If you do a Google search for the brand you have the mfg. should have the specs available.
For example, here is the diffuser I am using A single will handle 1.4-4.2 cfm, double will be double that for this particular diffuser. You should be able to find the same type info for your particular unit.
Keep in mind, your pump output will vary with the depth of water. Some pumps a lot (linear air pumps), others not so much (rocking piston or vane) depending on design. Your pump should have a performance curve also to tell you the output at a given back pressure (depth).
