According to Gruber:
'One naturally thinks that a seven-inch screen would offer 70 percent of the benefits of a 10-inch screen. Unfortunately, this is far from the truth. The screen measurements are diagonal, so that a seven-inch screen is only 45 percent as large as iPad’s 10-inch screen. You heard me right: just 45 percent as large.... 7.85 inches is closer to 8 inches than 7, and that the exact size of the purported iPad Mini display offers 66 percent of the surface area of a 9.7-inch iPad, not 45 percent."
Gruber's math appears to be off here...here is my math.
- 9.7-inch iPad has dimensions of 5.82-in by 7.76-in or 45.16 square inches
- 7-inch iPad has dimensions of 4.2-in by 5.6-in or 23.52 square inches
- 7.85-inch iPad has dimensions of 4.71-in by 6.28-in or 29.58 square inches
The 7.85-inch iPad would be 65.49% of the original iPad surface area
So the "45 percent as large" from Grubers post is incorrect.