About the Deck Board Calculator
Deck boards are the visible part of a deck, and in composite or hardwood they are also the most expensive part. They are bought by the piece in fixed lengths, so the question is not how many square feet the deck has but how many rows of boards it takes and how those rows divide into the lengths on sale.
This deck board calculator works out the number of rows from the deck's width, the board width and the gap between boards; the total run of board; how many boards that is in the length you buy; and how many screws or hidden clips will hold them down.
How to Use the Deck Board Calculator
Choose imperial or metric.
Enter the deck length — the direction the boards run, usually along the house — and its width, across the boards.
Enter the board width (the actual face width, 5½ in for a nominal 6 in board, 140 mm for most metric decking) and the gap between boards.
Enter the board length you will buy and the joist spacing.
Choose face screws or hidden clips, and adjust the waste allowance.
How the Boards Are Worked Out
rows = deck width ÷ (board width + gap), rounded up
run = rows × deck length
boards = run × (1 + waste %) ÷ board length, rounded up
crossings = rows × joists
screws = crossings × 2 (or clips = crossings)Step-by-Step Example
A 16 × 12 ft deck, 5½ in boards with 3/16 in gaps, 16 ft boards, joists at 16 in, face screws, 5% waste.
Rows: 144 in ÷ 5.6875 in = 25.3 → 26 rows
Run: 26 × 16 ft = 416 ft
Boards: 416 × 1.05 ÷ 16 = 27.3 → 28 boards
Joists: 16 ft ÷ 16 in + 1 = 13
Screws: 26 × 13 × 2 = 676In metric, a 4.8 × 3.6 m deck with 140 mm boards and 5 mm gaps has 25 rows and needs 27 boards of 4.8 m.
Board Widths and Gaps
The nominal size of timber is not its actual size. A "5/4 × 6" deck board is about 1 in thick and 5½ in wide; a "2 × 6" is 1½ × 5½ in. Always use the actual face width, which is printed on composite packaging and easy to measure on timber.
The gap lets water drain and debris fall through, and allows boards to move. Kiln-dried timber and composite are usually gapped at about 3/16 in (5 mm). Freshly treated lumber is often wet and will shrink, so many builders butt it tight and let the gaps open as it dries. Composite boards also need end gaps that vary with the temperature at installation; the manufacturer's guide gives the figures.
Board Length and Joints
The neatest decks use boards at least as long as the deck, so every row is a single piece. When the deck is longer, rows need butt joints. Each joint must fall on a joist, with both board ends fully supported — usually by doubling that joist or adding a block beside it — and joints in neighbouring rows should be staggered so they do not line up. Some designs use a breaker board across the deck at the joint line, which makes a feature of it.
Screws or Hidden Fasteners
Face screws go down through the board into each joist, two per crossing. Stainless or coated deck screws resist corrosion from treated timber and weather.
Hidden fasteners are clips that sit in grooves on the board edges and screw into the joist, leaving the surface clean. They are one per crossing and set the gap automatically, but need grooved boards.
At the start and end of the deck, and at butt joints, boards are usually face-screwed even with hidden systems.
Timber, Hardwood or Composite?
Pressure-treated softwood is the cheapest decking, but it needs cleaning and re-staining every year or two and can split and splinter as it weathers.
Hardwoods such as ipe, balau or cumaru are dense, very durable and beautiful, but heavy, hard to cut and costly; they need pre-drilled holes for every screw.
Composite and PVC boards need little maintenance — no staining or sealing — and do not splinter, but cost more up front, can get hot in full sun, and have strict rules on joist spacing, gaps and fixings that must be followed for the warranty to apply.
Whatever the material, lay the first board straight against a string line, check the spacing every few rows so the last board is not a sliver, and finish the edges with a fascia or picture-frame border for a tidy look.
Stairs, Borders and Fascia
The calculator covers the main deck surface. Stair treads are extra: each tread usually takes two or three boards the width of the stair. A picture-frame border — boards running round the edge at right angles to the field — adds its perimeter in board length, and fascia boards cover the rim joist. Add these to the order separately.
Understanding Your Result
Boards is the number of boards to buy, including waste.
Rows gives the rows across the deck and the total run.
Fasteners gives the screws or clips.
Deck gives the area and joist count.
Worth knowing covers joints between boards.
When Should You Use This Calculator?
Pricing decking in timber, hardwood or composite.
Comparing board widths or lengths.
Ordering screws or clips with the boards.
With the deck calculator, which gives the framing.
Common Mistakes
Using nominal board sizes. Use the actual face width.
Forgetting the gaps. They add up across the deck.
Joints between joists. Every butt joint needs support.
Too little waste on diagonal decks. Allow 10–15%.
Mixing batches of composite. Colour can vary between runs.