Decking Board Calculator
Estimate decking boards, linear metres, packs, waste allowance, joists and screws for a rectangular deck. Enter board width, gap, board length and laying direction so the result matches the way boards are actually fitted.
Deck Size and Board Details
Board Estimate
The estimate rounds up to full boards and packs. Check board defects, trims and steps before ordering.
Plan Boards Before You Price the Deck
A decking quote can look simple until the first board is cut. Board width, expansion gaps, board length, laying direction, offcuts, borders and steps all affect the order. This calculator starts with the deck footprint, then converts that footprint into rows of boards. It also estimates linear metres, packs, joists and screws so a shopping list is easier to check against a supplier quote.
The result is a material estimate for a rectangular deck, not a structural design. Ground conditions, post spacing, ledger fixing, balustrades, loading, drainage, ventilation and product warranty rules still need proper care. For raised decks, commercial use, hot tubs, public access or unusual spans, ask a qualified designer, structural engineer or competent installer to check the frame.
Three Choices That Change the Board Count
Board direction
If boards run along the deck length, rows are counted across the width. If boards run along the width, rows are counted across the length. The same deck can need a different board count when direction changes.
Gap size
A 6 mm gap is counted between rows, so it reduces the number of boards across a fixed width. Never reduce the gap below manufacturer guidance to make the count look cheaper.
Waste allowance
Simple square decks often use 5% to 10%. Diagonal layouts, picture-frame borders, steps and short offcuts can need 12% to 20% or more.
Formula and Method
deck area = deck length x deck width
board pitch = board width + board gap
rows across deck = ceiling(width across boards / board pitch)
board pieces before waste = rows across deck x ceiling(run length / board length)
boards to buy = ceiling(board pieces before waste x (1 + waste percentage))
packs to buy = ceiling(boards to buy / pack size)
joist lines = ceiling(run across joists / joist spacing) + 1
screws = rows across deck x joist lines x screws per fixing line
The calculator rounds up because part boards, half packs and partial joists cannot always be bought. The board estimate assumes straight runs and butt joints where the deck run is longer than the board length. If you plan staggered joins, add more waste and check that joins land on supported joists or noggins as required by the product.
How to Measure the Deck
Measure the finished deck platform, not the rough garden space. If boards will overhang the frame, measure the boarded surface. If a wall, step, planter or post cuts into the rectangle, either split the deck into rectangles or include extra waste. For decks with a picture-frame border, calculate the main field first, then add border boards separately because their cuts and direction differ.
Use millimetres for board width and gap because small differences matter across many rows. A 145 mm board with a 6 mm gap has a 151 mm pitch. Across a 3.6 m width, that works out as 24 rows. Changing to a 120 mm board with the same gap would need more rows, more screws and usually more joins.
Waste Allowance Guide
| Deck layout | Suggested waste range | Why it changes | Extra checks |
|---|---|---|---|
| Plain rectangle, boards run full length where possible | 5% to 10% | Few cuts and offcuts can often be reused. | Check board defects, delivery damage and trim cuts. |
| Rectangle where boards need butt joints | 8% to 12% | Joins must be placed on support, and short offcuts may not suit the next row. | Plan staggered joins before ordering. |
| Deck with picture-frame border | 10% to 15% | Border boards create mitres, returns and short cuts. | Add border length separately if using a different board. |
| Diagonal deck boards | 15% to 25% | Angled cuts create more waste, and usable offcuts are harder to place. | Use a detailed cutting plan. |
| Steps, benches, planters or access hatches | 12% to 20% | Small sections and trims increase cutting time and waste. | Estimate trim boards, fascia boards and fixings separately. |
Joists, Screws and Hidden Costs
- Joists: The joist estimate is a count of lines, not a full structural list. Real frames also need bearers, posts, noggins, blocking, hangers, fixings, damp proof separation and ground clearance.
- Screws: Two screws per board per joist is a common timber assumption. Composite systems may use clips and starter clips instead. Always follow the board supplier fixing method.
- Fascia and trims: Edge boards, fascia, stair nosings and end caps are not included in the board count unless they are part of the deck surface.
- Delivery and storage: Long boards may need special delivery, flat storage and careful handling. Add a spare board if colour batch matching matters.
Worked Example
A 4.8 m by 3.6 m deck uses 145 mm boards with 6 mm gaps, and the boards run along the 4.8 m length. The deck width is divided by the 151 mm board pitch, giving 24 rows. Each row needs two 3.6 m boards to cover the 4.8 m run if full-board joins are used, so the base count is 48 board pieces. With 10% waste, the order becomes 53 boards. If the boards are sold in packs of four, the calculator rounds this to 14 packs.
If the same deck uses 4.8 m boards, the base board count would be 24 pieces before waste. That may cost less and create fewer joins, but long boards can be harder to deliver, store and fit. The cheapest pack price is not always the cheapest finished deck if it creates many offcuts.
FAQ
Does the calculator include a frame?
It estimates joist lines and screws, but it does not design the frame. Posts, beams, footings, ledger boards, bracing and fixings need a separate structural check.
Can I use it for composite decking?
Yes for board count, as long as you enter the correct board width, gap and length. Replace the screw estimate with the clip system stated by the manufacturer.
Should I round down if the deck is nearly exact?
No. Round up. Boards can split, warp, arrive damaged or be rejected for colour and grain. A small spare allowance is normally cheaper than a second delivery.
Sources
- Timber Decking and Cladding Association. (2025). Decking guidance and technical information. TDCA. Accessed 18 May 2026.
- British Standards Institution. (2013). BS 8579: Guide to the design of balconies and terraces. BSI Standards Publication.
- Planning Portal. (2026). Decking: planning permission. Planning Portal. Accessed 18 May 2026.