BB-8's dimensions
At this time the only dimension we're reasonably sure of is that the diameter of BB-8's ball in the movie is 506mm, which is 19.92" (inches). If you're dealing in inches then 20" will look fine. I've explained below how I got all the measurements so that if you're making a different sized BB-8, you can still do what I did in order to get the proportions right or as close as you want to.
And here are more details for the head.
How the dimensions were obtained
To get dimensions I start with the one dimension of the actual BB-8 that I know, the diameter of the ball, which is 506mm/19.92". Everything else is gotten by analysing photos as demonstrated below.
Getting major body dimensions
To get the dimensions I first started with the photo below of J.J. Abrams with BB-8 which has BB-8 standing fairly straight on to the camera. I then brought it into a drawing program, assigned labels to the various dimensions and measured them in the drawing program using the arbitrary units the drawing program gave.
Since I knew that the ball's diameter (balld) is 506mm/19.92" I could say that 4.19 units = 506mm. Therefore to get any other dimension I simply multiply the units for that dimension by 506/4.19 for millimeters and 19.92/4.19 for inches. Or by doing the division that means multiply the units for that dimension by 120.76 for millimeters and 4.75 for inches.
If you're making a BB-8 that's a different size then measure the diameter of the ball, then do as I did above and divide that diameter by 4.19. Take that resulting number and multiply it by the head diameter unit below to get the head diameter.
balld: 4.19 units, 19.92", 508mm (ball diameter)
hd: 2.44 units, 11.6", 295mm (head diameter)
Dimensions of BB-8's head details
For the details of the head I started with a photo taken from the Star Wars Celebration in Anaheim in 2015 video where BB-8 walks out on stage. Just as I did for the above photo with J.J. Abrams, I brought BB-8's head photo into a drawing program and took measurements in arbitrary units.
Here are all the head units, as well as the dimensions in inches and millimeters. From the analysis further above of the J.J. Abrams and BB-8 photo I know that the head diameter (hd) is 11.6", 295mm. That means that the 12.70 units above is equivalent to 11.6" or 295mm. So as we did further above, that means for this head photo there are 11.6" per 12.70 units or 11.6/12.70 which is 0.91 inches/unit, or there are 295mm per 12.70 units or 295/12.70 which is 23 mm/unit. Using the same trick as further above, we then multiply all the units except the first one by 0.91 to get their dimensions in inches and by 23 to get their dimensions in mm, resulting in the list below.
hd: 12.70 units, 11.6", 295mm (head diameter)
hr: 6.35 units, 5.8", 146mm (head radius)
hh: 8.57 units, 7.8", 197mm (head height)
hbh: 1.35 units, 1.2", 31mm (head base height)
hch: 0.87 units, 0.8", 20mm (head cone height)
hcbd: 9.71 units, 8.8", 223mm (head cone bottom diameter)
Dimensions of curved things
For the dimensions of things that follow the curve of the ball I used the following technique. Using the example of the width of the orange circles, following the curvature of the ball, I again used a photo taken from the Star Wars Celebration in Anaheim in 2015 video where a circle was more-or-less exactly on one side of the ball. I then brought it into Open Office Draw (many drawing programs will do, or just print it out). I then rotated the photo such that a horizontal line went through the center of the circle. I next drew a line from the center of the ball to the top edge of the circle. The drawing program allowed me to measure the angle between the horizontal line and that edge line, and it was 35°. You can also just hold a protractor up to the screen or on a paper printout in order to measure the angle.
- 2x35=70 degrees
- A full circle is 360 degrees.
- Therefore, each circle is 0.19 (70/360) of the circumference of the ball.
- BB-8's ball 506mm/19.92" in diameter.
- Circumference is pi (π) times the diameter = 3.14159 x 506mm = 1589mm or 3.14158 x 19.92" = 62.58"
- Each circle is therefore 0.19 x 1589mm = 302mm in diameter, following the curvature of the ball or 0.19 x 62.58" = 11.89".
