The system of rating cooling in tons goes back a very long time and is very confusing to the unfamiliar...
Back in the days before electricity, homes and food were cooled using blocks of ice harvested from lakes and rivers during the winter months. This was a big and very lucrative business for many at the time. People back then liked being cool in the summer as much as you and I today.
This is where we first hear the term "tons" for cooling. This term refers to how much ice can be melted in a 24 hour period. Melted? Yes, melted. Air conditioners do not actually cool a space but instead they remove heat from the space, leaving it cooler as a result.
This cooling is measured in British Thermal Units (BTUs). One British Thermal Unit represents the amount of energy to raise the temperature or one pound of water by one degree (Fahrenheit) (at sea level).
A one ton air conditioner can remove 12,000 BTU per hour.
There is a very simple calculation that you can do to determine how large an air conditioner that you need (in tons).
Depending on how well your home is insulated, how good your windows retain cold/heat and if any shade is provided by trees and bushes, the formula to calculate the tonnage that you need is:
The square footage that you want to cool (your homes square footage). Multiply this number by 25. This will give you the total number of BTUs that you will need to cool your home.
Once you have your BTU requirement calculated, divide this number by 12,000 to get the number of tons that you will need. (Remember a one ton air conditioner can remove 12,000 BTU per hour).
A 2000 square foot home time 25 needs 50,000 BTU.
50,000 BTU divided by 12,000 equals 4.16 tons (round down to 4 tons)
If you have an air conditioner that is smaller than recommended by the above formula, do not be concerned. It is quite common for HVAC companies to install air conditioners that are slightly smaller than recommended. Why do they do that? Well, the theory is that a smaller air conditioner will run more often reducing the buildup of condensation therefore reducing rust and rot in the air conditioner.
We hope that this clears up and questions that you had about air conditioner ratings and tons...