Matt told me what this was today but I couldn't remember so I looked it up after I couldn't find it here. I imagine a lot of people that move here wonder as it's definitely noticeable.
The formula is Horsepower -(Elevation *.03 * Horsepower / 1000) with the part in parenthesis being the power loss.
So my ~65hp KTM is making the sea level equivalent of 55hp at 5,280' and only 42hp at 11,991'. My ~190hp BMW was making 160hp at 5,280 and 121hp at 11,991.
Or you can use this calculator