When it comes to measuring heart rate and calories burned, not all fitness trackers are created equal, based on the findings of a scientific study that pitted the top wearables on the market against each other.

The in-depth study conducted by researchers at Stanford University was published today, revealing that Apple Watch is the most accurate wrist-worn tracker on the market, but it’s still far from perfect.

In their paper titled “Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort,” researchers evaluated the Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn, and Samsung Gear S2.

Researchers concluded that Apple Watch achieved the lowest overall errors when measuring both heart rate and energy expended. Samsung did the worse overall with the Gear S2 reporting the highest amount of errors for both fields.

The study found that the wrist-worn trackers had the lowest error rate when a person was cycling. Walking provided the highest amount of errors.

Six of the seven devices tested all achieved a median heart rate error below 5%. Tracking calories burned is much tricker though. Not a single device tested had an error rate below 20 percent, so if you’re trying to use your Apple Watch or FitBit to count calories, you probably need to find a more accurate method.