How many decking boards do I need?
Enter your deck length and width, choose your board size and gap, and set a waste allowance — you'll get the number of decking boards to buy, the rows and boards per row, the lineal metres of decking and a screw estimate, instantly. For example, a 5 × 4 m deck in 140 mm boards with a 5 mm gap is 28 rows of full-length boards — about 31 boards with a 10% waste allowance.
Includes a 10% waste allowance for cuts, off-cuts and the odd bad board. Boards run the length; each row is 5 m board. The screw count assumes two face-screws per board at every joist — hidden-clip systems use about one clip per board per joist.
Boards = rows (deck width ÷ [board width + gap], rounded up) × boards per row (deck length ÷ board length, rounded up), plus a waste allowance. This sizes the DECKING only — joist size, beam size, footings and span are a structural design that depends on your load and span; confirm those with a designer and your local building authority.
Common questions
- How many decking boards do I need?
- Count the rows across the deck width, then the boards in each row. Rows = deck width ÷ (board width + gap), rounded up; boards per row = deck length ÷ board length, rounded up. Multiply the two and add 10% for waste. For example, a 16 × 12 ft deck in 140 mm boards with a 1/4 in (5 mm) gap is 26 rows of full-length boards ≈ 29 boards with a 10% allowance. The calculator above does it for you in metres.
- How do I calculate decking boards?
- It is a covering problem. Decking runs one way (the length); you lay row after row across the width with a small drainage gap between boards. Divide the deck width by (board width + gap) and round up for the number of rows, divide the deck length by the board length and round up for the boards per row, then multiply and add a waste allowance. Boards are sold in fixed lengths, so buying a board length that matches your deck avoids off-cuts.
- How much gap should I leave between deck boards?
- A drainage and expansion gap of about 1/8 to 1/4 in (3–6 mm) between boards is typical for pressure-treated and composite decking; some green (wet) timber is butted tight because it shrinks as it dries. The gap matters for the count because every board after the first uses up its own width plus the gap of deck width, so a wider gap means slightly fewer boards. Set the gap in the calculator to match your board type and the manufacturer's instructions.
- How many screws do I need for a deck?
- The common rule is two face-screws per board at every joist it crosses. So screws = 2 × rows × joist lines, where joist lines = deck length ÷ joist spacing + 1. A 16 × 12 ft deck with joists at 16 in on-centre is about 26 rows × 13 joist lines × 2 ≈ 676 screws. Hidden-fastener clip systems use roughly one clip per board per joist instead. The calculator estimates this for you once you set the joist spacing.
- How much waste should I add for decking?
- About 10% is a normal allowance, rising to 15% for a diagonal or herringbone layout, a deck with lots of angles, or a picture-frame border — anything that means more cuts and off-cuts. The allowance covers the waste from cutting boards to length, the occasional bowed or split board, and a few spares. Order the whole job in one delivery so the colour and batch match.
- Does this calculator size the joists and footings?
- No — it sizes the decking boards, lineal length and fasteners only. Joist size, beam size, footing size and how far each can span depend on the load, the span and the timber species/grade, which is a structural design set by span tables and your local building code. Use this tool to order the decking, then have the frame designed and confirm the requirements with your local building authority.
Want the full walk-through? Read the decking board guide → Framing the deck first? Try the framing calculator →
Reference & education only. Not professional, engineering, or code-compliance advice. Estimates are based on published model codes; local amendments and your Authority Having Jurisdiction (AHJ) govern. Always verify against the current adopted code and a licensed professional before doing work.
Last reviewed 2026-07.