To convert milliliters to liters, use the conversion ratio of 1 liter = 1,000 milliliters.
The cylinder holds 6,750 milliliters, so:
\[ \text{Liters} = \frac{\text{Milliliters}}{1,000} = \frac{6,750}{1,000} = 6.75 \text{ liters} \]
Therefore, the answer is 6.75.