Worked example: a 4 m × 3 m deck
140 mm boards with 5 mm gaps, 3.6 m boards, on joists at 400 mm centres.
| Item | Quantity | Working |
|---|---|---|
| Rows of boards | 21 | On the 140 mm board + 5 mm gap module across the 3 m width |
| Boards per row | 2 | A 3.6 m board does not span 4 m, so two boards run end to end |
| Boards to buy | 42 | 21 rows of two boards |
| Joists | 11 | At 400 mm centres — 33.0 m of timber in total |
| Decking screws | 924 | Two per board per joist crossing, so 4 boxes of 250 |
| Joist hangers | 22 | Two per joist where it meets a rim board or a ledger |
Boards are counted in rows, not by area
The board and the gap together are the module, exactly as the joint is with paving. A 140 mm board with a 5 mm gap needs 21 rows to cover the 3 m width, and rounding up to a whole row is what stops the deck ending up short at the last edge.
Why 42 boards and not 21
Each row runs the 4 m length of the deck, and a 3.6 m board does not span it — so every row is two boards end to end. That is 21 rows of two, which is 42 boards. Offcuts from the second board can be used to stagger the joints, but the count is still based on whole boards.
Joists and screws
Joists at 400 mm centres across a 4 m run come to 11, which is 33.0 m of timber. Screws run at two per board per joist crossing, giving 924 — 4 boxes of 250. Where a joist meets a rim board or a ledger it wants a hanger, which is 22 in this deck.
How the calculator works
- Enter the deck length and width, the board width and the gap.
- The board plus the gap gives the rows; the rows round up.
- The board length divides into the deck length to give boards per row.
- Joists come from the span and the joist centres.
- Screws and hangers follow the row count and the joist count.
Where to go next
Decking FAQs
How many decking boards do I need for a 4 m × 3 m deck?
42 boards of 3.6 m — 21 rows with two boards end to end in each row.
How many joists does a deck need?
11 for this deck, at 400 mm centres, which is 33.0 m of timber in total.
How many screws for a deck?
Two per board at every joist crossing, so 924 for this deck — 4 boxes of 250.
Do I need joist hangers?
Where a joist meets a rim board or a ledger, yes — 22 for this deck. A joist carried on a post or a beam does not need a hanger but will want a post shoe or a bracket.
How do I allow for the gap between boards?
The board plus the gap is the module the calculator counts: 140 mm boards with 5 mm gaps give the 21 rows this 3 m width needs. A wider gap means fewer boards but more shadow underneath.
Guidance only. The quantities above are planning estimates from the ProjectCalc calculators, not a quote and not a specification. Check your own measurements and your supplier's current product data before ordering.