Deck Calculator
Calculate the deck area and the number/length of deck boards needed, given the board width and gap.
- Free to use
- Accurate results
- No registration required
- Works on all devices
Enter your deck's length, width, and board size to find how much decking material you need.
Your result
92.4
Total board length needed
Board rows across width21
Deck area12
AI explanation
Formula
Board rows = deck width ÷ (board width + gap), rounded up; total board length = rows × deck length × (1 + wastage%)Worked example
4m × 3m deck, 140mm boards, 5mm gap, 10% wastage
| Field | Value |
|---|---|
| Deck length (m) | 4 |
| Deck width (m) | 3 |
| Board width (mm) | 140 |
| Gap between boards (mm) | 5 |
| Wastage allowance (%) | 10 |
| Deck area | 12 |
| Board rows across width | 21 |
| Total board length needed | 92.4 |
Assumptions
- Assumes deck boards run the full length of the deck, laid side by side across its width with a fixed gap for drainage/expansion.
Frequently asked questions
Why leave a gap between deck boards?
A small gap (typically 3-6mm) allows for drainage, airflow, and slight expansion/ contraction of the boards with temperature and moisture changes.
How do I get the total number of boards, not just total length?
Divide the total board length by your board's individual length to get the number of full-length boards needed (round up, since boards are typically sold in fixed lengths).
Does this include joists or substructure material?
No — this calculates decking (surface) boards only, not the substructure below.