Glass · Safety critical
Glass balustrade thickness calculator
A free-standing glass balustrade is a cantilever, so the interlayer decides everything. Stress at the base is σ = 6·F·h/t² and deflection at the top is δ = 4·F·h³/(E·t³) — and t is the effective thickness, which for the same 12+12 laminate can be 15 mm or 24 mm depending on how much shear the interlayer transfers. That is a factor of four in deflection. This page gives you both bounds and never pretends to know which one you have.
Glass balustrade thickness chart
The thinnest laminated glass for a free-standing balustrade — clamped at the base, no handrail, line load on the top edge — that passes in the calculator above: deflection within height / 65 and bending stress, with the line load × 1.5, within the design strength for a 5-second load, both at the conservative bound, with no shear transfer through the interlayer. Two equal plies in mm, interlayer not counted. The loads are the UK National Annex values the calculator offers.
| Line load, kN/m | 900 mm 35.4 in | 1000 mm 39.4 in | 1100 mm 43.3 in | 1200 mm 47.2 in |
|---|---|---|---|---|
| 0.36 — stairs in a single dwelling | 10 + 10 | 10 + 10 | 10 + 10 | 10 + 10 |
| 0.74 — residential, offices | 12 + 12 | 12 + 12 | 12 + 12 | 15 + 15 |
| 1.5 — shops, assembly areas | 15 + 15 | 15 + 15 | 15 + 15 | 19 + 19 |
| 3.0 — crowd loading | 19 + 19 | 19 + 19 | 19 + 19 | 25 + 25 |
Heat-strengthened and fully tempered give the same thicknesses: deflection governs almost every cell, and tempering makes glass stronger, not stiffer. Only at 3.0 kN/m does stress govern heat-strengthened glass in some cells, and it still passes at the same thickness. At 1100 mm, the 25 mm absolute deflection limit instead of height / 65 does not change any of them.
This is the intact glass only. Whether the barrier still holds with one ply broken often decides the make-up, and it is a separate check: the broken-ply case below.
How this is calculated
A free-standing balustrade is a vertical cantilever, clamped at the bottom, pushed horizontally near the top. Everything is worked out per metre of width, which is how the line load is given.
The effective thicknesses are the two bounds of a laminate, exactly as in the thickness calculator:
Design strength follows EN 16612, with kmod = 0.663 · t−1/16 for a load lasting t hours — which gives 1.00 at five seconds, 0.89 at thirty and 0.74 at ten minutes.
| Glass type | fbk | Design strength, 5 s |
|---|---|---|
| Annealed | 45 N/mm² | 25.0 |
| Heat-strengthened | 70 N/mm² | 45.8 |
| Fully tempered | 120 N/mm² | 87.5 |
The broken-ply check, which almost nobody does
This is the part that matters and the part most online calculators skip entirely. A balustrade is not judged only on whether it holds when it is intact. It is judged on what happens when one ply breaks — because glass does break, and a barrier that lets someone through when it does has failed at its only real job.
The page checks the remaining ply on its own, carrying the full line load, and reports the stress. Read it as a warning light, not as a certificate:
- Tempered glass all but disappears when it breaks. It dices into small fragments, and the residual capacity comes from the interlayer holding those fragments, not from the glass. A calculation of "the remaining ply" flatters it badly.
- Heat-strengthened breaks into large pieces that stay bonded and keep interlocking, which is why barrier standards lean towards it despite the lower strength.
- Real post-breakage performance is demonstrated by test, to BS 6180 and EN 12600, not by arithmetic. Codes generally allow a reduced load and a longer duration for the residual case; this page does not attempt to pick those for you.
The US mode: a check based on the IBC
Press United States — IBC above, or open the page in that mode, and it works in inches, pounds and psi, with the guard loads of the International Building Code and its section on glass in handrails and guards, 2407. It is a separate calculation: nothing of EN 16612, kmod or the load factor 1.5 goes into it.
- Loads. 50 lb/ft along the top and a 200 lb concentrated load, not at the same time: Section 1607.8 in the 2018 edition, 1607.9 in 2021 and 2024. One- and two-family dwellings take the 200 lb load only; I-3, F, H and S areas closed to the public with fewer than 50 occupants, 20 lb/ft; roofs not intended for occupancy, the 200 lb load only, from 2024. The IBC leaves position and direction to ASCE 7; the page puts both loads horizontally on the top edge, the worst place for a cantilever, without load factors.
