Home Container gardening & soil volume
How many liters of soil does a window box hold?
Published by Potting Ledger
For a straight-sided window box, multiply inside length, inside width and usable fill depth in centimeters, then divide by 1000. A 60 × 20 cm box with 18 cm of fill holds 21.6 liters of potting mix before any plants go in.
Find the floor and mark the soil line
Hooking a tape over the outside lip counts plastic, decorative ledges and occasionally an entire water tank. Find the bottom of the planting compartment. In a self-watering box this is usually a separator above the reservoir. Measure upward to where you want the mix to finish, leaving space at the top for watering.
For example, a planting compartment 21 cm deep with a 3 cm watering gap has 18 cm of usable depth. With a 60 × 20 cm interior, each extra centimeter of soil depth would add 1.2 liters. A decorative rim a few centimeters above the soil line makes a surprisingly large difference to a bag estimate.
Round bags after adding the boxes
Three empty boxes of 21.6 liters each need 64.8 liters combined. If the available bags contain 20 liters, four bags provide 80 liters and leave a nominal 15.2 liters. Rounding up to two bags for every box would put six bags in the cart. Count all boxes first and round the total once.
Subtract the volume of substantial intact root balls if the boxes are being planted immediately. If the box narrows substantially toward the bottom, the upper opening overstates its capacity. Treat the result as an upper estimate or measure the volume with a known-size scoop as you fill.