Abstract: An automatic variable demand flow regulator regulates water flow into a building based on variable supply and demand pressures. The regulator is positioned between a building and a water supply meter for the building. The regulator includes a piston that moves in response to pressure at both the supply side and the demand side of the regulator, and the piston is brought to rest when equilibrium is reached and the supply side and demand side pressures are balanced. When demand is low the piston is in a closed position and flow through the regulator is minimal. When demand increases to a condition where flow through the supply side of the regulator cannot provide equal pressures on both the supply and demand sides, the piston will automatically move toward the demand side to convey additional water to the demand side until the pressures are equalized on both sides of the piston.