On our site, all the Field of View (FOV) numbers are the image Horizontal FOV (HFOV), which is the degree width that the lens sees. If you see a lens advertised as having a certain FOV, make sure to see if it's horizontal FOV (HFOV) versus diagonal FOV. Diagonal FOV will always be larger so companies typically mis-advertise a lens as having a much larger FOV than it really does (most customers don't know this so companies get away with it). There are also at least 4 ways to calculate the FOV on a fisheye lens, adding to the confusion.
On the Hero 4/3+ Black camera, the Sony IMX117CQT sensor has a usable full-sensor size of 6.248mm x 4.686mm (we're using this site as reference). This is the dimension used for full-sensor 12MP photos and the 4:3 video modes. All the HFOV numbers on the lenses on our site are based on this 4:3 ratio full-sensor. When shooting in other resolutions (such as 2.7k and 1080p) the lens will have a larger HFOV (smaller capture area means wider FOV). Keep this in mind because lens manufacturers may say a lens has a certain FOV but it may be for a smaller camera sensor. GoPro doesn't necessarily crop the sensor to get the smaller resolution media, so check this article if you're curious about what happens in the different modes (and which is best to use when).
As mentioned above, there are multiple ways to calculate FOV with a fisheye lens: Equisolid Angle Projection, Equidistance Projection, Orthogonal Projection and Stereographic Projection. Each of the calculations use the lenses focal length, which GoPro has not listed anywhere. When doing tests of stock vs our lenses, we feel the stock lens has slightly less FOV (higher mm) than our 2.9mm lens:
Using the 6.248mm x 4.686mm sensor size and a 2.92mm lens: Equisolid Angle Projection = 128.5284012d Equidistance Projection = 122.5972707d Orthogonal Projection = 147.3604278d Stereographic Projection = 112.1030264d