What equation does the Instinct 2 use for resting calories?

The equation was never published, afaik. Just do not mistake the Resting Calories for BMR (Basal Metabolic Rate). It is rather the RMR (Resting Metabloic Rate), which is slightly higher than BMR. Some online RMR calculators indeed show lower RMR than Garmin, but there ae also some of them estimating it higher. For example the RMR calculated at RMR Calculator | Resting Metabolic Rate Calculator (omnicalculator.com) shows slightly higher than what Garmin uses. The difference in my case is just 40 Calories though. Their calculation is based on the Harris/Benedict algorithm.

The only official information from Garmin about the RMR is at Calorie Terminology | Garmin Customer Support, which may explain why the Resting Calories are slightly higher than the RMR shown by other sources. 

Resting Calories

Your resting calories are calculated using an estimate of your Resting Metabolic Rate (RMR). This is the number of calories you burn while performing basic functions like breathing and circulation. RMR is based on your age, height, weight and gender. To determine your resting calories, we slightly increase your RMR estimate to account for sedentary to light movement throughout the day.

I tried the calculator link above and my Garmin resting calories are 260 more than that calculator. 
The calories are much higher than my old fitbit charge. I have been testing both watches out on each wrist and garmin seems significantly higher.

6171885 said:

The calories are much higher than my old fitbit charge. I have been testing both watches out on each wrist and garmin seems significantly higher.

Resting Calories have nothing to do with the device. Those are not measured by your Garmin Instinct. Resting Calories are calculated from your age, gender, weigh, height, and possibly slightly corrected by your activity class. It is a constant value (as long as the entry parameters do not change). So if you Garmin Connect shows too high Calories, then it has little to do with the device. I'd tell some of the entry parameters may be wrong.

Only Active Calories are being measured by the device, and depend greatly on your Resting HR, Max HR, and the HR throughout the day. Other factors have some influence too.

I understand that resting calories is a fixed not variable.

I have checked that the same data, gender, age, height and weight are correct. Yet the difference between online calculators and what garmin says is 260 cal. Activity level set to 0. I consider that a big difference.

The garmin total seems to multiply the figure from the above calculator by around 1.17. This is with activity class set to 0.

6171885 said:

The garmin total seems to multiply the figure from the above calculator by around 1.17.

Not in my case. The calculator shows the RMR 40 Calories higher than Garmin.

Firmware on it is 11.18

I will contact support… see what they can tell me

Attualmente e stato aumentato il valore del’rmr oppure l’instinct 2 mantiene questa differenza di calcolo (circa 200 kcal in meno rispetto ad altri garmin)?