Deck Board Calculator
Enter the deck size, which way the boards run, the board width and gap, and the board length you plan to buy.
How it works
Rows = (deck width in inches + gap) ÷ (board width + gap), rounded up, so n boards and n − 1 gaps cover the width. Boards per row = deck length ÷ board length, rounded up. Boards = rows × boards per row. Extra = boards × allowance, rounded up. With no gap this matches Trex's method: deck depth (in) ÷ board width (in), rounded up.
Worked example: Trex: a 20 × 12 ft deck with 5.5 in boards, 20 ft long: 144 ÷ 5.5 = 26.18, rounded up to 27 boards; 5% extra rounded up is 2, so 29 boards. TimberTech: a 192 sq ft deck ÷ 7.2 sq ft per 16 ft board = 26.6, so 27 boards.
Assumptions
- Gap default 3/16 in, the minimum width-to-width gap in Trex's 2024 installation guide. Other products and wood decking differ; use your product's instructions.
- Board width is the actual width; Trex and TimberTech examples use 5.5 in. Allowance default 5%, Trex's rule of thumb; TimberTech's example uses the deck area divided by board area.
- Straight boards, square to the deck. Diagonal and picture-frame layouts, stairs, fascia and butt joints over joists use more boards.
- Joists, beams, footings, fasteners and railings are not included. Deck structure is governed by local codes; check local codes and get a permit where required.
Frequently asked questions
How many deck boards do I need for a 12 × 20 deck?
Trex's example: with 20 ft boards running the 20 ft way, 144 in ÷ 5.5 in = 26.18, so 27 boards; adding 5% gives 2 more, 29 boards. With a 3/16 in gap it is 26 rows.
What gap should deck boards have?
It depends on the product. Trex's 2024 installation guide lists a minimum width-to-width gap of 3/16 in; follow your decking's instructions.
How much extra decking should I buy?
Trex suggests about 5% extra as a rule of thumb. Diagonal patterns and many joints need more.
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Sources
Formulas and reference values are from these sources. Page content last updated 2026-10-19.
- Trex: How many deck boards do I need? (worked example with 5% extra)
- Trex 2024 decking installation guide (minimum 3/16 in width-to-width gap)
- TimberTech: How to calculate how much decking you need (area method example)
- NIST Guide to the SI (SP 811), Appendix B.9: conversion factors (foot = 0.3048 m, ton, pound)