Window boxes look best when they are full, but overcrowding leaves plants competing for a small volume of soil. Knowing exactly how many plants fit helps you buy the right number of plugs and pots.
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Enter the inside measurements of your box and the spacing you want. The calculator places plants along one row or two staggered rows, draws the layout from above and works out how much potting mix fills the box.
usable length = inside length − (2 × end gap) plants in row 1 = floor(usable length ÷ spacing) + 1 plants in row 2 (staggered) = floor((usable length − spacing ÷ 2) ÷ spacing) + 1 soil volume = length × width × (depth − space below rim) bags = ceil(soil volume × boxes ÷ bag size)
Staggered rows are offset by half a spacing so the plants interlock in a zigzag. Soil volume uses inside dimensions; tapered boxes hold a little less than the result.
A 76 cm box with 5 cm end gaps leaves 66 cm usable. At 15 cm spacing a single row fits 5 plants. Switching to two staggered rows adds a second row of 4, for 9 plants. Filling the 76 × 20 × 20 cm box to 2.5 cm below the rim takes 26.6 L of potting mix.
With a 2-inch gap at each end and 6-inch spacing, a single row fits 5 plants. In an 8-inch-wide box two staggered rows fit about 9 compact plants.
Multiply the inside length, width and depth (minus the space you leave below the rim). A 30 × 8 × 8-inch box filled to 1 inch below the rim holds about 0.97 cubic feet, or roughly 27.5 litres.
At least 15–20 cm (6–8 in) of soil depth suits most herbs and annuals. Deeper boxes dry out more slowly.
Yes, compact herbs tolerate closer spacing in containers if you water and feed regularly, but give vigorous herbs such as mint their own container.