The 125mm rule, in one line
Openings in a required barrier must not let a 125mm sphere through. That's the child-safety provision in the NCC, it applies to the whole barrier — not just the pretty bit in the middle — and the number is small for a reason: a toddler's head is bigger than 125mm, their body isn't.
So the design job isn't "space the balusters evenly". It's "make sure the widest opening anywhere in this barrier is 125mm or less". Even spacing is how you get there, not the goal.
Why the count rounds up, always
Say you've got a 2400mm bay and 42mm balusters. Run the sum:
- n ≥ (2400 − 125) ÷ (42 + 125) = 2275 ÷ 167 = 13.62
- Round up: 14 balusters, making 15 gaps
- Gap: (2400 − 14 × 42) ÷ 15 = 1812 ÷ 15 = 120.8mm — under the limit
Now round 13.62 down to 13, the way a calculator that isn't paying attention would:
- Gap: (2400 − 13 × 42) ÷ 14 = 1854 ÷ 14 = 132.4mm
Every gap in that bay is over the limit, by 7.4mm, in a barrier whose whole job is stopping a child falling. One baluster saved. That's why this tool rounds up and won't be argued with.
Thinner balusters need more of them
Counterintuitive but obvious once you see it: a fat baluster eats span, so fewer gaps fit. Same 2400mm bay, different balusters:
| Baluster | How many | Resulting gap |
|---|---|---|
| 65 × 65 timber | 12 | 124.6 mm |
| 42 × 42 timber | 14 | 120.8 mm |
| 30 × 30 timber or steel | 15 | 121.9 mm |
| 16 mm stainless rod | 17 | 118.2 mm |
| 12 mm stainless rod | 17 | 122.0 mm |
Worth knowing before you price a job in slim stainless rod because it looks better: 17 rods instead of 14 timber balusters is about twenty per cent more holes to drill and fix, in every bay.
Setting them out on site
Centre to centre is the gap plus one baluster width. For the 2400mm example that's 120.8 + 42 = 162.8mm, with the first baluster's centre sitting 141.8mm off the post face — that's one gap plus half a baluster.
Cut a spacing block exactly one gap wide and work along the rail with it rather than marking every position. Faster, and the error doesn't accumulate the way a run of pencil marks does. Check the last gap against the block before you fix the final baluster — if it's tight, your bay wasn't the width you measured.
Measure every bay separately. Posts are rarely dead accurate over a long run, and a bay 15mm wider than its neighbour can push the gap over on its own.
When you need a barrier at all
The trigger in the NCC is a fall of more than 1 metre. Below that, a barrier is a choice; above it, it isn't. Heights to design to:
| Where | Minimum barrier height |
|---|---|
| Balcony, deck, landing, walkway | 1000 mm above the floor |
| Along a stair flight | 865 mm above the nosing line |
And where the fall is more than 4 metres there's a further rule about climbable elements: horizontal members between 150mm and 760mm above the floor are restricted, because a wide horizontal rail at that height is a ladder to a small person. Design the infill vertical up high.
The gaps everyone forgets
- Under the bottom rail. It's an opening in the barrier and it counts. A bottom rail sitting 140mm off the deck fails on its own, with perfect baluster spacing above it.
- Beside the posts. The gap at each end of a bay is a gap. This calculator counts n + 1 gaps for exactly that reason.
- Between stair treads. An open-riser stair has to pass the same sphere test between its treads — see the stair calculator.
- At a gate or an opening panel. The clearances a gate needs to swing are gaps too.
Wire and glass are different problems
Don't use this calculator for a wire balustrade. Wire deflects under load, so the opening is tested with a push on it, not just measured at rest — which is why wire systems specify their own maximum post spacing and a set wire tension. Follow the system's own instructions.
Frameless and semi-frameless glass has no baluster gaps, but it has its own rules: the glass type and thickness come under AS 1288, and the gaps between panels and underneath them still have to pass the sphere test. Get the panel schedule from the glazier, not from a spacing sum.
Balustrade sorted. Now get the job paid for.
Yamate turns your materials and hours into a quote your customer accepts on their phone, then an invoice, then a payment — GST worked out, follow-ups chased for you.
Try Yamate free → Free to try · no card · sign in with a link, no password to rememberBalustrade spacing FAQ
What is the maximum gap between balusters in Australia?
125mm. Openings in a required barrier must not let a 125mm sphere pass — that includes the gap under the bottom rail and the gaps beside the posts, not just between balusters.
How many balusters do I need?
For a 2400mm bay with 42mm balusters, 14 — giving a gap of 120.8mm. Take the span, subtract 125, divide by the baluster width plus 125, and round up. Enter your own span above.
Why can't I round the baluster count down?
Because fewer balusters means wider gaps. Rounding 13.62 down to 13 on a 2400mm bay takes the gap from 120.8mm to 132.4mm, which is over the limit in every gap in the bay. Always round up.
When do I need a balustrade in Australia?
Where the fall is more than 1 metre. Design to a minimum barrier height of 1000mm on a balcony, deck or landing, and 865mm measured above the nosing line along a stair flight.
What height should a deck balustrade be?
1000mm above the deck surface is the published minimum for a balcony or deck. On the stair flight itself it drops to 865mm above the nosing line. Higher is fine; lower is not.
Does the gap under the bottom rail count?
Yes. It is an opening in the barrier like any other and gets measured the same way. It is one of the most common things missed on an otherwise well-built balustrade.
Can I use this for a wire balustrade?
No. Wire deflects when pushed, so the opening is assessed under load rather than measured at rest. Wire systems set their own maximum post spacing and wire tension — follow those.
Other calculators you'll want
General information for Australian trades, and an estimate only — not tax, financial or legal advice. It is not engineering, building or design advice either: a calculator works out a quantity from the figures you enter, and anything that needs sign-off still needs your engineer, certifier or building surveyor. The 125mm opening limit and the barrier heights here follow published Australian guidance on the NCC barrier provisions. The NCC and the Australian Standards it references are published and sold by their publishers and are not reproduced here; editions change. This calculator works out spacing — it does not sign a barrier off. Confirm any barrier you are going to build with your certifier or building surveyor, and follow the manufacturer's instructions for a proprietary wire or glass system.