The water pressure of a home or commercial building is measured in Pounds-Per-Square-Inch (psi). Plumbing fixtures operate most efficiently between 30-80 psi. You can expect problems outside of that range…
 

Too Little Pressure (less than 30 psi)

Your plumbing does not hold pressure in the pipes. Low water pressure while taking showers or when running more than one fixture or appliance at once are among the most common signs.
Common Causes: MISSING/FAILING PRESSURE REGULATOR, MAIN WATER VALVE NOT OPEN COMPLETELY, BUILD-UP IN PIPES, LEAKING PIPES, MUNICIPAL WATER SUPPLY PROBLEMS

Too Much Pressure (greater than 80 psi)

The pressure in your pipes is building up. This can cause unnecessary stress on your plumbing which could result in breaks/leaks. It also poses a danger to your water heater causing it to rely heavily on its TPR Valve. Aside from these dangers, too much pressure can result in the waste of up to 10,000 gallons per year!
Common Causes: MISSING/FAILING PRESSURE REGULATOR, MUNICIPAL WATER SUPPLY PROBLEMS

DIY – There are two ways you can check your water pressure yourself…

The most accurate method is to buy a pressure gauge from your local hardware store and hook it up to a hose faucet. Check the pressure when all other faucets and water-using appliances are turned off to get a baseline reading. In general, you want the household plumbing to provide between 30 and 80 psi.

There’s an even simpler way to check for low water pressure. Simply turn on the shower and sink faucets in a bathroom, then flush the toilet. Watch the water flow in the shower – if it appears to drop significantly when the toilet is filling, you can assume that your home’s water pressure is low.