VolunaroDeck calculator
How the calculation works
- Rows: the deck width plus one gap, divided by the board width plus one gap, rounded up.
- Joists: one every joist spacing along the deck length, plus one at the far end.
- Cut plan: if one board covers the deck length, each row is one board. If not, the boards are joined end to end with each butt joint centered over a joist, and neighbouring rows use different patterns so their joints never share a joist.
- Boards: the pieces of every row are cut from full-length boards, reusing offcuts and allowing for the saw kerf. We then add the spare-board margin for defects and mistakes.
- Fasteners: (rows × joists + one extra set per butt joint) × fasteners per board at each joist.
- This is a material estimate, not a structural design. Joist, beam and post sizes and spans depend on your local code.
Worked example
A 16 × 12 ft deck with 16 ft boards, so each row is a single board with no butt joints, plus the default spare boards.
- Deck area
- 192 sq ft
- Rows of boards
- 26
- Deck boards to order (cut plan + spares)
- 29
- Boards used by the cut plan
- 26
- Butt joints (each over a joist)
- 0
- Installed board length
- 416 ft
- Offcuts left over
- 0.0 ft
- Joists (including both ends)
- 13
- Joist length
- 156 ft
- Fasteners
- 676
Default values and sources
| Value | Default | Type | Sources |
|---|---|---|---|
| Actual width of a 5/4 × 6 deck board | 5.5 in | Typical published value | True ValueWestern Forest ProductsNIST |
| Gap between boards | 0.1875 in | Our default (editable) | — |
| Typical maximum joist spacing, boards perpendicular to joists | 16 in | Typical published value | Simpson Strong-Tie (Structural Engineering Blog)American Wood Council |
| Fasteners per board at each joist | 2 | Typical published value | DeckoratorsTrexTrex |
| Saw kerf (width of each cut) | 0.125 in | Our default (editable) | — |
| Spare boards on top of the cut plan | 10 % | Our default (editable) | — |
Limitations
- Material quantities only: this calculator does not size joists, beams, posts or footings. Follow your local code and the decking manufacturer's span and fastening guide.
- Assumes boards run perpendicular to the joists, with butt joints centered over a joist and staggered between neighbouring rows. Diagonal layouts need closer joist spacing and more waste.
- The default of two fasteners per joist suits face-screwed boards; hidden-clip systems often use one per joist. Change it to match your system.
- The 16 in maximum joist spacing is a typical limit for 5/4 or composite boards laid perpendicular; many composite boards require closer spacing. Your manufacturer governs.
- Some decking needs doubled joists or blocking at butt joints: check your manufacturer's installation guide.
Results are estimates for planning and ordering, not professional, engineering or legal advice. Confirm quantities, sizes and local building requirements with your supplier or contractor before you buy or build.