If you have a heavier door that weighs more than 150 pounds with one spring still wound you will need to use 2 scales and position them under the first row of hinges from each end of the garage door.
How much wight garage door.
Based on this rating a 1 2 hp opener be able to lift about 185 pounds and a 3 4 hp opener should be able to lift over 370 pounds.
Like many other motors garage door openers are rated with horsepower hp or horsepower similar hps for dc motors.
Garage door weight is determined by size material steel gauge thickness insulation and glass.
Therefore you need a pair of 210 pound springs even though your total door weight is 210 pounds.
It should be noted that since the cable loops around a pulley the lifting force of a 210 pound spring is only 105 pounds as seen by the garage door.
The average modern steel single car door weighs around 125 pounds while double car doors weigh about 200 pounds.