Deck Calculator

Lumber is sold by nominal size and delivered at actual size. A "2×6" leaves the mill at 2 × 6 in green and rough, then dries and is planed to 1.5 × 5.5 in. A "5/4×6" deck board is 1 × 5.5 in. The nominal figure survives only as a name.

Half an inch on a 6 in board is 8%, and on a 12 ft wide deck that is the difference between 25 rows and 26. Every dimension in this calculator is actual, measured width — if you are entering a custom profile, measure a board rather than reading the label.

Boards are laid with a gap between them for drainage and for the wood to move. The gap is not decorative: without it, water sits on the joint, debris packs in, and boards that swell have nowhere to go but up.

The counting consequence is that each board occupies its own width *plus* one gap — the gap that follows it. So the repeat is 5.5 + 0.1875 = 5.6875 in, and that is what the deck width divides by. Dividing by the board width alone overstates the row count, which is the friendlier direction to be wrong in but still wrong.

How big the gap should be depends on the timber. Kiln-dried wood is laid at 3/16 in because it will barely move. Wet pressure-treated stock is often laid tight, because it shrinks as it dries and opens its own gap — lay wet stock with a 3/16 in gap and you may end up with 3/8 in. Composite decking follows the manufacturer, and its gaps are usually larger to allow for thermal movement.

If the deck is longer than the boards, every row has to be made from two or more pieces meeting end to end. That joint must land on a joist — half a board end cantilevered in mid-air will move, cup and eventually fail — which sometimes means adding blocking.

Butt joints also waste material. A 16 ft run from 12 ft stock takes two boards and leaves 8 ft over, and that offcut is rarely long enough to be the second piece of another row. Buying 16 ft stock for a 16 ft deck halves the board count outright.

Where joints are unavoidable, stagger them. Rows of joints lined up across the deck read as a seam and draw the eye; scattering them across different joists makes them nearly invisible.

The surface is the visible part and the small part. Under it sit beams sized for the span, posts, concrete footings below the frost line, a ledger bolted and flashed to the house, joist hangers, and often railings and stairs — each of which is its own calculation.

The ledger connection deserves particular mention: it is the single most common cause of catastrophic deck failure, and the failures are almost always either fastening (nails instead of through-bolts or structural screws) or flashing (water tracking behind the ledger and rotting the rim joist).

Joist spacing is a structural matter too. A 5/4 board wants joists at 16 in; many composite boards need 12 in, and diagonal decking needs the spacing reduced because the board crosses joists at an angle and spans further between them.