# Facade Glazing Calculator: Vision Glass, Spandrel and WWR

> A facade glazing take-off divides a curtain wall into its repeating modules, then measures the vision glass people look through and the opaque spandrel that hides each slab edge. Those two areas give the window-to-wall ratio, the number of glass units and spandrel panels, and the sealant run around the glass.

## What it calculates

A curtain wall is built from one module repeated across the facade and up the building. Once
the module width and the split between vision glass and spandrel are fixed, most of the
quantities follow from simple geometry. For one rectangular elevation the calculator gives:

- the **number of modules** on each floor;
- the **gross facade area**, the **vision glass area** and the **spandrel area**;
- the **window-to-wall ratio** (WWR) of that elevation, with a warning if it is above the
  check value you set;
- the number of **vision glass units** and **spandrel panels**, including wastage and attic
  stock;
- the **perimeter sealant run** around the vision units.

It is a take-off tool for early planning and for checking quantities in a tender, not a
facade design.

## Inputs

**Facade width** is the length of the elevation in metres, measured along the glazing line.

**Number of floors** is how many storeys the curtain wall covers on that elevation.

**Floor-to-floor height** is the distance from one slab level to the next. The calculator
treats every floor as the same height; enter a podium or a double-height lobby as a
separate run.

**Module width** is the centre-to-centre spacing of the vertical mullions. Common modules are
set by the architect's grid, the glass sizes that can be made and handled, and the internal
partition layout.

**Vision glass height per floor** is the height of the see-through band on each floor. The
rest of the floor-to-floor height is treated as spandrel.

In the **assumptions** panel are the wastage and attic stock percentage and the WWR check
value. The check value defaults to 40% and is labelled as the ECBC 2017 prescriptive maximum;
confirm the value that applies to your building with the energy consultant before relying on
it.

## Method

| Quantity | Formula |
| --- | --- |
| Modules per floor | ceil(facade width ÷ module width) |
| Facade area | facade width × floors × floor-to-floor height |
| Vision area | min(modules × module width, facade width) × vision height × floors |
| Spandrel area | facade area − vision area |
| Window-to-wall ratio | vision area ÷ facade area |
| Vision glass units | modules × floors × (1 + wastage), rounded up |
| Spandrel panels | modules × floors × (1 + wastage), rounded up |
| Sealant per vision unit | 2 × (module width + vision height) |
| Sealant, all vision units | sealant per unit × modules × floors |

The vision width is capped at the facade width so that a narrow last module does not add
glass that is not there. If the vision height entered is more than the floor-to-floor height,
it is capped and the spandrel is zero. The sealant run is measured on the net number of
units, without wastage, because spares are not installed.

## Assumptions

| Assumption | Default | Notes |
| --- | --- | --- |
| Module width | 1.5 m | Set by the architect's grid |
| Vision glass height | 2.4 m | The rest of each floor is spandrel |
| Wastage and attic stock | 5% | Breakage plus spares for the owner |
| WWR check value | 40% | ECBC 2017 prescriptive maximum; confirm the applicable value |
| Floor heights | Equal | Enter different heights as separate runs |

Every one of these can be edited. The calculator holds no glass prices or rates.

## Limitations

- **One flat elevation at a time.** Corners, curved facades, sloped glazing and setbacks
  need to be measured separately and added.
- **No openings.** Entrance doors, louvres, vents and openable windows replace some vision
  units and need their own counts.
- **One vision band per floor.** Some designs add a transom, a clerestory light or a
  fritted band; split the vision height accordingly or measure from the elevation drawing.
- **Frame area is ignored.** The vision area is the module grid, not the visible glass
  inside the frames. Energy calculations and daylight studies use the real glass and frame
  dimensions.
- **No structural or glass specification.** Glass make-up, thickness, the mullion and
  transom sections, brackets and anchors are designed by the facade engineer for the
  building's actual loads and conditions.
- **No compliance verdict.** The WWR here is for one elevation and the check value is an
  input. Compliance is assessed by the energy consultant for the whole building.

