Decking Calculator
Decking boards are sold in fixed lengths and laid in rows with a gap, so the answer depends on the board width, the gap and which way the boards run as much as on the deck size. Enter the deck, choose the board and joist spacing, and this calculator counts the rows, the linear metres with waste, the boards, the joists and the screws, rounded up to what you can actually buy.
Your measurements
Check your order
Your outdoor material order
- Mark the footprint and check finished levels, depth or board layout as applicable.
- Confirm sales units, delivery access and the supplier’s allowance for settlement or compaction where needed.
- Measure edging and finishing runs separately and check the tools you can reuse.
Tools for this job
Check the deck footprint, board lengths and planned gaps before ordering. Measure diagonals when laying out a rectangular deck.
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- Check room dimensions and cuts
Short tape measure
STANLEY 25-foot tape measure
For checking lengths, widths and short runs before ordering. This is a 25-foot tape; use matching units in the calculator and check the scale on the selected model.
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Long tape measure
Komelon 6611 100-foot open-reel tape
For measuring longer outdoor runs and marking out areas. Keep the tape straight and use the same units throughout; this listing is for the 100-foot model.
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Already have suitable tools? Reuse them. These are optional product suggestions, not a complete equipment list or a hands-on comparison. Amazon may redirect Amazon.com links to local products or search results. Amazon.nl links search for alternatives, which may differ in brand, size and specification. Check suitability, seller, delivery and any import costs on your chosen store before ordering.
How many decking boards do I need?
Decide which way the boards run, then divide the deck dimension across the boards by the board width plus the gap. That gives the number of rows. Each row is as long as the deck in the other direction, so rows multiplied by that length is the linear metres of decking. Add the cutting allowance, divide by the length of the boards you are buying and round up: that is the number of boards.
The joists run at right angles to the boards, so their length is the dimension across the boards and their number is the run length divided by the joist centres, plus one for the far end. The screw count follows directly: every row crosses every joist, and each crossing takes two screws. A deck of modest size needs several hundred screws, which is why they are sold in boxes of 200.
The formula
Board sizes, gaps and joist centres
Board lengths are 2.4, 3.6 and 4.8 m in the UK, 12 and 16 ft in the US. Buy the length that matches a whole number of runs where you can: 4.8 m boards on a 4.8 m deck have no joints at all.
| Board | Width + 5 mm gap | Rows per 3.6 m | Usual joist centres |
|---|---|---|---|
| Softwood, 120 x 28 mm | 125 mm | 29 | 400 mm |
| Softwood, 145 x 28 mm | 150 mm | 24 | 400 mm |
| Hardwood, 140 x 21 mm | 145 mm | 25 | 400 mm |
| Thick softwood, 145 x 38 mm | 150 mm | 24 | 450 to 600 mm |
| US 5/4 x 6 or 2 x 6 (5.5 in) | 144.7 mm | 26 | 16 in (406 mm) |
| Composite, 140 mm | 140 mm plus the maker's gap | 25 or so | 300 to 400 mm, per the manufacturer |
Worked example
A 4.8 m x 3.6 m deck, boards running along the 4.8 m length, 145 mm boards with a 5 mm gap, 4.8 m board lengths, joists at 400 mm centres and a 10% allowance.
Each row takes 145 + 5 = 150 mm, so 3.6 ÷ 0.15 = 24 rows. Linear metres = 24 × 4.8 = 115.2 m, plus 10% = 126.7 m. Boards = 126.7 ÷ 4.8 = 26.4, rounded up to 27 boards. Joists = 4.8 ÷ 0.4 = 12, plus 1 = 13 joists, each 3.6 m long, which with the two 4.8 m rim boards is 56.4 m of joist timber. Screws = 24 × 13 × 2 = 624, so 4 boxes of 200.
Turn the boards to run across the 3.6 m width instead and you get 32 rows of 3.6 m: the same 115.2 m of board, but with 3.6 m boards there are no end joints at all, and the frame becomes 10 joists of 4.8 m instead of 13 of 3.6 m. Try both in the calculator: the direction changes the joist count and the offcuts more than it changes the number of boards.
The frame underneath
- Joists: 47 x 150 mm treated softwood is the usual section for spans up to about 2.5 m at 400 mm centres; longer spans need a bearer underneath or a deeper joist. Ground-level decks often use 47 x 100 mm.
- Rim or ledger boards: the two outer members the joists fix to, the same section as the joists. They are included in the timber total above.
- Posts and footings: a post at each corner and every 1.8 m or so along the bearers, set in concrete. The concrete calculator works out the bags with its post hole shape.
- Noggins: short pieces between the joists at mid-span on anything over 2 m, cut from the offcuts.
- Weed membrane and gravel under a low deck: see the gravel calculator.
Screws, gaps and joints
Use two screws at every crossing, 20 mm in from each board edge, and choose a screw about 2.5 times the board thickness: 64 mm for a 28 mm board. Stainless steel or coated decking screws only; plain zinc rusts and stains within a year. Pre-drill hardwood and the ends of every board, or they split.
Lay the boards with the gap set by a 5 mm spacer and remember that fresh treated softwood shrinks: a 5 mm gap laid wet can open to 8 mm by the end of the first summer, and a board laid tight can close to nothing. Where two boards meet end to end, cut both to land on the centre of a joist, screw both ends, and stagger the joints so they never line up in neighbouring rows. Finish with a decking oil or stain, using the paint calculator for the quantity.
Sources and assumptions
Board rows use the board width plus gap. Total linear length is increased by waste and divided by stock length; joists and screws are then estimated from the selected spacing and crossings.
- Timber Decking and Cladding Association: decking specification: The association discusses timber moisture, drainage gaps and two fixings at joist crossings. The guidance makes clear that material and installation details matter.
Before using this estimate
- Total linear length is not an optimised cutting plan. End joints require suitable support and can increase both timber and fixing quantities.
- Joist count is not a span or load calculation. Beams, posts, footings, ledger connections, balustrades and composite-board requirements need a separate deck specification.
Source notes reviewed . See how we check calculations and handle corrections.
Planning your order
Check the inputs before buying: measure the surfaces you need, then apply an allowance and round to whole packs. The guides include worked examples you can follow alongside this calculator.
Published by OrderEnough. This is a planning estimate: check the product specification and the assumptions above before ordering. Last updated . Read how we source our figures on the methodology page.