One cubic metre is 264.1720523581 US gallons. The pair spans the gap between the metric unit used for water metering and the American unit used for water billing, which is exactly where it matters.
About the units
The cubic metre is the SI unit of volume, equal to a thousand litres. The US gallon is 231 cubic inches, 3.785411784 litres. Neither is a natural subdivision of the other, so the conversion factor is untidy. American water utilities complicate it further by billing in hundreds of cubic feet, abbreviated CCF (C for the Roman hundred, CF for cubic feet), or in thousands of gallons, while the meters themselves may read in either. A CCF is 748.05 US gallons and 2.832 cubic metres, which means an American water bill can involve three different volume units before it reaches a price.
The exact factor
The derivation is 1,000 litres ÷ 3.785411784 = 264.172052358148 US gallons. In imperial gallons a cubic metre is 219.969. Useful anchors: a cubic metre of water weighs one tonne and is 264 US gallons; a typical domestic swimming pool of 50 m³ holds 13,209 US gallons; and a 1,000-litre bulk container is one cubic metre and 264 gallons.
Where you meet this conversion
Water supply and wastewater, irrigation, and industrial process volumes. Pool chemical dosing instructions are written per thousand gallons in American products and per cubic metre in European ones, and the dosing error from confusing them is a factor of 3.79. Fuel storage tanks, tanker capacities and chemical shipments are specified in cubic metres internationally and gallons inside the United States. Agricultural irrigation rates use both, sometimes on the same equipment.