To know how much soil an urban planter box needs, calculate the interior volume of the box: soil volume = interior length × interior width × soil depth. Use the same unit for all three measurements. A 4 ft × 2 ft planter filled to 12 in (1 ft) holds 8 cubic feet of soil. Since 1 cubic yard is 27 cubic feet, that is about 0.30 cubic yards, or roughly 227 liters. If you measure in meters, length × width × depth gives cubic meters; multiply by 1,000 for liters.
Most urban planters should not be filled to the very top. Leave 1–2 inches (2.5–5 cm) of headspace for watering and mulch, and stop the soil about 1 inch below the rim. Add 5–10% extra soil to allow for settling and to fill around roots. For a self-watering planter, follow the manufacturer’s fill line and keep the reservoir separate. Drainage holes are essential; do not use a solid gravel layer unless the planter is designed for it, because it can reduce the soil volume available to roots.
Depth matters more than total volume for plant health. Herbs and shallow-rooted flowers often need 8–12 inches (20–30 cm) of soil; leafy greens and many vegetables need 12–18 inches (30–45 cm); tomatoes, shrubs, and deep-rooted plants need 18–24 inches (45–60 cm) or more. Use a lightweight container mix rather than heavy garden soil so the planter drains well and stays manageable.
To estimate bags, divide the needed cubic feet by the bag size. For example, the 8-cubic-foot planter needs four 2-cubic-foot bags, plus a little extra for settling. In metric, a 1.2 m × 0.6 m × 0.3 m planter holds 0.216 m³, or 216 liters, before the 5–10% adjustment. In short: measure inside dimensions, multiply length × width × depth, subtract headspace, and add 5–10%—that is the amount of soil your urban planter box needs to fill it properly.