Case Study — Finby
Saving up with friends, without spreadsheets or anyone chasing payments in the group chat. I designed the app and, on my own and in days, turned the file into a prototype you can use on a phone: 12 navigable screens, from invite to withdrawal.
A consórcio is a very Brazilian way of saving up: a group pays a monthly instalment into a shared pot, and each month one member takes the prize. Among friends it has always run informally, with one person tracking who paid and chasing whoever is late. Finby, which I designed, moves that agreement to the phone and takes the debt-collector role off the group.
But an 80-screen file can’t show whether the numbers add up, or what it’s like to invite someone who isn’t on the app yet. So I turned the design into a prototype you can use: 12 navigable screens, with the same flows and the states the file skipped. I did it alone, with AI agents on production and the product decisions staying with me, in days, not months.
What I did
01 — The problem
When friends save up for a trip or a graduation, someone becomes the treasurer. That person tracks who paid and chases whoever is late, usually in the group chat.
Finby puts that agreement in an app. You set the prize and the instalment and invite whoever you like. Who gets paid when comes from a random draw or an order everyone agrees on, and the money earns 100% of the CDI rate while it waits.
02 — The prototype
It has 12 screens, and what you do is saved. Create a consórcio and it shows up on the home. Invite someone and they join as a pending sign-up. Or let the tour walk through the screens.
Jump to
Use it like a phone: create a consórcio, invite someone and reorder who gets paid. The data is fictional and stays in your browser. The interface is in Portuguese, as in the original.
03 — Decision: hierarchy
Before any colour, I settled the order of information in wireframes. In an app about money among friends, people want to know how much they’ll pay and when their turn comes. Those answers come first on every screen, and the finish doesn’t change that order.
01 — My consórcios
Decision: Yours first
The home opens with what’s yours: the consórcios you’re in sit at the top. What your friends did comes after, in a feed.
In the finish, each consórcio gets its own theme, and the winner of the month becomes a card with amount and date.
02 — The consórcio
Decision: Numbers that never disagree
On the details screen, the first question is how much the group has saved. Right below come who’s in and when the next draw is.
Every amount comes from the same maths, so one screen never contradicts another. The next draw says how long is left.
03 — Who’s joining
Decision: Nobody left out
Building the group means picking people, and not all of them are on the app. The list has search and a shortcut to invite whoever is missing.
Selection shows in mint, with a check and AA-contrast text, and the count is announced to screen readers. People without an account join as pending.
04 — The prize amount
Decision: The maths in plain sight
Whoever creates a consórcio thinks about the prize first. So the amount dominates the screen, with the instalment right below.
The instalment is recalculated on every tap, with the maths written out: "You and 5 friends · 6 months, one prize a month". Buttons that would take the amount below zero are disabled.
05 — Withdrawing
Decision: Prevent the error
Withdrawing should feel like the banking app people already use. It fits on one screen, with a keypad that types from the cents up.
An amount above the balance disables the button with a message. The balance only changes after the review step.
04 — Decision: identity
The design is mine, so the rule was to follow it. The brand violet and Sarabun stayed. I changed what made the app feel like a bank statement: cool greys became cream and emoji became photos of real people.
The brand violet became a ten-step scale and the cool background turned cream. The mint uses dark text to pass AA contrast.
Six grainy gradients, one per theme. The creator picks it, and it follows the consórcio on its card and header.
The file’s typeface, with clearer hierarchy. In money amounts, "R$" and the cents are smaller so the value reads first.
A single stroke instead of the file’s mix of styles. The tab bar, which was icon-only, got labels.
The emoji on the success screens became warm photos of real people, and each consórcio got a photo of its goal.
The file showed each component in a single state. The prototype has default, selected, disabled and a visible focus.
05 — Decision: one flow
In the file, creating a private consórcio and creating a public one were two flows with nearly the same screens. I merged them into one wizard, one question per screen, that only splits at the entry criteria. The summary shows everything before publishing and lets you fix it right there, without stepping back.
06 — Inviting and organising
A consórcio among friends stalls if everyone has to download the app first. The invite goes out through the channel the person already uses, like WhatsApp, and they show up as a pending sign-up until they join. Admins drag members to set the payout order. The Friends tab had no designed screen, so I built it from the system’s components.
07 — Decision: trust
Among friends, trust is already there. In a public consórcio, it has to come from somewhere. People who pay on time level up, and whoever creates a public consórcio can require a minimum level to join.
The profile shows that history. Admins see how many people are still missing, and the balance reaches your bank account in three taps.
08 — How I worked
I wrote the brief and the visual system, and had AI agents build the screens in parallel. Then I reviewed each one against my design: copy, states, contrast and the numbers, which have to match from screen to screen.
AI sped up production. The product and design decisions stayed with me, and that’s how I take an app from Figma to a testable prototype in days, not months.
Outcome
Tools
I take it from design to a prototype you can use, in days, and handle what the file doesn’t show, from states to numbers that add up.