Choose the board direction
Boards run along either the entered length or width. Across the perpendicular dimension, rows round up using (deck width across boards + gap) ÷ (board face width + gap). The edge board may need ripping.
07 / Outdoor surfaces
Decking board calculator
Estimate surface boards for a rectangular deck using board direction, actual face width, side gap and stock length.
Know what goes into your estimate
Count rows before buying lengths
A board layout estimate with visible assumptions.
Allocate stock per row
Divide the distance along the boards by stock board length and round up. Multiply by rows. Each row receives its own stock boards, so spare lengths from one row are not reused in another.
Review the actual build
Add an editable percentage of extra boards and round up. Prepare the real supported joint and cutting plan. The calculator does not count substructure, fasteners, stairs, fascia or picture framing.
How board width changes a decking estimate
Examples use a 4 × 3 m deck, boards along the 4 m side, 4.8 m stock boards and a 5 mm side gap. The gap is an example only. Offcuts are not reused, and substructure, fasteners and supported joints need separate planning.
| Board face width | Rows | Boards before extra | Boards with 10% extra |
|---|---|---|---|
| 90 mm | 32 | 32 | 36 |
| 120 mm | 25 | 25 | 28 |
| 140 mm | 21 | 21 | 24 |
These are examples, not product specifications. Enter your own measurements and product details in the calculator above.
Common questions
How many boards for a 4 × 3 m deck?
For boards running along the 4 m length, 140 mm board faces and 5 mm side gaps give 21 rows. With 4.8 m stock boards, each row uses one board: 21 before allowance. Adding 10% rounds the purchase up to 24 boards.
Why does a longer deck sometimes need many more boards?
If a row exceeds one stock board, it receives another whole board. Offcuts are not pooled. This is a conservative stock allocation, not an optimal cutting plan; a designed layout may use materials differently.
Does it calculate safe joist or bearer spans?
No. It only counts surface boards. Deck height, foundations, structural spans, connections, loads and permits need separate assessment under the chosen system and local requirements.
Is the example gap suitable for my boards?
The 5 mm example is editable. Required gaps vary by material, temperature, fastening system and manufacturer. Enter the side gap specified for your boards; end-to-end expansion gaps are outside this model.
Where can I check decking guidance?
Trex’s board quantity guidance discusses board dimensions and layout. Its spacing guide explains why product-specific gaps matter. Room & Root is independent of Trex.