Planter Box Soil Calculator

Deck planter soil volume and plant capacity calculation ensures your container has enough growing medium for healthy roots, adequate drainage, and structural weight you can safely place on a deck.

Soil volume formula (rectangular planter box): Usable Soil Volume = Length × Width × (Depth − Headspace − Drainage Layer)

Leave about 1.5 inches of headspace at the top or every watering runs straight over the rim.

Skip the gravel drainage layer. It is traditional advice and it makes drainage worse, not better. Water does not move readily from a fine material into a coarse one, so it collects at the bottom of the soil and sits there until that layer is saturated. A gravel course simply raises that soggy zone closer to the roots while stealing depth they could have used. Fill the box with mix, put screen over the drainage holes to stop it washing out, and leave it at that. The field is set to 0 for this reason; change it if you have a container you cannot drill.

Plant spacing and capacity: Number of Plants = Floor Area ÷ Per-Plant Space Requirement

Recommended minimum soil depth by plant type:

Weight calculation, which is the part that matters for deck safety:

Total Weight = (Soil Volume × Wet Soil Density) + Planter Weight + Plant Weight + Water Weight

• Wet potting mix density: 55–65 lbs per cubic foot (about 880–1,040 kg/m³)
• Premium lightweight potting mix: ~45 lbs/cubic foot
• Deck load capacity: residential decks typically rated 40–60 lbs per square foot

Always check deck structural capacity before placing heavy planters. A 24″ × 48″ × 18″ planter holds about 11 cubic feet of mix once you take off the headspace, and wet that is roughly 660 lbs of soil alone, before the box, the plants or a day of rain. It sits on 8 square feet, so it is putting about 82 lbs on every one of them.

Worked example: Deck planter, measured inside: 36 inches long × 18 inches wide × 16 inches deep. No gravel layer.

• Effective soil depth: 16 − 1.5 headspace = 14.5 inches
• Soil volume: 36 × 18 × 14.5 = 9,396 cubic inches ÷ 1,728 = 5.44 cubic feet
• Bags: 6 at 1 cu ft, or 3 at 2 cu ft
• Soil weight when wet (60 lb/ft³): 5.44 × 60 = 326 lbs
• Footprint: 36 × 18 inches = 4.5 sq ft, so the soil alone puts 73 lb per square foot on the deck

That last figure is the one that matters and the one nobody works out. A residential deck is typically built for 40 to 60 lb per square foot spread evenly, and this planter exceeds it on the patch it sits on before you add the box, the plants or the rain. Put it directly over a joist or, better, over a post, never in the middle of a span. Measure the inside of the box, not the outside: a 2-inch thick cedar planter loses about 20% of its apparent volume to its own walls.