How to Calculate Decking Boards for a Deck in 5 Steps

Decking boards in one line

Decking boards are simple: deck area, divided by board area, plus waste, rounded up to whole boards. A 16 ft by 12 ft deck is 192 square feet, one 5.5 in by 12 ft board covers 5.5 square feet, and with 10% waste the deck needs 39 boards, against 35 boards exact. The rest of this guide is about getting those three inputs honest: the deck size, the board size, and the waste allowance.

Step 1: Measure the deck, length and width

Measure the deck in feet, length and width, at the deck itself — not at a plan nearby, since decks are built to the ground, not to the drawing. The worked example is 16 ft long and 12 ft wide, a common backyard deck, and 16 times 12 is 192 square feet. If the deck is not a rectangle, split it into rectangles, measure each one, and add the areas: an L-shaped deck is two rectangles, and the board count follows the total area, not the longest side.

Step 2: Work out the area of one board

Work out the area of one board from the board you are actually buying: board width in inches, divided by 12, times board length in feet. A 5.5 in board, 12 ft long, covers (5.5 ÷ 12) × 12 = 5.5 square feet. Do not use the nominal board width off the shelf label without checking: a board sold as 6 in often measures 5.5 in on the face once the rounded edges are allowed for, and that half inch, across dozens of boards, moves the count by whole boards.

Step 3: Divide the deck by the board

Divide the deck area by the board area for the exact board count: 192 ÷ 5.5 = 34.9, which is 35 boards exact once rounded up, since boards are whole. This is the count with no waste at all — a deck boarded perfectly, with no end cuts, no breakage and no miscuts — and no real deck is boarded that way. Keep the exact count in mind anyway: it is the floor of the order, and the gap between 35 and the final order is the waste, not a mistake.

Step 4: Add waste and round up

Add the waste allowance to the deck area, then divide again and round up: 192 × 1.10 = 211.2 square feet with 10% waste, and 211.2 ÷ 5.5 = 38.4, rounded up to 39 boards. Waste is the end cuts, the breakage and the miscuts, and 10% fits a rectangular deck, while a deck with angles, stairs or a picture-frame border wants 15%, since every angle consumes boards. Round up at the end, not at the start: rounding the exact count first and then adding waste double-counts the rounding and over-orders.

Step 5: Order, lay and check the deck

Order the boards — 39 boards for the example — and check the deck before laying it: every board the right size, the joists spaced for the board span, the first row straight, since a crooked first row runs crooked across the whole deck. Then count what boards do not cover: joists, screws, railing and stairs. For the frame under the deck, the joist calculator sizes the joists, so the deck and the frame are ordered together, and the deck is not boarded onto a frame that was never counted.

Decking boards in numbers

Suppose the deck is 16 ft long and 12 ft wide, the boards are 5.5 in wide and 12 ft long, and the waste allowance is 10%. Deck area: 16 × 12 = 192 sq ft. Board area: (5.5 ÷ 12) × 12 = 5.5 sq ft. Exact boards: 192 ÷ 5.5 = 34.9, rounded up to 35. Area with waste: 192 × 1.10 = 211.2 sq ft. Boards with waste: 211.2 ÷ 5.5 = 38.4, rounded up to 39. Order: 39 boards, plus joists, screws and railing bought separately.

Common mistakes

The most common mistake is using the nominal board width instead of the face width, which under-counts by a board or more on a full deck. The second is measuring only the longest side and forgetting the width, which orders for a strip instead of a deck. The third is skipping waste, which boards the deck to the exact board and runs short at the last row. The fourth is rounding up before adding waste, which double-counts the rounding and over-orders. The fifth is forgetting the frame, which boards a deck onto joists that were never counted.

Worked example, start to finish

A backyard deck, 16 ft long and 12 ft wide, 5.5 in boards, 12 ft long, 10% waste. Deck area: 192 sq ft. Board area: 5.5 sq ft. Exact boards: 35. Area with waste: 211.2 sq ft. Boards with waste: 39. Result: order 39 boards, and buy joists, screws and railing separately. If the same deck used 7.25 in boards instead, the need would fall to 30 boards with waste, but the deck boarded would be identical.

What changes the answer most

The deck area changes the answer most: every square foot of deck is another fraction of a board, and a wrong measurement rebuilds the whole order. The board size is second: moving from 5.5 in to 7.25 in boards cuts the order from 39 to 30, since every board covers more deck. The waste allowance is third, and it changes only the top of the order, never the deck itself — moving from 10% to 20% waste adds 3 boards, from 39 to 42. The deck shape is fourth, and it changes only the waste, never the area maths.

Frequently asked questions