Decking Joist Calculator
Count simple joist lines and linear metres from the real frame dimensions and the specified maximum joist centres for your chosen decking system.
Estimate joist lines and timber length
Use dimensions from the actual frame layout or current drawing. Enter the joist centres required by the decking manufacturer/specification—this tool does not choose them for you.
How to calculate decking joist quantity
For a simple rectangular field of parallel joists, the calculator divides the distance across the joists by the maximum centres you have specified. It rounds the number of spaces up, then adds the end joist line.
Measure the length of each simple joist line from the actual frame layout.
Use the maximum centres permitted for the exact decking, board direction and project specification.
Convert net linear metres to stock only if you know the real merchant length.
Net joist metres = joist lines × joist run length. Optional extra is applied only to the purchasing-length target, never to the joist-line count.
Worked quantity example
Suppose the simple joists run 4.00 m and must be set out across 5.00 m. If the exact decking/system specifies a maximum of 400 mm centres, 5.00 ÷ 0.40 = 12.5 spaces, so the quantity estimate needs 13 spaces and 14 joist lines.
Those 14 lines contain 56.00 linear metres of joist before any separately chosen purchasing allowance. This does not determine whether a 4 m joist run is structurally permissible.
Why there is no default joist spacing
Current manufacturer guidance varies by product and board orientation. Trex UK says common composite installations can use joists at up to 400 mm centres, while diagonal decking may require tighter spacing. Cladco currently specifies different maximum spacing for different systems, including 400 mm for its standard composite guidance and 300 mm for Nordeck. The correct value therefore belongs to the chosen decking/system—not to a generic calculator.
Trex UK decking installation guidance · Cladco installation guidance · Wickes exterior joist example
This is not a structural joist-span calculator
Quantity estimate only. This tool does not select joist depth/width, timber grade, treatment, allowable span, bearer or post spacing, cantilevers, connections or fixings. Follow the structural design, drawings, Building Regulations where applicable, and the exact decking/subframe manufacturer instructions.
Stock-length conversion is only a purchasing cross-check
If you enter a supplier stock length, the calculator divides the adjusted linear-metre target by that length and rounds up. That does not prove that the individual joist runs can be cut from that many pieces. A run longer than one stock piece, restricted joints, unusable offcuts and doubled/trimmer members can all increase the real order.
Continue the decking estimate
Decking Board Calculator
Estimate surface-board rows and linear metres separately from the frame.
Timber Length Calculator
Convert an already known timber requirement into merchant stock-length equivalents.
Decking Materials Estimator
Work through boards, joists and separately measured frame extras as one project workflow.
Frequently asked questions
Does this calculator tell me what joist centres to use?
No. Enter the centres specified for your exact decking product, board direction and frame design.
Does it select the joist size or maximum span?
No. It is deliberately a quantity estimator. Joist section, strength grade, supports and allowable span require the appropriate design information.
Does the joist-line count include rim joists or doubled members?
No. The count represents repeated parallel field joist lines only. Add perimeter members, doubled joists, trimmers, noggins and special framing explicitly from the real design.