Use singular verbs with units. This usage is so because the measurement results in a quantity of a substance, e.g., 2 litres (better written as 2 L) is a quantity of water with the water still being a non-count noun. To avoid this problem, use the measured quantity as the object of verb as illustrated below:
a. Add 2 L of water and stir.
b. Tie 10 m of the rope around the neck of the animal and tether it.
Never write the word name of a unit of measure after a number to represent the result of measurement, e.g., 5 metres is incorrect. Wrte 5 m. However, when used as an adjective, both the symbol and word name are correct, e.g., 5 km journey and 5-kilometre journey. Do not insert a hyphen when the unit symbol is used.