FinTech app · UX research & UI · 2026
Kalm
A calm way for Dutch investors
to see what they own and what
box 3 will take.
TYPEConcept case study
SCOPEResearch, personas, user flow, wireframes, UI
PLATFORMiOS app
TOOLFigma
01 — The problem
Investing is easy to start.
Knowing what you owe is not.
In box 3, the Netherlands taxes a deemed return on savings and investments: not the return you actually made, but a percentage set by the government, which changes every year.
My hypothesis: investing apps show what a portfolio is worth, but leave people guessing about tax until their return is due. Kalm puts that number next to the portfolio, with the working one tap away.
02 — Box 3 in 2026, in four numbers
€59,357
Tax-free allowance per person
6%
Deemed return on investments like ETFs
1.28%
Deemed return on savings (provisional)
36%
Tax on that deemed return
Source: SRA, “Box 3: percentages, vrijstellingen en bedragen 2026”
03 — A worked example
Five steps from €90,000
to one clear number
1 · You own
€90,000
€80,000 in ETFs,
€10,000 in savings2 · Deemed return
€4,928
6% of €80,000
plus 1.28% of €10,0003 · Above allowance
€30,643
€90,000 minus €59,357
4 · Taxable return
€1,678
The share of €4,928
that falls above it5 · Box 3 tax
≈ €604
36% of €1,678
That is about 0.67% of everything this person owns. Kalm shows this one number first, and the five steps only when someone asks how.
Illustrative: single person, 2026 rules, no debts
04 — Personas
Three investors, one shared worry
Sarah, 29
UX writer, new investor
“I opened a broker account, then avoided it because of the tax.”
Wants: to know she isn’t doing it wrong.
Hates: jargon.
Daan, 41
Teacher, monthly ETF saver
“I just want one number when the tax return is due.”
Wants: a reminder and an answer.
Hates: opening five apps.
Mei, 35
Data analyst, spreadsheet person
“Show me the maths, or I won’t trust it.”
Wants: every step traceable.
Hates: black boxes.
05 — Research insights
The barrier isn’t maths.
It’s fear and words.
Tax fear delays investing
Beginners postpone or avoid investing because they’re afraid of getting box 3 wrong.
1 January surprises people
Few know that box 3 looks at one snapshot date, not the whole year.
Jargon is the wall
Words like “forfaitair rendement” stop people before the numbers do.
Trust needs the working
Even confident investors re-check numbers in a spreadsheet before believing an app.
How might we make box 3 feel as simple as checking a balance?
06 — Design principles
- 1
One number first
The estimate leads; detail waits.
- 2
Show the working
Every number can be opened and traced.
- 3
Plain words, two languages
Dutch and English, with tax terms explained.
- 4
No hype, no advice
It explains, never tells you what to buy.
07 — User flow
Three set-up steps,
then one home screen
Set-up 1
Welcome
Language, get started
Set-up 2
Who is filing
Single or fiscal partner
Set-up 3
Add what you own
Value on 1 January
Home
Overview
Total, mix and box 3 estimate
From the overview, everything is one tap away
Tap the estimate
How we got this
Five steps to the numberTerm explained
Plain-words sheet, tap any termWhat-if tab
What if
Monthly amount sliderLearn tab
Learn
Short explainersGear icon
Settings
Language, filing, privacy
08 — Wireframes to UI
Structure first,
calm second
I tested the layout in grey before adding colour. The one dark card on each screen is always the number that matters.
09 — All nine screens
From welcome to settings. Every screen, its wireframe, the user flow and the design system are in the Figma file.
10 — How I’ll know it works
Success is someone saying,
“oh, that’s all it is.”
Understanding
Can people explain their box 3 tax in one sentence after two minutes?
Trust
Do they check the working, and does it match their own numbers?
Calm
Do they rate the tax return as less stressful than before?
Let’s connect: verakarulina1@gmail.com