To figure out how much soil you actually need, start by measuring the inside of the planter box, not the outside. You need the internal length, internal width, and the usable soil depth. For a rectangular planter, use this formula: length × width × depth = soil volume. Keep all measurements in the same unit, such as feet or meters. For example, a planter that is 4 feet long, 2 feet wide, and 1.5 feet deep has a volume of 4 × 2 × 1.5 = 12 cubic feet. If the box has a false bottom, a built-in reservoir, or a thick drainage layer, measure only the depth that will actually be filled with soil.
Choose the soil depth based on what you plan to grow. Shallow-growing herbs and greens often need about 6–8 inches, most flowers and small vegetables need around 8–12 inches, and larger vegetables such as tomatoes need 12–18 inches or more. Leave about 1–2 inches of space below the rim so you can water without soil spilling over. Then add 5–10% extra soil for settling and compaction after watering. In the 12-cubic-foot example, 10% extra gives you about 13.2 cubic feet. Since bags are sold in fixed sizes, round up: about 14 bags of 1 cubic foot, or 7 bags of 2 cubic feet, depending on the bag size.
If you prefer metric, measure in meters and multiply length × width × depth to get cubic meters, then multiply by 1,000 to get liters. To convert cubic feet to liters, multiply by about 28.32. To convert cubic feet to cubic yards, divide by 27. Also check the weight before filling a balcony or roof planter. A moist container mix can weigh roughly 30–45 pounds per cubic foot, so 13 cubic feet may weigh several hundred pounds. Use a lightweight container or potting mix rather than heavy garden soil, make sure drainage holes work well, and water thoroughly after filling. You will likely need to top off the soil after the first few waterings as it settles.