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The calculator works the way you would sketch a section: you stack the layers in order, from the outside face on the left to the room side on the right, then ask it what that stack does. Nothing is assumed for you — if a layer is not in the strip, it is not in the calculation.

1. 1
   Set who you are allowed to buy from

   The WORLD chip in the top bar sets the sales region and ticks the manufacturers you want offered. It starts on World with every Glass Technique‑approved supplier ticked. Anything outside that scope simply does not appear in the pickers, so you cannot accidentally specify a product you cannot source.

   Watch out — If a brand you expected is missing from a picker, this chip is almost always the reason — check it before assuming the product is not in the catalogue.

2. 2
   Add layers from the ribbon

   The eight icons under the top bar are the layer types: Glass, Coating, Interlayer, Gap, Smart, Mesh, Blind and Polycarbonate. Click one, pick a manufacturer, then a product. The layer lands in the build‑up strip below.

   Watch out — Order matters and it reads left to right, outboard (outside face) on the left, inboard (room side) on the right — the same way you would draw the section.

3. 3
   Coat a pane, if you need to

   A coating belongs to a pane, not to the gap between panes. There are two ways to add one: the Coating step inside the glass picker while you are choosing the pane, or the small circle that appears when you hover a pane already in the strip. Either way you then choose which face of that pane carries it, and the app works out the surface number (#1 to #6) from the finished geometry.

   Watch out — The picker shows only coatings from the same brand as the pane, because a coating is sputtered or pyrolytically applied onto that maker’s own glass. There is an override tick if you genuinely mean to mix brands.

4. 4
   Fix anything the validator flags

   The banner above the build‑up turns green when the make‑up is sound and red or amber when it is not. Each issue says what is wrong in plain words, and where the fix is obvious there is a one‑click button to apply it.

   Watch out — Red issues block Calculate. Amber ones are warnings — you can proceed, but read them first.

5. 5
   Press Calculate

   Calculate sits in the sticky bar at the bottom of the right‑hand rail, so it stays reachable however long the panel grows. Results fill the metric tiles in the rail, and the spectral chart opens in the full‑width band underneath the build‑up.

   Watch out — Nothing recalculates on its own. If you change a layer, press Calculate again — the tiles keep showing the previous run until you do.

6. 6
   Read it through the right lens

   The seven mode buttons — Standard, Optical, Thermal, Acoustic, Fire, Structural, Safety — do not change any number. They highlight the tiles that matter for that question and dim the rest, so a thermal conversation is not competing with a colour‑rendering figure for your attention.

The eight layer types

A number on its own does not tell you whether you can put it in a contract. Next to every safety and fire classification the app shows a small coloured badge saying where that value came from and how much weight it will carry. There are five, and the difference between the green one and the rest is the difference between a declaration and a recommendation.

Provenance is not the same as data completeness

Separately from the badge, every product in the catalogue carries a single letter grading the data we hold about it. The letter is about the input; the badge is about the output. A product graded D — the best grade, straight from the manufacturer — can still produce a predicted pendulum class, because nobody has put that make-up in a pendulum rig. Good data in does not manufacture evidence that was never gathered.

The app produces two very different documents from the same calculation, and they are held to different standards of proof on purpose.

What the DoP declares, and what it gates

Some characteristics are always declared because they are always calculated from measured product data under a harmonised standard. Others cannot be established without physically testing the assembly, so they wait for a certificate.

A certificate only counts for what it was tested on

Every certificate you record carries the make-up the test house tested, and the calculator checks it against the build-up in front of it before applying anything. An exact match is certified and declarable. Anything else is set aside, the class falls back to the calculated value, and a banner above the build-up names the certificate and the difference — a thickness, a treatment, a different interlayer.

NPD stands for No Performance Declared. It means nobody has stated a value for that characteristic. It is a blank, not a zero, and it is not a fail. The app labels it not assessed on screen for exactly that reason.

So when you see NPD, the question to ask is not “what went wrong?” but “does this project need that characteristic declared?” If it does, there are only two honest routes:

1. Change the glass. Toughen it, laminate it, or specify a product that carries the class you need. A missing pendulum class on annealed glass is telling you something real about the glass.
2. Get it tested and record the report. Enter the class, the report reference and the test house on the Lab Certificates page. The value turns green and becomes declarable.

A coating is a microscopically thin layer of metal or metal oxide bonded onto a glass face at the factory. A low-E (low-emissivity) coating reflects heat back into the room and is what turns an ordinary double-glazed unit into an insulating one. A solar control coating rejects heat from the sun before it gets in. The difference a coating makes is not marginal — on a typical unit it roughly halves the U-value.

Where a coating can sit

On a glass face, and only on a glass face. Choose the pane, then which of its two faces carries the coating; the app numbers the surfaces for you from the finished make-up. Surfaces count from the outside in, so in a double-glazed unit #1 is the weather face, #2 is the cavity side of the outer pane — where a low-E coating normally goes in the UK — #3 is the cavity side of the inner pane, and #4 faces the room.

Why some coatings will not calculate

Some coatings appear in the list with an amber data pending tag and produce no result. The product is real and the manufacturer sells it — we simply do not hold the figures the calculation needs: light transmittance, solar factor, emissivity, and ideally a measured spectrum across the wavelength range.

We list them anyway, for two reasons. You need to know the product exists so you can ask for it. And listing it with an honest tag is far better than the alternative, which is inventing a plausible number. Coatings we can calculate are sorted to the top of the list, so the working options are always the first thing you see.

What the Tv and g figures in the picker mean

The values shown against each coating in the list are the manufacturer’s published figures for their standard reference unit: 4 mm glass, a 16 mm cavity filled with 90% argon, 4 mm glass, coating on surface #2. They exist so you can compare one coating against another on equal terms.

They are not the answer for your make-up. Your real numbers come out of Calculate, using your actual thicknesses, cavity width and gas fill. We audited the whole coating table against measured panes in August 2026 and sixteen of the nineteen entries we could check landed within 2.5 percentage points of the measured result — the table is sound, but it is a comparison tool, not a substitute for running the calculation.