People asked me about what the closing ratio is and what it tells us. Closing ratio is defined as the cross sectional area of the ram piston (cylinder) divided by the cross sectional area of the ram shaft. The closing ratio is used to determine Ram closing pressure which will overcome wellbore pressure acting to Ram body.
Closing Ratio = Ram Piston Area ÷ Ram Shaft Area
Before going into the detailed calculation, we would like to show you where the cylinder and the ram shaft are in BOP. In Figure 1, the yellow shaded parts demonstrate these two areas which will be used to calculate the closing ratio.
Figure 1 – Shaffer SL-Ram BOP
Example: Ram has a piston cylinder of 12 inch and 4” of ram shaft (see Figure 2).
Ram piston area = (π x 122 ) ÷ 4 = 113.1 square inch
Ram shaft area = (π x 42 ) ÷ 4 = 12.6 square inch
Closing Ratio = 113.1 ÷ 12.6 = 9.0
Figure 2 – Basic Diagram of Rams
How To Use Closing Ratio To Determine Minimum Operating Pressure
When you know the closing pressure of the BOP ram, you can use the figure to determine the minimum operating pressure. The following equation is used to determine the minimum operating pressure from the accumulator unit (koomey).
Minimum Operating Pressure = Working Pressure ÷ Closing Ratio
Example: What is the minimum operating pressure would be needed to close the ram against 10,000 psi maximum anticipated pressure on BOP? Please use the ram details from the example above.
Minimum Operating Pressure = 10,000 ÷ 9 = 1,111 psi
With operating pressure of 1,111 psi, hydraulic force will equal to force acting from the wellbore in this case (see Figure 3).
Figure 3 – Force Acting at Ram Shaft and Force At Piston
In this case, a standard accumulator (3,000 psi system) with minimum operating pressure of 1,200 psi is good enough to shut the well in with 10,000 psi surface pressure.
Reference book: 
Well Control Books