Deck Board CalculatorFree online other calculator — instant results, no sign-up
Count full-length boards across a deck, including gaps.
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About the Deck Board Calculator
The Deck Board Calculator estimates how many full-length boards are needed across a deck’s width, allowing for gaps between adjacent boards. Enter the deck width in metres and the board width and gap in millimetres to calculate a whole-board count. With the default inputs of 4 m, 140 mm and 5 mm, the result is 28 boards.
- Deck width (m)
- The distance across the deck, measured perpendicular to the boards, in metres.
- Board width (mm)
- The actual face width of each decking board, excluding the gap, in millimetres.
- Gap (mm)
- The clear spacing between adjacent decking boards, in millimetres.
Formula and method
N = ceil((1000W + g) / (b + g)); with the defaults, N = ceil((1000 × 4 + 5) / (140 + 5)) = 28 boards.
- NThe whole number of full-length boards needed across the deck width.
- WThe deck width in metres.
- bThe board face width in millimetres.
- gThe gap between adjacent boards in millimetres.
- ceilRound upward to the next whole number, leaving an exact whole number unchanged.
Convert the deck width from metres to millimetres. A layout of N boards has N − 1 internal gaps, so its total width is Nb + (N − 1)g. Solve for the smallest whole number of boards that reaches or exceeds the deck width by dividing the converted deck width plus one gap by the board width plus one gap, then rounding upward. Using 4 m, 140 mm and 5 mm gives 28 boards.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Deck width4 m
- Board width140 mm
- Gap5 mm
Results
- Result28 boards
How to use the Deck Board Calculator
- 1Measure the deck width perpendicular to the direction the full-length boards will run.
- 2Enter that measurement in Deck width (m); the default is 4.
- 3Enter the actual board face width in Board width (mm); the default is 140.
- 4Enter the planned spacing between adjacent boards in Gap (mm); the default is 5.
- 5Read the whole-board result and check whether the final board will need trimming to fit the deck edge.
Reading your result
The result row reports the number of full-length boards required across the entered deck width, including the effect of internal gaps. The default result of 28 boards means 28 board positions are needed across the deck; it does not mean every board will fit at its full face width. The final board may need to be ripped narrower.
Limitations
This is a width-based layout estimate, not a complete decking purchase list. It assumes straight, parallel boards of uniform width, consistent gaps, and one board long enough to span each run. It allows gaps only between boards, not at walls or perimeter edges. It does not account for deck length, butt joints, diagonal layouts, borders, obstructions, cutting waste, defects or spare boards. Use actual board dimensions and follow the decking manufacturer’s spacing guidance, including allowances for moisture movement. Deck and board widths must be positive, and the gap must not be negative.
Common questions
Why do the default inputs give 28 boards?+
For a 4 m deck width, 140 mm boards and 5 mm gaps, the calculator evaluates ceil((1000 × 4 + 5) / (140 + 5)). Rounding upward gives 28 boards so the layout reaches the full deck width.
Does the calculator add a gap after the last board?+
No. It counts only the gaps between adjacent boards, so there is one fewer gap than boards. Any required perimeter clearance must be accounted for when determining the width to enter.
Does 28 boards mean I can install every board without trimming?+
No. The result is rounded upward, so the last board may need to be cut along its length to fit the remaining width. Check the edge layout before fixing the boards.
Can I use the result to order boards without knowing the deck length?+
Not reliably. The count assumes each board spans the full run, but this tool does not check available board lengths. Confirm the required length and add any boards needed for joints, waste and defects before ordering.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
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Results are estimates for informational purposes and are not financial, medical or legal advice.