CASE STUDY · Quarter
How we shipped a safe-to-spend calculator, IRS-accurate tax estimates, and bank sync for US freelancers in one focused product.
Building a money OS for freelancers who hate tax surprises
PROBLEM
The problem
Freelancers in the US carry a tax liability that employees never see: 15.3% self-employment tax on top of income tax, paid quarterly. Most freelancers either overpay (parking too much in reserves) or underpay (facing a penalty at filing). Spreadsheets help but rot immediately after the first big invoice. We wanted one number on screen that answered: what can I actually spend today, right now, without wrecking my quarterly estimate?
APPROACH
Our approach
We started with the tax math. IRS Publication 505 lays out the quarterly estimated-tax system clearly; the hard part is parameterizing it for variable income. We built a pure-function tax engine (no I/O, no clock) that takes year-to-date income, filing status, and state and returns a liability estimate. Golden-vector tests against IRS worksheet examples give us confidence the math is correct before any UI exists.
Bank sync came next. SimpleFIN Bridge provides read-only access to US bank accounts without asking users to hand over credentials to a third party. The integration is intentionally shallow: we pull balance and transactions, derive income from credits above a configurable threshold, and store nothing more.
The UI reduces to a single number: safe to spend. The calculation is balance minus tax reserve minus a user-defined runway target. Every screen is designed to answer one question; we cut anything that needed a second thought.
OUTCOME
What shipped and what we learned.
Quarter launched in private beta with 47 active freelancers. Safe-to-spend accuracy (within 5% of actual quarterly tax owed) held across all test accounts. The bank-sync onboarding flow completes in under 90 seconds for most users. We removed three screens from the original design after watching session recordings show users skipping them entirely.
STACK
Technology stack
SCREENS
What it looks like.
