65% of internet users in Brazil only go online through their phones. Is your website ready for them?

I wanted to start a conversation about a piece of math almost nobody does before pasting that magic website-building prompt.

Let me start with a number, because it caught me off guard. The TIC Domicílios 2025 survey, by Cetic.br, shows that 99% of internet users in Brazil go online through a mobile phone — and 65% of them, around 106 million people, go online exclusively through it. Notice the word: exclusively. No computer at home, no second screen, no tablet. Just the phone. And when you look only at the D and E income classes, that number climbs to 87%.

Yeah.

Now put that next to the craze of the moment: building websites fast with AI. I'm not here to bash the tool — I work with AI all day and love what it has unlocked. What worries me is the bill that gets left behind. When the result looks gorgeous on a full-size monitor, the person looks at it, likes it, approves it, and ships it without ever opening it on a phone — and the ones who pay the price are exactly that majority. A website that breaks on mobile is a minor annoyance for someone with two screens. For someone with only one, it's a closed door.

Mobile data costs money, and we forget that on fiber Wi-Fi

There's a second layer that anyone who works all day on fiber never feels. Going online through a phone costs money, and the data comes in small packages that expire in days. More than half of Brazilian mobile users are on prepaid plans. The numbers lay it bare: 39% of people who own a phone, around 64 million, saw their data package run out at least once in the previous three months, and among prepaid users that figure reaches 54% (TIC Domicílios 2025). And mobile data is expensive by comparison: in the first quarter of 2026 a gigabyte on mobile cost an average of R$ 5.46 against R$ 0.25 on fixed broadband, roughly twenty-two times more, according to Anatel, the Brazilian telecom regulator. When the Wi-Fi disappears, what's left is metered data — and that's when page weight turns into a bill.

A heavy website does more than load slowly. It eats the data allowance of people who buy data in metered packages. That giant hero image the AI dropped in without optimizing, the three autoplay videos, the huge font file that downloads along with everything else: each one of those is money leaving someone's pocket. The person waits, spends what they had budgeted for the month, and often gives up before the page even finishes loading.

Why websites come out like this

AI delivers the easiest case to show off: wide screen, big images, everything spacious and beautiful in the screenshot. Mobile and page weight are constraints, and constraints only make it into the equation if someone asks. Most viral prompts don't.

Add to that the habit of reviewing everything on desktop. There, the page feels light, because it's running on your Wi-Fi and your full-size screen. The 5G from the ads is not the reality of someone catching an unstable signal on the outskirts of town or in the countryside. We approve thinking of the best possible connection and deliver to whoever is on the worst one.

What to do — and it fits in the same five minutes

The good news is that fixing this is cheap. Here's the recipe I use:

• Ask for mobile-first, in so many words. Tell the AI to start from the 375-pixel screen and only then expand to desktop. When the phone is the starting point, desktop becomes the easy case.
• Watch the weight. Ask it to optimize images, load what appears first, and cut what nobody asked for. Then, in your own browser, simulate a slow 3G connection and see how long the page takes to open.
• Measure at a real phone width, and check whether the text overflows and whether the buttons are within thumb's reach.
• Test on the device before celebrating. Status bar, notch, and weak signal only show up on a real phone. Until you've opened it on yours, it's "probably works".

This has happened to me, on my own portfolio: the entire site was solid on the phone, except the navigation, which broke badly. I almost blamed the phone's cache before sitting down and measuring. If it slips past me, and I do this all day, imagine someone shipping their first website trusting only what they see on the monitor.

Designing for the Brazilian internet means designing for people who only have a phone in their hand and count every megabyte until the allowance runs out. The speed AI brought is real and I'm not giving it up. It just can't cost the majority of the country its way in. Before approving the next website built in the blink of an eye, do the math that matters: does it open — and open light — for whoever is on the other side with a phone and a small data plan?

Um beijo e até a próxima!

Cover photo: Robin Worrall / Unsplash, with treatment.

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Sources

• TIC Domicílios 2025 — Cetic.br / NIC.br / CGI.br (data collected from March to August 2025; 27,177 households and 24,535 individuals aged 10 or older): 99% of internet users go online through a mobile phone and 65% do it exclusively (87% in the D and E income classes); 52% of mobile phone owners are on prepaid plans; 39% of phone owners, around 64 million people, saw their data package run out at least once in the previous three months (54% among prepaid users).
• Anatel — Economic and Financial Panorama of the Telecommunications Sector, Q1 2026 (released 2 July 2026): average price per gigabyte of R$ 5.46 on mobile against R$ 0.25 on fixed broadband. The indicator is a proxy — revenue divided by traffic — not a list price; and the price per gigabyte fell 10.9% over twelve months while average consumption rose 21%.
• The mismatch concept ("who designs × who uses") inspired by David Arty · Chief of Design.