How are decking boards calculated?

Rows = width / (board width + gap). Running metres = rows × deck length.

What gap between boards?

Usually 3–5 mm to allow the wood to expand.

Decking Calculator

Calculate the running metres and number of decking boards from deck size, board width and gap.

Deck length: 5.00 m

Deck width: 4.00 m

Board width: 120 mm

Gap between boards: 4 mm

182 m

Running metres (+10%)

Board rows	33 pcs
Running metres	165 m
Area	20.0 m²

ℹ️ Rows run across the deck width. Add 10% waste for cuts.

This decking calculator works out how many boards and how many running metres you need from the deck size, board width and the gap between boards. It also shows the number of rows and the deck area.

How the calculator works and what it’s for▾

How it works

The tool divides the deck width by the combined width of one board plus its adjacent gap, which gives the number of board rows needed. Multiplying the rows by the deck length yields the total running metres.

For instance, a 120 mm board with a 5 mm gap takes up 125 mm across, so a wide deck needs surprisingly many rows.

Inputs and results

Enter the deck length and width in metres, plus the board width and gap in millimetres. You get the number of board rows, the running metres and the deck area.

The headline figure shows the running metres with a 10% waste margin already added, so you can order boards directly.

Who it is for

It is useful when building or replacing a deck and planning your yard work. It helps you estimate material costs before a trip to the hardware store.

Tips

Run the rows across the deck width. Add 10% waste for cuts, since board ends often have to be sawn and joints consume material. Also check the real board width, as it directly affects the number of rows.

🔄 Reviewed June 2026