The overall accuracy per each filling of the bottle is approx. one to two fluid ounces and the accuracy of each individual measurement is about one fluid ounce, meaning that if you had two drinks of 5 fl oz, it could be recorded as 4 fl oz and 6 fl oz (each measurement will have a certain degree of error, however, as mentioned above the sum of errors per each filling of the bottle is expected to be in the range of two ounces).
The H2OPal tracker does need to be properly calibrated to ensure accurate results.
It needs to be recalibrated if you're switching to a different bottle.
We also recommend calibration every few months, of if you notice that the measurements aren't accurate, or if your tracker has experienced a significant physical impact. You can learn more about how to recalibrate your H2OPal here: https://outofgalaxyinc.zendesk.com/hc/en-us/articles/115000842168-Recalibrating-H2OPal
In more scientific terms, the precision of any individual measurement is approx. one fluid ounce (40 ml). The accuracy, based on conducting over a hundred measurements on a recently calibrated H2OPal is also within one fluid ounce (30 ml).
The following is an article about how to use H2OPal for better accuracy:
Articles about how measurements and syncing work in H2OPal: