To convert gallons to liters, you can use the conversion factor that 1 gallon is approximately 3.8 liters.
Given that you have 18 gallons, the calculation would be:
\[ 18 \text{ gallons} \times 3.8 \text{ liters/gallon} = 68.4 \text{ liters} \]
So, 18 gallons is approximately 68.4 liters, rounded to the nearest tenth.