For standard spur gears (based on tooth fillet radius = 0.35/P) From AGMA Information Sheet 225.01.
Number of teeth in mating gear
 
Number of teeth, N (a) 20-degree full-depth
| where: | 
| A - Load applied at highest point of single-tooth contact (sharing). | 
| Load applied at tip of tooth (no sharing). | 
Number of teeth in mating gear
 
Number of teeth, N (b) 25-degree full-depth
| A - Load applied at highest point of single-tooth contact (sharing). | 
| Load applied at tip of tooth (no sharing). |