SettingsSettings for this calculationUS
The length of railing between two posts.
Measure one post-to-post span at a time — repeat for each section of your railing.
Tools needed: Tape measure
The width of a single baluster (spindle), from the product spec.
Standard square wood balusters are commonly 1.5 in (3.8 cm) x 1.5 in; metal balusters are often thinner.
Balusters needed
17 balusters
The count is the fewest spindles that keeps every clear gap at or under the largest gap shown: 100 mm (3.94 in) when you work in metric, the sphere England's Part K and Canada's code use, and 4 in (101.6 mm) in imperial, the IRC's. The figure your own building rule sets is the one that governs, and some jurisdictions set stricter guard requirements.
- Rail section length
- 96 in
- Actual gap between balusters
- 3.92 in
- Largest gap allowed
- 4 in
They open the calculator with your figures already in it
Deck Baluster Calculator: 17 balusters — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- US: International Residential Code R312.1.3 — openings in a required guard must not allow passage of a 4 in (101.6 mm) diameter sphere; the imperial page solves to it
- UK: Approved Document K (2013), Section 3 — guarding in a building likely to be used by children under five should not have openings that allow passage of a 100 mm (3.94 in) sphere; the metric page solves to it
Inputs used
- Rail Section Length
- 8 ft
- Baluster Width
- 1.5 in
Intermediate steps
- Rail section length
- 96 in
- Actual gap between balusters
- 3.92 in
- Largest gap allowed
- 4 in
Confidence note: The count is the fewest spindles that keeps every clear gap at or under the largest gap shown: 100 mm (3.94 in) when you work in metric, the sphere England's Part K and Canada's code use, and 4 in (101.6 mm) in imperial, the IRC's. The figure your own building rule sets is the one that governs, and some jurisdictions set stricter guard requirements.
What this calculation does not cover
- The count covers one post-to-post bay and includes no spares, so a railing made of several sections has to be worked out and ordered bay by bay, with your own allowance on top for a spindle that splits at the fixing hole.
- Spacing is solved as one repeating baluster-plus-gap unit, which assumes every spindle is exactly the width entered and that the two end gaps against the posts finish the same as the gaps in the middle; setting out from a centred spindle, or leaving a wider gap at one post, breaks the figure shown for 'Actual gap between balusters'.
- Rail section length is taken as the clear opening between posts exactly as typed, with nothing deducted for post faces, newel trim or rail-end brackets, so a centre-to-centre measurement overstates the run and buys more spindles than the real opening needs — squeezed into the true span, the finished gaps close up tighter than the breakdown reports.
- The largest gap follows the unit system the page is worked in, not the place the guard is built: 100 mm (3.94 in) in metric, the sphere of England's Approved Document K and Canada's National Building Code, and 4 in (101.6 mm) in imperial, the IRC's. A Canadian guard set out in inches is still held to 100 mm, so work it in metric; where a code allows a larger sphere, such as Australia's 125 mm (4.92 in), this count can hold more spindles than that code asks for, never fewer.
- Nothing in the result speaks to strength or fixing: baluster thickness, whether each spindle is screwed, pocketed or clipped to the rails, and the load the infill has to resist are all absent from the arithmetic, which counts spacing and nothing else.
- Only the horizontal gaps between spindles are sized here — the gap beneath the bottom rail, the clearance where the guard meets the deck surface and the height of the guard itself are never computed, so a bay that passes on spindle spacing can still leave an opening somewhere the count never looked.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Part of bigger jobs
This trade is one line of a job takeoff. Run the whole job and every other trade comes back with it, off the same measurements.
Or plan the space itself: measure it once, doors and windows included, and this figure comes back worked out on that space, with what goes with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-28 · v1.1.0
Regulatory standards & verification citations2
- US: International Residential Code R312.1.3 — openings in a required guard must not allow passage of a 4 in (101.6 mm) diameter sphere; the imperial page solves to it
- UK: Approved Document K (2013), Section 3 — guarding in a building likely to be used by children under five should not have openings that allow passage of a 100 mm (3.94 in) sphere; the metric page solves to it
Which documents these citations point at
- International Residential Code — R312.1.3 (United States)One- and two-family dwellings and townhouses up to three storeys — structure, fire safety, energy, plumbing, mechanical and electrical in a single document.
- Approved Documents to the Building Regulations (England) — K (United Kingdom)Practical guidance on meeting the Building Regulations, one lettered part per subject — Part A structure, Part B fire, Part K protection from falling, Part L conservation of fuel and power, and the rest.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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Your workspace
Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.
Tools and safety for this job
To fit the handrail and balustrade. Generic types, no brands, no prices.
Protection this work requires
- Hardwood dust is a recognised carcinogen: extract at the tool and wear a P2/N95 respirator when cutting or sanding indoors.
- Saws, grinders and breakers run above 85 dB, where hearing damage accumulates and does not recover: defenders or plugs for every cut, not just the long ones.
Show the 4 tools this job needsHide tools
Essential
Cordless drill/driver
Mitre saw
Spirit level
Tape measure
Also needed as materials: drill and driver bits, mitre saw blades.
what each carpentry tool is for, and the spec that decides which to buy where one does.