Glass Cutting Calculator
Cut list optimization from sheet glass and required pieces.
About this calculator
This calculator is a structural deflection and stress checker for a single rectangular glass panel under uniform wind load. It follows the simplified plate-theory approach from ASTM E1300, Standard Practice for Determining Load Resistance of Glass in Buildings — the glass-strength standard the International Building Code itself incorporates by reference: deflection and bending stress both depend on an alpha coefficient that varies with the panel's aspect ratio (long side over short side), which this calculator interpolates from a lookup table of Timoshenko plate-theory values rather than solving the full plate equations directly. Deflection scales with wind pressure and the fourth power of the short span, and inversely with Young's modulus (fixed at 70 GPa for soda-lime glass) and the cube of thickness — meaning thickness is by far the most powerful lever for reducing deflection.
Fixed/clamped edge supports are modeled as roughly five times stiffer than simply-supported edges, cutting deflection to 20% and stress to 50% of the simply-supported case. The result is checked against the L/60 deflection limit — the industry rule of thumb that a glass panel's center deflection under load shouldn't exceed 1/60th of its short span — and compared to an allowable stress that depends on glass type: annealed, heat-strengthened, and fully tempered glass carry very different design strengths (20, 50, and 80 MPa respectively), yielding a safety factor. This is an engineering estimation tool for early sizing; a stamped calculation from a licensed structural engineer is still required before glazing is installed under code, particularly when the safety factor comes out close to 1.0.
Inputs
Results
Max deflection (mm)
5.54
Figures current as of 2016. Source: ASTM International, ASTM E1300-16, Standard Practice for Determining Load Resistance of Glass in Buildings; adopted by reference in the International Building Code, Section 2404 (load resistance for glass not tested/certified under IBC 1710.5.1)
How to Use This Calculator
- Enter the panel Short Side and Long Side in mm and the Glass Thickness in mm.
- Enter the Design Wind Pressure in Pascals and select the Glass Type (annealed, heat-strengthened, or tempered).
- Select the Support Condition (simply supported or clamped on all four sides).
- Review Maximum Deflection and whether it passes the L/60 deflection limit.
- Check Maximum Bending Stress vs Allowable Stress and Safety Factor to confirm structural adequacy.
How the result changes with Thickness (mm)
| Thickness (mm) | Max deflection (mm) |
|---|---|
| 3 | 44.34 |
| 4.5 | 13.14 |
| 9 | 1.64 |
| 15 | 0.35 |
What each input means
- Short side (mm)
- Shorter dimension of the glass panel.
- Long side (mm)
- Longer dimension of the glass panel.
- Thickness (mm)
- Glass thickness in mm.
- Wind pressure (Pa)
- Design wind pressure in Pascals. 1 kPa ≈ 21 psf.
- Glass type
- 0 = Annealed, 1 = Heat-strengthened, 2 = Fully tempered.
- Support condition
- 0 = Simply supported (4 sides), 1 = Fixed/clamped (4 sides).
What each result means
- Max deflection (mm)
- Maximum center deflection under the applied wind load.
- Deflection limit L/60 (mm)
- Allowable deflection based on span/60 criterion.
- Passes deflection check
- 1 = within L/60 limit, 0 = exceeds limit.
- Max bending stress (MPa)
- Maximum bending stress in the glass plate.
- Allowable stress (MPa)
- Design allowable stress for the selected glass type.
- Safety factor
- Ratio of allowable stress to actual stress. Should be > 1.0.
- Panel area (m²)
- Panel area in square meters.
- Panel weight (kg)
- Panel weight in kilograms.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersShort side (mm) = 1000, Long side (mm) = 1500, Thickness (mm) = 6, Wind pressure (Pa) = 1000 = 6 input(s) provided
- Calculate Max deflectionMax deflection = deflectionM * 10005.54 = 5.54
- Calculate Deflection limit L/60Deflection limit L/60 = (a * 1000) / 6016.67 = 16.67
- Calculate Passes deflection check1 = 1
Figures and sources
- ASTM E1300 — Standard Practice for Determining Load Resistance of Glass in Buildings, incorporated by reference into the International Building Code for glass strength design (2016) — ASTM International, ASTM E1300-16, Standard Practice for Determining Load Resistance of Glass in Buildings; adopted by reference in the International Building Code, Section 2404 (load resistance for glass not tested/certified under IBC 1710.5.1)
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does thickness matter so much more to deflection than to stress?
Deflection is inversely proportional to thickness cubed (t³) in the formula, while bending stress is only inversely proportional to thickness squared (t²) — so doubling glass thickness cuts deflection to one-eighth of its original value but only cuts stress to one-quarter. This is why increasing thickness is the single most effective lever for passing the L/60 deflection check, often more effective than it is for improving the stress safety factor.
How much does switching from simply-supported to fixed/clamped edges actually help?
The calculator applies a supportFactor of 0.2 to deflection (roughly five times stiffer) and a stressSupportFactor of 0.5 to bending stress when Support Condition is set to fixed/clamped on all four sides. In practice this means the same panel, wind load, and thickness will pass the L/60 deflection limit far more easily when clamped than when simply supported, since clamped edges resist rotation and reduce how far the center of the panel can bow outward.
Why does the same wind pressure produce a very different safety factor for annealed versus tempered glass?
The maximum bending stress (maxStressMPa) is identical for any glass type at a given geometry and load — it depends only on the plate dimensions, thickness, and wind pressure, not glass type. What changes is the allowable stress used to compute safetyFactor: 20 MPa for annealed, 50 MPa for heat-strengthened, and 80 MPa for fully tempered, so the same physical stress yields a safety factor four times higher for tempered glass than for annealed glass of identical thickness.
What does the L/60 deflection limit actually represent, and why is it based on the short side?
L/60 means the panel's maximum center deflection shouldn't exceed 1/60th of its span — the serviceability convention used alongside ASTM E1300's simplified plate-theory approach, meant to keep the glass looking and performing rigid rather than visibly sagging or straining its seals. The calculator computes the limit from the short side (a) because that's the span the plate-theory deflection formula is built around; the aspect ratio (long side over short side) is what determines the alpha coefficient used elsewhere in the same formula.
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