One US gallon is exactly 3.785411784 litres; one UK imperial gallon is exactly 4.54609 litres. To convert a per-gallon price into the same currency per litre, divide by the matching gallon size. For example, 4.00 per US gallon is about 1.057 per litre, while 4.00 per UK gallon is about 0.880 per litre. Trip cost is fuel volume multiplied by a price for that same volume unit; this unit conversion does not convert currencies.
Match the denominator
If fuel required is 36 litres and the price is €1.65 per litre, multiply directly to get €59.40. If the displayed station price is per gallon, do not use the same multiplication. Convert the price to per litre or convert the required volume to the same gallon.
Name the gallon standard
Divide a US-gallon price by 3.785411784 to obtain price per litre. Divide a UK-gallon price by 4.54609. A bare word “gallon” is not enough for a reproducible estimate. Use the market and label on the pump or receipt to select the standard.
Keep currency separate from conversion
The currency label identifies the result but does not perform an exchange-rate conversion. Enter the fuel price in the currency you want the answer to use. For cross-border planning, obtain a current price and exchange rate separately, then document the date and assumptions behind the estimate.