Deck Board and Screw Calculator
Deck Board and Screw Calculator
Deck dimensions to board rows, lineal metres of decking and screw count, with the gap between boards included in the pitch. Boards are assumed to run across the width, the usual orientation.
The pitch is board width plus gap, so a nominal 140 mm board at a 5 mm gap covers 145 mm of deck per row. Gaps let the timber drain and move: 5 to 6 mm is typical for dry material, less for wet-treated lumber that will shrink as it dries. Joist spacing of 400 mm centres suits most residential decking; 300 mm is used for diagonal lays or thinner boards.
How to use this tool
Give it the deck depth the boards run across, the width each board spans, the board width and the gap between boards. It returns the number of board rows, the lineal metres to buy including waste, and the screw count based on joist spacing.
The gap is part of the board
Decking is laid on a pitch, not a width. A nominal 140 millimetre board with a 5 millimetre gap consumes 145 millimetres of deck per row, and over a 4.2 metre depth that difference is the gap between 29 rows and 30. Estimating on board width alone under-orders by roughly the gap fraction, which is 3 or 4 percent, enough to leave the last row short.
The gap exists for good reasons: water has to drain off the surface and the timber has to move. Five to six millimetres suits dry material. Wet-treated lumber is the exception worth thinking about, because it shrinks as it dries, so it goes down tighter and opens up later; laying it with a full dry gap can leave gaps wide enough to lose things through by the second summer.
A worked check you can repeat on paper
A deck 4.2 metres deep and 3.6 metres wide, 140 millimetre boards, 5 millimetre gaps. The pitch is 145 millimetres, so 4.2 divided by 0.145 is 28.97, which rounds up to 29 rows. Each row is 3.6 metres long, so that is 104.4 lineal metres of decking before waste. At 400 millimetre joist centres a 3.6 metre board crosses 10 joists, and at 2 screws per joist that is 29 times 10 times 2, or 580 screws. Buying a box of 500 would be one short trip.
Mistakes that catch people out
Ignoring the gap. As above, a systematic under-order. Fix: the pitch is board plus gap, which the tool computes and displays.
Buying lineal metres as if they were square metres. They are different quantities; 104 lineal metres of 140 millimetre board is about 15 square metres. Fix: the tool gives both the area and the lineal figure.
Forgetting board length waste. Boards come in fixed lengths and a 3.6 metre span cut from a 4.8 metre board wastes 1.2 metres unless the offcut is used. Fix: the waste percentage, and where possible buy lengths that divide into the span.
Underestimating screws. Two per joist per board adds up fast; 580 is not unusual for a small deck. Fix: the screw line in the table.
How this was checked
The pitch, row count and screw arithmetic were verified on the worked example above, and the rounding was tested at an exact fit: a 2.9 metre depth at a 145 millimetre pitch returns exactly 20 rows with no spurious extra row.
Related Calculators and Reference
Posts and footings under the deck are the Fence Post and Materials Calculator, which does the same concrete arithmetic. Deck area and awkward shapes are the 2D Area Calculator, and comparing board prices across lengths is the Unit Price Comparison Calculator.