Correct me if I’m wrong; your furnace and water heater are discharging exhaust gasses into a common 5” diameter vent pipe, which has been downsized to 4” diameter vent pipe before penetrating the ceiling. This is wrong and dangerous; you not supposed to downsize the vent pipe!
If both of your appliances are natural draft type / with draft hoods (no induced draft motors to force exhaust gases into the vent pipe) you don’t need to use Gama Venting Tables for calculations.
The simple formula to determine the diameter of the vent pipe is 100% of the larger pipe diameter (measured at the draft hood’s collar) plus 50% of the smaller vent pipe. If your furnace uses 4” and WH 3” diameter vent pipe, the minimum required common vent diameter would be 4”+1 ½” = 5 ½” - the closets available size would 6”.
If both of those natural draft appliances require 3” diameter vent pipes, the result of this calculation would be 4 ½” and you can use 5” diameter vent pipe (they don’t make half sizes).
Assuming that your furnace utilizes induced draft motor for venting you’re supposed to use Gama Venting Tables, but the information you provided is not complete to make those calculations. Also, the connector between the water heater, furnace, and that 5” common vent pipe is important, it must be “Y” shaped, look at the picture beneath the video in my article: furnace and water heater venting
For calculations go to this link - Gama tables
and look at the examples starting on page 20
, pick the one that applies to your installation and just change the numbers / use appropriate table listed in that particular example. First pages of Gama tables contain information on how to use them and more info about appliance venting.
If there’s something you don’t understand or if you have any trouble with getting the final number, please let me know. Without those few missing values it looks like you are looking at a 5” diameter vent pipe.
Just click "comment" below if you need more help.