point, not the scale. You can check the scale with a static test: set the crank arm horizontal, hang a known weight on the pedal and compare the torque the power meter shows with the calculated value. For a 172.5 mm crank and 20 kg the torque is 20 × 9.81 × 0.1725 ≈ 33.8 Nm. Not every power meter or app shows live torque, so this is mainly a test for crank and spider meters. With pedals it's easier to compare power against a second, trusted source.

Summary

A zero offset takes a few seconds and protects you from an error that flows into your zones, TSS and every conclusion you draw from training. Do it before rides that count, after a temperature change and after every fitting, note the result and react to big jumps. If you have Favero pedals, we cover fitting them step by step in how to install Favero Assioma pedals, and what your leg balance tells you in left–right power balance.

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