Leandro de Medeiros

Work

How I redesigned the core product of a tax platform with 15,000 users

I redefined the whole experience of Sittax Simples, a tool that automates tax calculation for small businesses under Brazil's Simples Nacional regime.

Role
Senior UX Designer
Company
Sittax
Period
2024 to present
Team
Design, product and engineering
Sittax Simples dashboard with filing indicators, monthly summary, filings by sector, company table and deadline notifications
The Sittax Simples home dashboard: in one place, accountants see which companies are ready to file, which have inconsistencies and which deadlines are coming up. Sample data.
  • 255khours of work saved by Sittax customers in 2025
  • R$504Min improper taxes avoided by Sittax customers in 2025
  • 6,000active customer firms on the platform
  • 15,000users on the platform

Context

Sittax Simples is Sittax’s main product. It automates tax calculation, a task accounting firms do every month, for many clients at once and against legal deadlines. The people using it are accountants and their teams: they know the tax rules in depth and have no time to lose with the interface.

I was responsible for the product’s UX and UI, from research to delivery, working directly with the business teams.

The problem

The biggest pain in tax calculation has always come down to three things: seeing the data clearly, checking the numbers and trusting the result before filing. To get there, accountants relied on a large number of spreadsheets to gather invoices and calculate taxes.

The goal of the redesign was to take that work out of spreadsheets. The product now fetches and calculates invoices automatically, suggests adjustments when something doesn’t match, and lets accountants issue the payment slip and file the return in one click.

What I did

  • Ran user research: interviews, discovery sessions, data analysis and usability tests with clickable prototypes.
  • Reorganized the information architecture: navigation, search, labeling and the product’s interface patterns.
  • Redesigned the flows and interface, built on the design system I created for the company’s 6 products.
  • Introduced the functional specification document (FSD) as the standard for documenting screens, produced with Claude, Figma and Notion, to make handoff to engineering and QA easier.
Sittax Simples tax calculation screen with the DAS amount calculated by Sittax and from XML invoices, the savings, net revenue, the audit checklist and each tax's share of revenue
Improved version of tax calculation, the core screen of the product, launched in 2026: the DAS amount calculated by Sittax side by side with the calculation from XML invoices, the savings, an audit of the company's pending issues and each tax's share of revenue. Company name replaced with an example.

Decisions and trade-offs

The hardest decision was convincing stakeholders that, in this product, showing less is better. Instead of putting every piece of data on screen, I kept each screen lean and focused on what actually helps accountants make decisions.

That was the constant work across the whole product: when there was too much information, I split the flow into steps that follow the user’s mental model, so they could read and analyze the data more clearly.

Results

The redesign spanned the period in which Sittax grew from 1,000 to 6,000 customer firms. In 2025, the year the new Sittax Simples main dashboard launched, I contributed to a platform that saved customers 255,000 hours of work and avoided R$504 million in improper taxes, according to Sittax.

In 2026, tax calculation, the core step of the product, got an improved version built on the same approach.

What I’d do differently

I would negotiate more time for discovery. A considerable share of the effort went into changing the final prototypes based on user testing, and some of those changes could have been caught earlier with deeper research at the start.