## Worked example

A 30 m wide elevation, six floors at 3.6 m floor-to-floor, a 1.5 m module and 2.4 m of
vision glass on each floor, with 5% wastage and the default 40% check value:

| Step | Calculation | Result |
| --- | --- | --- |
| Modules per floor | ceil(30 ÷ 1.5) | 20 |
| Facade area | 30 × 6 × 3.6 | 648 m² |
| Vision area | min(20 × 1.5, 30) × 2.4 × 6 | 432 m² |
| Spandrel area | 648 − 432 | 216 m² |
| Window-to-wall ratio | 432 ÷ 648 | 66.7% |
| Vision glass units, 1.5 × 2.4 m | ceil(20 × 6 × 1.05) | 126 |
| Spandrel panels, 1.5 × 1.2 m | ceil(20 × 6 × 1.05) | 126 |
| Sealant per vision unit | 2 × (1.5 + 2.4) | 7.8 m |
| Sealant, all vision units | 7.8 × 120 | 936 m |

The facade divides into exactly 20 modules, so there is no narrow module. The WWR of 66.7%
is above the 40% check value, and the calculator shows a warning. That does not mean the
building fails: it means this elevation is heavily glazed and the energy consultant should
look at it early. Options they may explore include more spandrel, glass with a lower solar
heat gain, shading, or demonstrating compliance by the whole-building performance route.

To see what a taller spandrel does, change the vision height to 2.1 m. The vision area falls
to 378 m², the WWR to 58.3%, and the spandrel panels become 1.5 m high.

## How engineers use it

A facade consultant or architect uses this kind of take-off at concept and schematic stage,
before the elevation is fully drawn. It answers quick questions: how many units of each size,
how much spandrel to price, and roughly where the WWR lands compared with the energy brief.

Once the module and the vision band are agreed, the take-off moves to the elevation drawings
and the panel schedule. Each unit gets a mark, a size and a glass type, corner and special
units are listed separately, and the sealant and gasket lengths are taken from the system's
own details. The vision and spandrel areas also feed the energy model, where the glass
properties such as solar heat gain coefficient and visible light transmission are tested
against the code and the cooling load.

The calculator is also useful for checking a tender. If a bid lists far fewer vision units
than the module grid produces, or no attic stock at all, it is worth asking why.

## When a professional design must replace it

Use the calculator to estimate quantities, compare module options and see how the vision
height changes the window-to-wall ratio. Replace it with a facade engineer's design before
any glass or framing is ordered. That design sets the glass make-up and thickness, the frame
sections, the brackets and anchors, the movement joints and the fire-stopping at each slab
edge, all for the building's actual site and height. The window-to-wall ratio and glass
performance are confirmed by the energy consultant for the building as a whole.

## Frequently asked questions

### What is the difference between vision glass and spandrel?

Vision glass is the transparent part of each floor that occupants see through, usually from just above the floor to just below the ceiling. Spandrel is the opaque band that covers the slab edge, the ceiling void and the raised floor or upstand. It is often glass too, but backed with an insulated panel, so it lets in no daylight.

### How is the window-to-wall ratio worked out here?

The calculator divides the vision glass area by the gross facade area of the elevation you enter. The energy consultant works out the ratio for compliance across the building's walls as the energy code defines them, so treat this figure as a check on one facade, not a verdict on the building.

### Why does the calculator add wastage to glass units?

Glass breaks in transit, during handling and on site, and an owner usually keeps a few spare units of each size as attic stock for replacements. Large insulated units have long lead times, so ordering spares with the main batch avoids waiting weeks for one broken panel. The default is 5% and can be changed.

### Does this tell me what glass to use or how thick it should be?

No. Glass make-up, thickness, heat strengthening, the frame sections and their fixings depend on wind pressure, span, safety and energy requirements for the actual building. The facade engineer designs these. This calculator only measures areas and counts units.

### What if the facade width is not a whole number of modules?

The calculator rounds the module count up and tells you how wide the last module on each floor is. Areas use the real facade width, but unit counts and sealant treat the narrow module as a full one. In a real design the architect usually adjusts the module so the facade divides evenly or places the odd module at a corner.

## Sources

- [ECBC 2017](https://beeindia.gov.in/) — Building envelope: vertical fenestration and window-to-wall ratio, prescriptive method

---

Source: https://hagerstone.com/calculators/facade-glazing-calculator