Deck Board Calculator

Count decking boards, linear feet and board feet for a deck, including the gap between boards.

Draft — figures checked, awaiting an editorial read. Not indexed.

Formula: Rows = deck width ÷ (board width + gap). Boards = rows × (deck length ÷ board length). Board feet = thickness × width × linear feet ÷ 12.

What this calculates

This turns deck dimensions into a shopping list: how many boards to buy, how many linear feet that is, and how many board feet if you are buying lumber priced that way. It accounts for the gap between boards, which sounds like a detail and is not — an eighth of an inch across twenty-six rows adds more than three inches of coverage, which is most of another board. It also reports board feet separately, because softwood decking is usually sold by the piece while hardwood and rough-sawn stock is sold by the board foot, and the two are not interchangeable.

How it works

The number of rows is the deck width in inches divided by the effective width of one board, which is its actual width plus the gap. Actual width is not nominal width: a 2x6 measures 5.5 inches, not 6. Each row then needs enough boards to span the deck length, rounded up to whole boards, and the two multiply to give the total. Board feet uses the trade formula — nominal thickness in inches, times actual width in inches, times length in feet, divided by twelve. The waste allowance is applied to the board count at the end.

Worked example

A 16 ft by 12 ft deck with 5/4x6 boards (5.5 in actual) and a 1/8 in gap. Effective board width is 5.625 in, so 12 ft × 12 = 144 in ÷ 5.625 = 25.6, rounded up to 26 rows. With 16 ft boards, each row takes exactly one board, so that is 26 boards. Add 10% waste and you buy 29. That is 464 linear feet, and at 1.25 in nominal thickness, 1.25 × 5.5 × 464 ÷ 12 = 266 board feet.

Common mistakes

Using nominal dimensions as actual ones is the most expensive mistake here, and it always under-buys — calculating rows on a 6 inch board when it really covers 5.5 makes the deck about 9% short. Forgetting the gap does the same thing more subtly. And if your boards run diagonally rather than square to the joists, add closer to 15% waste rather than 10, because every board gets cut twice at an angle.

Common questions

What gap should I leave between deck boards?

About 1/8 in for wet pressure-treated lumber, which shrinks as it dries, and up to 1/4 in for kiln-dried or composite boards that will not. The gap sheds water and debris; boards laid tight tend to trap both.

What is a board foot?

A volume unit: one square foot of lumber, one inch thick nominal. Thickness in inches × width in inches × length in feet ÷ 12. Hardwood and rough lumber are usually priced this way rather than by the piece.