Planter Box Wood Calculator

Calculate a framed planter box using 5/4 deck boards and 4x4 corner posts.

Overall Length Overall Width Overall Height Deck Board Width Slat Gap 4x4 Post Actual Size Top Rail Height Bottom Rail Height Cap Overhang Lumber Length Waste Factor

Schematic Build Diagram

Step-by-Step Planter Box Build Plan

Build a framed planter box using 4x4 corner posts, 5/4 deck boards, top and bottom rails, and a mitered cap frame.

Step 1: Cut the 4x4 Corner Posts

Cut four 4x4 posts to the finished height of the planter box.

4 pieces @ finished planter height
Use actual 4x4 size: usually 3.5" x 3.5"
Keep all posts exactly the same length

Step 2: Attach the Bottom Rails

Connect the posts with bottom rails. These create the lower frame and hold the planter square.

2 long rails between the front/back posts
2 short rails between the side posts
Clamp square before fastening

Step 3: Attach the Top Rails

Add the upper rails between the 4x4 posts. This creates the framed panel opening for the vertical deck boards.

2 long top rails
2 short top rails
Keep rails flush and level

Step 4: Install the Vertical 5/4 Deck Board Slats

Cut vertical deck board slats to fit between the top and bottom rails. Space them evenly across each panel.

Use 5/4 x 6 deck boards
Actual board width is usually about 5.5"
Use a 1/8" spacer between boards

Step 5: Build the Other Three Sides

Repeat the same rail-and-slat process on the back and both ends of the planter box.

Front panel
Back panel
Left side panel
Right side panel

Step 6: Add Bottom Supports

Add bottom support boards inside the planter. These support the soil, liner, or insert box.

Use 5/4 deck boards or 2x material
Leave small drainage gaps
Add landscape fabric or a plastic liner

Step 7: Add the Top Cap Frame

Add a mitered top cap around the planter. This is what gives the planter the finished furniture-style look.

Use 5/4 deck boards
Miter corners at 45°
Allow about 1" overhang if desired

Step 8: Sand, Seal, and Finish

Sand sharp edges, round over exposed corners, and apply an exterior finish if desired.

Ease edges with sandpaper or router
Use exterior-rated finish
Add liner before filling with soil