Backhoe Bucket

How to calculate required GPM for hydraulic pump?

Looking to get a PTO driven hydraulic pump to run a 3pt backhoe on my tractor. How do I go about figuring out what size I need. The backhoe has fairly large cylinders on it. They are may be 2x larger than the ones on the loader which runs off the internal system and it operates very slow. It seems that common pto pumps come in 11 or 21 GPM. That seems to be quite a spread so I would like to get the correct one the first time. The tractor is rated at 40hp so I should have plenty of power at any RPM.

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