If you're on a well, residential well systems normally cycle between 40-60psi. There is a small pressure gauge at the base of your water holding tank.

On municipal water, the pressure can either be nicely regulated, or it can be all over the place. One friend up in MA had 90psi! The high pressure was killing plumbing fixtures on his street. You can call your water service and ask the psi. But 40-60 is a steady target range.

Valve manufacturers usually publish a flow vs psi chart, similar to the chart below:

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You might find it on a manufacturers website.

You don't need to worry about your 2GPM shower head. The valve you choose will be capable of passing more than 2GPM through to the head, but the shower head itself will restrict the outflow to 2GPM.

Example in the chart below, as PSI increases, the GPM flow out of the tub spout increases since the tub spout has no internal flow restrictor. But the shower head eventually caps out at 2.5GPM, since it's a "2.5GPM shower head".

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Hi LARemodel, for the shower head your plumber might to be able to change/add a flow restrictor to get you to 2.0 gpm. If you want one that has the 2.0 gpm handheld right out of the box, we used the Waterworks Studio Ludlow at 12gpm/2gpm -- it has 2 lever handles but otherwise is floor-mounted, free-standing, with handshower, and the trade discounts off the website prices can be substantial.

Good luck!

Thanks for the responses. I have city water - I checked with my local water department and my water pressure range is 74-84 PSI.

I'm looking at Brizo, Graff, Grohe, Kallista (some of their newer shower heads are 2.0 GPM), and Newport Brass.

I'm used to having a mixer for cold and hot water in the tub, so not sure that I want to go back to splitting it, plus would mean two holes in the floor for a f/s tub filler.

I realize I can have the plumber add a flow restrictor but I think there are enough products to choose from that meet the Calif standard for shower flow rate. At least it gave me an excuse to cut some of the pricey items from my list (Dornbracht, Fantini, Hansgrohe Axor Organic, Hansgrohe Puravida, and Vola).