Select at least 3/4-inch copper pipe for your water supply. This will ensure you get optimal water pressure. If your water supply comes in at low pressure consider putting in a 1-inch copper pipe. To check the water supply pressure ask the local water company for a water supply pressure reading.
Use 1/2-inch copper pipes for the bathroom. Use this size for all fixtures in the bathroom. This includes the toilet, sink, bathtub, shower and any other fixture installed.
Install 3/4-inch copper pipes for the kitchen sink and any other fixtures in the kitchen.
Use 3/4-inch copper pipes for the water heater and any water tank you might have. It is important that you do not use a smaller pipe for the water heater as this will decrease the pressure to the hot water in the whole house if you use a smaller size.
Use 1/2-inch copper pipe for all other fixtures you will add to your house. This can include washing machines, outside water hoses and extra sinks in the house.