Decking calculator
Boards, joists, screws and the frame for a timber deck — with a span check that tells you whether your joists will actually carry it before you buy them.
How this calculator works
The span check is the part that matters
Board quantities are arithmetic and nobody gets hurt if they are slightly out. Joist sizing is different: undersized joists at too wide a spacing give you a deck that feels alive underfoot, then sags, and in the worst case fails. It is also the decision that is impossible to correct later without taking every board up.
So this calculator checks your deck width against the clear span the chosen joist can actually manage at the chosen spacing, and tells you when you need an intermediate bearer. Adding one bearer halves the span, which usually lets you drop a joist size — often cheaper than the bigger timber, and always stiffer.
| Joist | 400 mm centres | 450 mm centres | 600 mm centres |
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These are planning figures, not a design. Actual capacity depends on the timber grade, its moisture content, how the deck is supported, and what you put on it — a hot tub is an entirely different problem from a table and four chairs. A raised deck, a deck attached to the house, or anything more than about 600 mm off the ground brings building regulations into play in most countries. Check locally, and get a raised structure designed.
Board count comes from linear metres, not from board count per row
Dividing each row by the board length and rounding up per row over-orders badly, because it throws away every offcut. In practice the offcut from the end of one row starts the next. This calculator totals the linear metres you need, adds the cutting waste, and then divides by the board length — which is how a merchant's estimator does it and is typically one or two boards fewer on a deck this size.
The one case where per-row rounding is right is a board pattern with no butt joints allowed — if you want every row to be a single unbroken board, your deck length is limited to the board length and you order one board per row. Set the cutting waste to 0 and multiply rows by one yourself.
The gap between boards is not optional
Timber moves with moisture. A 5 mm gap lets water drain and lets the boards swell without buckling against each other. Board width plus gap is what determines the row count, so the gap directly changes how many boards you buy — on a 3 m wide deck with 144 mm boards, moving from a 5 mm to a 3 mm gap adds a row.
If you are laying pressure-treated boards that arrived wet, gap them tighter — they will shrink as they dry. Kiln-dried or composite boards go the other way and need the full gap or more.
Common questions
Which way should the boards run?
Across the direction of the joists, always — that is structural, not aesthetic. Beyond that, running boards away from the house makes a narrow deck look longer and lets water run off rather than pooling against the wall. Running them parallel to the house suits a wide, shallow deck. Whichever you choose, the joists must run the other way, which is why this calculator asks for length and width separately.
Do I need noggins between the joists?
Yes on anything but the smallest deck. Noggins stop the joists twisting under load, which is what makes a deck feel solid rather than springy. One row down the middle is the minimum; two rows on a deck over about 3 m wide. They also give you something to fix board ends to where two boards meet.
How many screws per board?
Two at every joist crossing, positioned about 20 mm in from each edge. One screw lets the board cup and twist. The calculator counts two per crossing and adds 10% for the ones you strip or drop through the gaps — and you will drop some through the gaps.
Should I use hidden fixings?
They look better and they eliminate the water traps that screw heads create, which genuinely extends board life. They cost several times as much, take much longer, and most systems need a specific board profile with a groove. If you go this way, the screw count here does not apply — use the clip manufacturer's figure, which is normally one clip per board per joist.
What about the ground underneath?
More important than the timber. A deck on soft or uneven ground moves regardless of how well it is built. You want a stable, free-draining base with the posts or pads bearing on something solid, a membrane to keep weeds down, and enough clearance under the frame for air to circulate — trapped damp under a deck is what rots the joists from beneath.
Related
- Concrete calculator — for the post holes and pads under the frame
- How to measure properly
- Fixings by substrate — for attaching a ledger to the house wall
Method last reviewed 2026-09-18. Estimates for planning. Structural decks — raised, attached to a building, or carrying unusual loads — should be designed by a qualified person.