Deck Materials Calculator
Work out deck boards, joists, screws and cost from the deck size.
About
Deck Materials Calculator
A deck estimate has three parts that behave differently. The decking is an area problem, the framing is a spacing problem, and the fasteners follow from both. Adding a flat percentage to a square footage figure gets all three wrong at once, which is why the number from the back of an envelope rarely matches the pile on the driveway.
Decking: the gap is part of the coverage
Deck boards do not cover their own width. Each board carries a gap alongside it, and that gap is part of the space it occupies. A nominal 5/4 × 6 board measures 5½ inches across the face; with a 3/16 inch gap it covers 5.6875 inches. Calculating on 6 inches under-buys by about 5 percent, and calculating on 5.5 over-buys slightly. Over a 400 square foot deck the difference between the two is roughly a dozen boards.
How large the gap should be depends on what the boards are made of and how wet they are when you lay them.
- Wet pressure-treated lumber is often laid tight, with no gap at all, because it shrinks as it dries and opens its own spacing.
- Kiln-dried lumber has already shrunk, so it needs a deliberate gap, commonly 3/16 of an inch.
- Composite decking expands and contracts with temperature rather than moisture, and the manufacturer specifies both a side gap and an end gap. Those figures are a condition of the warranty, so follow the instructions rather than the convention.
Joists: spacing is set by the decking, not by preference
Sixteen inches on centre is the common case for wood decking laid perpendicular to the joists. It is not a universal answer.
Most composite decking requires 16 inches on centre for a straight lay and drops to 12 inches when boards run diagonally, because the diagonal increases the unsupported span between bearing points. Some capped composites require 12 inches everywhere. Thicker stock over short spans occasionally allows 24 inches, but that is the exception rather than something to assume.
The joist count itself is a small piece of arithmetic that people routinely get wrong by one. Divide the run by the spacing to get the number of gaps, then add one for the joist that closes the far end. A 16 foot run at 16 inch centres has twelve gaps and thirteen joists.
Span limits are a separate question from spacing, and they are set by code rather than by taste. The International Residential Code publishes span tables for deck joists by species, size and spacing, and the local building department may amend them. A 2×8 southern pine joist at 16 inch centres spans considerably further than the same size in spruce.
Fasteners
Face-screwing takes two screws per board at every joist crossing. That number climbs quickly: a 16 by 12 foot deck runs to well over a thousand screws, which is three or four boxes rather than the one most people buy.
Hidden fastener systems change the count entirely, using one clip per joist crossing instead, and they add materially to the cost. They also change the installation sequence, because the last board usually has to be face-screwed regardless.
Use fasteners rated for the treatment chemistry in the lumber. Modern pressure treatment is far more corrosive to plain steel than the older CCA treatment was, and the standard requirement is stainless or a coating specifically approved for contact with treated wood. Ordinary zinc-plated screws will corrode through in a few seasons.
What this calculator does not cost
The figure above covers decking and framing. A real deck budget also includes posts, footings, post bases, joist hangers, ledger flashing, structural fasteners, railing, stairs and finish. Railing in particular often runs to a third of the total once balusters, posts and caps are counted, and stairs are disproportionately expensive per square foot because of the stringer work.
Two items are worth singling out because they are safety-critical rather than cosmetic. The ledger connection to the house, which has to be through-bolted and flashed rather than nailed, is the single most common point of catastrophic deck failure. And footings must reach below the frost line for your area, which the local building department publishes.
Common questions
Frequently asked questions
Divide the deck width by the coverage of one board, where coverage is the face width plus the gap. Multiply by the number of boards needed to span the length, then add waste. The calculator does this from your dimensions, including the gap.
Wet pressure-treated boards are often laid tight because they shrink as they dry. Kiln-dried lumber usually takes 3/16 of an inch. Composite decking must follow the manufacturer instructions, which specify both side and end gaps and form part of the warranty.
Sixteen inches on centre is standard for wood decking laid perpendicular. Most composite decking requires 16 inches straight and 12 inches for a diagonal lay. Check the decking manufacturer span table, because it, not preference, sets the requirement.
Divide the run length by the spacing to get the gaps, then add one for the closing joist. A 16 foot run at 16 inch centres needs 13 joists, not 12.
Two per board at every joist crossing when face-screwing. A 16 by 12 foot deck typically needs over a thousand screws, so buy by the box rather than the handful.
No. Modern pressure treatment is far more corrosive than the older CCA formulation. Use stainless steel or fasteners specifically rated for contact with treated wood, or they will corrode through within a few seasons.
Ten percent for a plain rectangle. Fifteen percent where boards run diagonally, where there is a picture-frame border, or where the deck has angles or cut-outs around a tree or post.
It depends on species, size and spacing, and it is set by code rather than by judgement. The International Residential Code publishes deck joist span tables, and your local building department may amend them. Southern pine spans noticeably further than spruce at the same size.
No. It covers decking and framing only. Railing often runs a third of a deck budget once posts, balusters and caps are counted, and stairs cost more per square foot than the deck itself.
Perpendicular to the joists, which usually means parallel to the longest wall of the house. Running them diagonally looks good and is entirely possible, but it needs closer joist spacing and wastes noticeably more material.
Method
How this is worked out
Before you act on this: This estimates quantities and deflection, it does not design a structure. Soil conditions, loads, spans and local code requirements decide whether a design is safe, and above modest sizes that judgement belongs to an engineer and your building department rather than to a calculator.
The formula on this page follows:
- International Residential Code, floors and decks — Joist span tables and ledger attachment
- Call 811 before you dig — Free utility locating, required by law in most US states
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