How This Calculation Works
The deck board calculator estimates how many boards you need by dividing the deck width by the effective width of one board (board face width plus the gap between boards). This gives the number of rows across the deck.
If the deck is longer than a single board, the calculator determines how many boards are needed per row (deck length ÷ board length, rounded up) and multiplies by the number of rows. A waste factor (typically 10%) is then added for cuts and unusable off-cuts.
The screw estimate is based on approximately 350 screws per 100 ft² (or 38 per m²), which assumes two screws per joist at each board crossing with joists spaced 40 cm (16 in) apart.
Common Mistakes to Avoid
- Forgetting the gap. A 5 mm (3/16 in) gap between boards is necessary for drainage and wood expansion. Without it, boards can buckle in hot weather.
- Using nominal dimensions. A "5/4 × 6" deck board actually measures about 1 in × 5.5 in. Always use the actual face width in your calculation.
- Underestimating screws. Running out of screws mid-project means a trip to the store. Buy a box with at least 10% more than estimated.
- Not planning the substructure. This calculator covers decking boards, but you also need joists, beams, and posts. Plan the frame first so you know the exact deck dimensions.
Worked Example
Scenario: A 5 m × 4 m deck using 14 cm wide boards that are 3.6 m long, with 5 mm gaps and 10% waste.
Step 1 — Effective board width: 14 cm + 0.5 cm gap = 14.5 cm = 0.145 m.
Step 2 — Rows: 4 ÷ 0.145 = 28 rows (rounded up from 27.6).
Step 3 — Boards per row: 5 ÷ 3.6 = 2 boards per row (rounded up from 1.39).
Step 4 — Total boards: 28 × 2 = 56 boards.
Step 5 — With waste: 56 × 1.10 = 62 boards. Buy 62 boards and ~760 screws.