Planter Box Calculator

Western red cedar is the best balance — naturally rot-resistant, food-safe (no chemical treatment), 18-yr lifespan, $5.80/BF. Redwood lasts longer (25 yr) but costs 25% more. IPE hardwood lasts longest (50 yr) but is heavy + slow to work. AVOID PT pine for edibles unless using a heavy plastic liner — modern ACQ treatment chemicals aren't immediately toxic but slow leaching into soil is a concern.

Inner dimensions × volume formula. Common boxes: 18×18×18 = 4.5 cu ft = 3 bags. 36×18×18 = 5.4 cu ft = 4 bags. 48×24×24 = 12.5 cu ft = 8-9 bags. 72×48×30 = 50 cu ft = 33+ bags (consider bulk delivery at $35-65/cu yd). 1 cubic yard = 27 cu ft = saves ~$50 vs bagged at this scale. The calculator outputs exact bag count.

Recommended for ALL wood planters. Heavy plastic (6-mil polyethylene) + landscape fabric prevents direct soil contact with wood interior. Extends wood lifespan 30-50% by reducing constant moisture exposure. REQUIRED if using PT wood with edibles. NOT needed for composite/PVC planters (no rot concern). Adds $5-15 per box depending on size.

½″ diameter holes at 6″ OC on the bottom. Calculator outputs count based on soil volume — typically 4-12 holes for most box sizes. Cover each hole with a square of landscape fabric to prevent soil washout. Add 1″ gravel/perlite layer above the holes for sustained drainage. Without proper drainage, roots rot within 1-2 seasons.

ACQ-treated PT (post-2003) is considered safe for incidental contact but NOT directly for food-growing soil. Use a heavy plastic liner (6-mil polyethylene) to prevent soil contact with the treated wood — this is the standard recommendation from extension offices. Avoid older CCA-treated wood (pre-2003) entirely — it contains arsenic. Best practice: cedar, redwood, or IPE for edible planters.

Only for freestanding boxes < 24″ wide that you want to move (chase the sun, store for winter, rearrange). Heavy-duty casters rated 100+ lb each ($38 kit). Soil weight: 80 lb/cu ft → a 4 cu ft soil-filled planter = 320 lb (80 lb per caster). Not for raised garden beds (too heavy + defeats the purpose). NOT for railing-mounted (no clearance).

18 yrs typical with liner + drainage. Western red cedar's natural tannins resist rot + insects. Lifespan halved (8-10 yr) without liner because constant soil moisture eats away at the wood interior. Lifespan doubled (35+ yr) with: liner + raised on cleats (no ground contact) + annual cedar oil treatment + cover in winter. Cedar weathers to silver patina — don't bother staining.

Yes with precautions: (1) use felt pad / composite spacers under the planter to prevent dye transfer from wet soil to composite, (2) drainage holes shouldn't pool water on composite surface — use a drain saucer or drainage pad, (3) verify deck framing supports the load (8 cu ft soil + plants ≈ 700 lb concentrated load — heavier than a typical patio chair). Most composite warranties cover planter use; check yours.

Minimum 12″ deep for most vegetables (lettuce, herbs, beans). 18″ deep for tomatoes, peppers, root crops (carrots, beets). 24″ deep for deep-root crops (potatoes, parsnips). Width 18-24″ for most. Length flexible — many gardeners build long beds (4-6 ft) for vegetable rotation. Drainage critical for vegetables: roots rot fast in waterlogged soil. Use the Raised Garden Bed style preset for proper sizing.