- Strength. The 2018 and 2021 editions ask for a “factor of safety of four” and do not say on which strength. The 2024 edition writes the answer down: 3,000 psi for heat-strengthened glass and 6,000 psi for fully tempered. They are a quarter of the 12,000 and 24,000 psi mean modulus of rupture the National Glass Association publishes, and the page uses them in all three editions.
- Glass type. Laminated fully tempered or heat-strengthened glass, at least 1/4 in nominal. Single fully tempered glass only where nobody walks beneath, or where the walking surface is permanently protected from falling glass. Annealed glass is not allowed.
- Top rail. Structural glass balusters need an attached top rail or handrail that stays in place if one panel fails, unless the laminate has two or more plies of equal thickness and the same type and the panels are tested to ASTM E2353.
The laminate keeps the two bounds of the European side, no shear transfer and full shear transfer, and the verdict is read at the first. Interior glass beside a walking surface with free adjacent edges has one more rule, Section 2403.4: under 50 lb/ft at any height up to 42 in, the edges may not move apart by more than the panel thickness. The page reports it; whether it applies depends on the guard.
Where the model does not fit. The 45° spread of the 200 lb load is an engineering assumption, not the code's: a plate analysis or a test gives the real figure. Glass infill between posts, held on two or more edges, is not a cantilever and is not calculated here. Wind on exterior guards, the base shoe and its rotation, the fixings and the top rail itself are outside the page. The plies are metric sizes; US design uses the minimum thickness of each nominal size, which is a little less.
Read on 17 October 2026: the 2018 and 2021 text in the City of Seattle's adoption, in sections it does not amend (Chapter 24, Chapter 16); for 2024, the ICC's public record of code change S235-22 (committee action, final action); and the NGA's FM05-12 for the strength of glass. ASCE 7 and the ICC's own code viewer were not read.
What this page does not do
- Free-standing cantilever only. A continuous top handrail changes the model completely — the rail spans between panels and sheds load, and the glass sees far less.
- The clamp is assumed rigid. In real life the base channel rotates, and that rotation often adds more to the top deflection than the glass itself. Ask the channel supplier for their rotation figure and add it.
- Line load only. No uniformly distributed infill load, no point load, no wind, no crowd dynamics, no impact or soft-body test.
- No fixings, no bolted or clamped local stresses, and no check on the glass around holes.
- The interlayer bounds are bounds. Where you sit between them depends on the product, the temperature and the load duration — a stiff ionoplast interlayer on a cold day is near the top, soft PVB on a hot summer afternoon is near the bottom. If the design only works at the upper bound, it does not work.
Worked example you can check by hand
1100 mm free height, 0.74 kN/m, 12 + 12 laminated, heat-strengthened, 5 s.
So it passes on both counts at the conservative bound — but the deflection is at 96 % of its limit, which is exactly the kind of margin that disappears once the base channel rotates.
Questions people ask
- What thickness of glass do I need for a balustrade?
- For a free-standing balustrade clamped at the base, 1100 mm high, at the 0.74 kN/m line load of the UK National Annex for residential and office areas, the calculator gives 12 + 12 mm laminated heat-strengthened or tempered glass at the conservative bound: deflection within height / 65 and no shear transfer through the interlayer. At 1.5 kN/m it is 15 + 15; at 3.0 kN/m, 19 + 19. The broken-ply case is a separate check.
- Is toughened glass stiffer than heat-strengthened glass?
- No. Tempering raises the strength, not the stiffness: both have the same Young's modulus, 70 000 N/mm². A balustrade governed by deflection, as most are, needs the same thickness either way.
- Can I check a glass guard to the IBC?
- Yes, as a pre-dimensioning check: press United States — IBC above, or open syntaxflow.io/glass/balustrade/?std=ibc, which starts in that mode. It works in inches, pounds and psi with the IBC guard loads, 50 lb/ft and a 200 lb concentrated load applied one at a time at the top edge, and compares the stress at the base with 3,000 psi for heat-strengthened and 6,000 psi for fully tempered glass: the factor of safety of four of the 2018 and 2021 editions, written as limits in 2024. It also flags the glass type and top-rail rules of Section 2407. How the 200 lb load spreads across a panel is an assumption of the page, not of the code, and a licensed engineer must verify the guard under the edition your jurisdiction has adopted.