Set the direction before counting decking
Board direction changes the purchase count. Here the boards run along the entered deck length, and their rows fill the entered width. Measure the actual rectangular decking surface rather than the outer railing. Use the actual board face width: a nominal timber name may not equal its finished width. Gaps belong between adjacent rows. The formula includes one fewer gap than boards, and the outermost row may need ripping to fit.
Rows and stock pieces are separate quantities
Rows = ceiling((deck width + gap) ÷ (board face width + gap))For each row, the length is divided by the stock length and rounded upward. Multiplying rows by those pieces produces the basic stock count. The chosen spare percentage increases that count before a final rounding to whole boards. Metric deck dimensions use metres while face width and gap use millimetres. US mode uses feet and inches. Zero gap is permitted mathematically; select installation spacing from the actual product instructions rather than from the example values.
A four by three metre rectangle
With boards 140 millimetres wide and gaps of 5 millimetres, a three metre width requires 21 rows. Each four metre row can be cut from one four metre stock board. The basic count is therefore 21 boards. Ten percent spares brings that to 23.1, rounded to 24 boards. There are no length offcuts in this ideal example, although end trimming, damaged stock and the ripped outer row can still create waste.
Check joints, offcuts and nonrectangular details
When a row is longer than the stock, butt joints need suitable support. This calculator counts pieces without designing joist spacing, end blocking or a staggered seam pattern. It assumes offcuts from one row are not reused in another, so short stock can yield a conservative purchase count. A cutting schedule may recover some of that waste, while saw kerfs and required end cuts consume additional length. Picture-frame borders, diagonal layouts, stair treads and fascia are separate items. Split an irregular deck into rectangles or prepare a drawing before ordering. Use the displayed rows and pieces to inspect the assumptions, then adjust your order to match the manufacturer and the actual layout.
Frequently asked questions
Which way do the boards run?
They run along the length field. Swap length and width to estimate the same rectangle with the decking rotated ninety degrees.
Are offcuts reused?
No. Each row has its own rounded stock count. Reusing suitable offcuts through a cutting plan can reduce the purchase quantity.
Does the gap include the perimeter?
No. Gaps are counted only between rows. Any required perimeter clearance should be accounted for in the measured decking width.
What if the last row is narrow?
The row count rounds upward, so the final board may need ripping. Check the edge detail and fastening rules for that width.
Are fasteners and joists included?
No. This page estimates deck surface boards only. Supporting structure, joint blocking and fasteners require a separate design and takeoff.
Last reviewed: 2026-10-05 · How we calculate