Planter Box Size Calculator

For most herbs, planter box dimensions of 8 in deep by 8 in wide work well — that covers basil, cilantro, parsley, sage and oregano, which all thrive in 6 to 12 in of soil. A 36-in-long box holds 6 herb plants at 6-in spacing. Mint and rosemary prefer a bit more room, so give them a 12-in-deep box. Use the plant mode above to size a box for any herb count.

Match planter box depth to the crop: leafy vegetables like lettuce and spinach do well in 8 in of soil; root vegetables like carrots and onions need 12 in; fruiting vegetables like peppers need 12 in and tomatoes do best with 14 in or more. Width matters too — give fruiting vegetables at least 12 to 14 in of width per plant so roots can spread sideways.

Soil volume is length × width × usable depth. A 36×12×12-in planter box filled 1 in below the rim holds 2.75 cu ft of potting mix, which is 20.57 gallons (82.3 quarts or 77.9 liters). That equals 2 bags of 1.5-cu-ft potting mix — buy the second bag even if it looks close, because mix settles after the first watering.

Plan on one 1/2-in drainage hole per 12 in of box length, with a minimum of one hole. A 36-in-long planter box needs 3 half-inch holes spaced evenly along the bottom. Drill from the inside out to avoid splintering, and line the bottom with landscape fabric so soil stays in while water drains freely.

For a 36×12×12-in cedar box built from 1×8 boards: 4 eight-foot boards (25 lin ft total — sides stacked 2 courses high plus 9 bottom slats), 34 exterior screws, 11 sq ft of landscape fabric to line it, and 4 corner brackets. The empty cedar box weighs about 17.7 lb. Use the materials mode above for any other box dimensions.

A 36×12×12-in box holds 2.75 cu ft of mix, and wet potting mix weighs about 70 lb per cu ft — so the soil alone is 192.5 lb, plus 17.7 lb for the cedar box, about 210 lb loaded. Use 4 legs for boxes up to 48 in long and 6 legs for longer boxes, with legs 30 to 36 in tall to bring the soil near waist height. Brace legs taller than 30 in and confirm your deck can carry the loaded weight.

With 2-ft walkways between them, six 3×1-ft planter boxes fit on a 12×8-ft patio — 2 rows by 3 columns. Keep walkways at least 18 to 24 in wide so you can reach every plant to water and harvest. The layout mode above computes rows, columns and count for any patio size and box dimensions.

A 36×12×12-in cedar planter box costs about $94 in materials: $17 for 2 bags of potting mix, $48 for 4 eight-foot cedar boards, $6 for exterior screws, $9 for landscape fabric and $14 for 4 corner brackets. Buying lumber in 8-ft lengths and cutting it yourself is the biggest saver versus pre-cut kits. Adjust quantities and local prices in the cost mode above.

A planter box is a container: it has a solid bottom with drainage holes, is usually smaller, and often sits elevated on legs or a deck railing for decoration and easy reach. A raised bed is a bottomless frame sitting directly on the ground, typically 4 ft wide so you can reach the middle from both sides. If you are gardening at ground level in rows, use our raised bed size calculator; if you want containers for a patio, porch or windowsill, this planter box calculator is the right tool.