How the numbers are worked out
Volume is length × width × depth for a rectangular planter and π × radius² × depth for a round one, using inside measurements. One liter is 1,000 cubic centimeters, one cubic foot is about 28.3 liters, one US gallon about 3.79 liters and one US dry quart about 1.10 liters. Bags are rounded up to whole bags.
The weight figures use the numbers in our guide to a balcony raised garden bed: University of Maryland Extension puts soilless potting mix at about 10 pounds per cubic foot dry, and a thoroughly watered mix can approach 60 pounds per cubic foot, roughly 1 kilogram per liter. The wet figure is an upper estimate built from those sources, not a measurement of your mix.
Watch out: The weight shown is the soil only. The container, plants, water in a saucer and people standing nearby add more. On a balcony or roof, ask your building manager or landlord what load the structure is designed for before you fill large planters.