Measure the diameter of the piston of the hydraulic cylinder. If the end of the cylinder has a saddle or other fitting, measure the actual piston diameter, and not the fitting, because the fitting could be larger than the piston.
How to Calculate Hydraulic Cylinder Tonnage
A hydraulic cylinder uses a working fluid to transmit a force. Because liquids are close to incompressible, very large loads can be applied with very little loss. Hydraulic cylinders are used in many applications, from household products to large industrial presses. The maximum load that a hydraulic cylinder can apply is based on the diameter of the cylinder and the pressure capacity of the hydraulic pump.
Instructions

Calculating Hydraulic Cylinder Tonnage

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Calculate the crosssectional area of the piston by squaring the diameter, multiplying the result by pi (3.14), then divide this result by 4. For example, for a piston with a 5 inch diameter, the cross sectional area is (5” * 5” * 3.14)/4 = 19.625 sq. inches.

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Calculate the cylinder tonnage by multiplying the cross sectional area as calculated above by the pressure capacity of the hydraulic pump, as listed in the pump specifications. For example, using the example cylinder above and a 1,000 psi pump, the cylinder capacity is (19.625 sq. inches) *(1,000 psi) = 19,625 lbs. To convert from pounds to tons, divide this result by 2,000 to get 9.8 tons.

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References
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