How we do the sums and write the guides

This page explains two things: how the calculator works and how we write the guides. That way you know what to expect from each, and what not to.

Last updated: 17 October 2026.

What the calculator does

The calculator does a simple sum: width × height. You pick the window type, enter the sizes in millimetres and the number of pieces. The site converts the sizes to square metres (m²) and adds up every window on your list.

• Each window type has size limits. A fixed window, for example, runs from 300 to 1800 mm wide. If you enter a value outside the limit, the calculator warns you and will not add the window to the list.
• The drawing shows the window in the proportions of the sizes you entered. It is a sketch to see the type and shape, not a design.
• The material, colour, glass, shutter and fitting you choose go into your request. They help the companies understand what you want. On their own they do not change the area.

And prices?

Right now (October 2026) the calculator shows no prices. We do not show prices yet: we will only do so when we can justify the figures. What the calculator gives you is the area.

If the calculator ever starts to show prices, this is how the sum will work, and this page will be updated:

1. It starts from a price table per m², set by us, with a range (minimum and maximum) for each window type (fixed, tilt-and-turn, sliding, balcony door) and each material (PVC and aluminium).
2. It multiplies by the window’s area. If there is a minimum billing area, the larger of the two is used.
3. A percentage for colour and another for triple glazing are added, only when you chose those options. If you choose “not sure” for glass, the top of the range includes triple.
4. The shutter and the fitting only count if you answer yes. If you answer “not sure” they stay out of the figure.
5. If you choose “not sure” for the material, the range covers both materials, from the cheaper to the dearer.
6. The total only appears if every window on the list has a price. A partial total would mislead.

That table is not a national average. It would always be an estimate for planning, not a quote.

What the calculator does not know

Many things decide the final price and do not fit in a form:

• the real size of the opening, which is only measured on site;
• the state of the wall, the sill and the old window;
• access (floor, scaffolding, stairs);
• removing the old windows and the waste;
• sealing and making good, inside and outside;
• VAT and payment terms.

So anyone who fixes a price without seeing the site is guessing. Use the calculator to get ideas and tidy measurements, and always ask for a detailed quote. To find out whether the quote you received is complete, use the quote checker.

What happens to your list of windows

The list you make in the calculator stays in your browser until you send something. When you ask for proposals, we send the list (type, material, colour, glass, shutter, fitting, sizes and quantity) together with your name, phone and town to companies in your area, so they can reply with proposals. The details are in the Privacy policy.

How we write the guides

1. We start from a real question from someone replacing windows: how to measure, what to ask for, what “Uw” means, why there is water on the glass.
2. We look for sources. We prefer ADENE, the Diário da República, standards and technical documents, and manufacturers’ data sheets. Price sites and forums are only a hint, never proof.
3. We write in plain words. We explain each technical term the first time it appears. The Portuguese text is European Portuguese.
4. We cite sources at the end. The most recent guides have a “Sources” section with the pages we read and what each one supports.
5. We give no prices without a source. When we talk about cost, we explain what pushes it up or down. We only give figures that come from a source we name, with the date we read it.
6. We say what we could not confirm. If we found no reliable source, the text says it depends and what to ask the installer, instead of stating it.

The role of artificial intelligence

We use artificial intelligence tools to help research, organise and draft. What goes on the site is checked against the sources we cite. We do not claim that the guides were reviewed by an engineer or an architect. They are general information, written with care, and they do not replace a technician’s opinion or an installer’s visit.

Independence and limits

• We do not make or install windows. We do not recommend brands or companies in the guides.
• If you ask for proposals, we pass your request to companies in your area. JanelaCerta.pt may receive payment for passing it on. For you, the service is free and without obligation.
• Rules, grants and standards change. In the most recent guides, when we talk about legislation or grants, we give the date we read the source. Always check the official site before you decide.

How to ask for a correction

Did you find a mistake, out-of-date information or a sentence that makes no sense? Write to [email protected] with the page address and what is wrong. If the mistake is ours, we fix it and update the page.