I started planning for early retirement around 2024. The idea was straightforward: save aggressively, invest, and eventually stop working well before 60. But every time I sat down with a retirement calculator, the same problems came up. They wanted me to pick a single retirement age, assume one flat tax rate, and pretend I could access all my money on day one. None of that is how early retirement actually works.
If you retire at 48, you have roughly 11 years before your 401(k) and traditional IRA unlock at 59 and a half. During that gap, you need to live on taxable accounts, Roth contributions, maybe some side income. That "bridge period" is the hard part, and I could not find a single free tool that modeled it properly.
So I built one.
The Spreadsheet That Got Out of Hand
It started as a Google Sheet. I had three columns for each year: my taxable brokerage (what I could spend right away), my Roth IRA (contributions accessible, gains locked), and my 401(k)/IRA (completely locked until 59 and a half). I called them P1, P2, and P3, short for Priority 1, Priority 2, Priority 3. The idea was simple: spend P1 first, tap P2 contributions if needed, and let P3 grow untouched until it unlocks.
That spreadsheet worked for a while. Then I wanted to add Social Security at different claiming ages. And a Roth conversion ladder. And tax brackets that change over time. The formulas got unmanageable fast. I had 47 tabs at one point. That is not a tool. That is a liability.
I decided to turn it into a real calculator. Not a startup, not a SaaS product, just a single-page web app that did what my spreadsheet was failing to do.
What It Actually Does
The core idea is still P1/P2/P3 priority-based withdrawal. You add your real accounts, categorize them by access level, and the calculator draws them down in order. It handles the bridge period naturally because it knows which money you can touch and when.
From there, it grew based on questions I kept running into during my own planning:
- Roth conversion ladders. If you convert traditional IRA money to Roth, those dollars become accessible after a 5-year wait. The calculator tracks each year's conversion separately and knows when each batch "seasons."
- Progressive tax brackets. Not just a flat percentage. Actual 2025 IRS brackets with standard deduction, long-term capital gains rates, and the Net Investment Income Tax. You can see how your effective rate changes as income drops in retirement.
- Monte Carlo simulations. Instead of assuming 7% returns every year, it runs 1,000 randomized scenarios using historical market patterns. The chart shows the spread of possible outcomes so you can see whether your plan holds up in a bad sequence of returns.
- Historical stress tests. What if you retired right before the 1929 crash? Or 1966, or 2000? The stress test replays actual market history against your exact plan.
- SEPP/72(t) early access. Some people use Substantially Equal Periodic Payments to pull from retirement accounts before 59 and a half without a penalty. The calculator models that with IRS-approved methods.
- Spending guardrails. Variable withdrawal rates that adjust based on portfolio performance. Spend more in good years, tighten up in bad years.
- Multi-region support. US, UK, Canada, Australia, Eurozone, and India. Each region gets its own terminology, pension system, and account types.
Everything runs in the browser. No server, no database, no login required. Your financial data never leaves your device.
How It Was Built
BridgeToFI is vanilla HTML, CSS, and JavaScript. No React, no frameworks, no build step. You open the file, it works. I chose this deliberately because I wanted the tool to be fast, simple to host, and impossible to break with a dependency update.
The three simulation engines (deterministic, Monte Carlo, and historical stress test) share the same withdrawal logic so they produce consistent results. I verified the tax math against IRS Revenue Procedure 2024-40, and the calculator has an 86-point automated test suite covering tax brackets, IRMAA surcharges, long-term capital gains, and Social Security taxation.
The site itself is hosted on Cloudflare Pages. The entire calculator loads in one request. No cookies, no analytics, no third-party scripts.
The Advisor Portal
This one surprised me. After the calculator had been live for a couple of months, I started getting emails from financial advisors asking if they could use it with clients. The free version worked fine for individuals, but advisors needed things like client management, scenario saving, branded PDF reports, and team access.
So I built an advisor portal on top of it. Supabase for authentication and data storage, Stripe for billing, Microsoft SSO for enterprise firms. Advisors can load a client's plan, run scenarios, and generate a report with their firm's logo and compliance disclaimer. The free calculator stays free. The advisor features are a separate product.
Things I Got Wrong
Plenty. The first version had a flat tax rate and nothing else. It took me an embarrassingly long time to realize that progressive brackets change the entire strategy. A married couple with a $31,500 standard deduction filing jointly can convert a significant amount to Roth at very low effective rates. That insight only becomes visible when you model brackets properly, and I was missing it for weeks.
I also underestimated how much the withdrawal sequence matters. Drawing from the wrong account first can cost you thousands in unnecessary taxes over a 30-year retirement. Getting that logic right, especially when Roth conversions, RMDs, Social Security, and pension income all interact, took more iterations than anything else in the project.
The UI was also a mess early on. Financial calculators tend to look like tax forms, and my first versions were no different. Cramming 40+ inputs onto one page without overwhelming people took a lot of rethinking. The simple/advanced mode toggle was the breakthrough. Most people only need five inputs to get a useful projection. The advanced stuff is there for people who want it, but it stays hidden until you ask for it.
Why It is Free
I get asked this a lot. The short answer is that I built it for myself, it costs almost nothing to host, and putting it behind a paywall felt wrong for something that helps people plan their financial future.
The longer answer is that the FIRE community already has a culture of sharing tools and knowledge freely. The blogs, forums, and spreadsheets that helped me learn this stuff were all free. Building a good calculator and keeping it open felt like paying that forward.
It is not a business. It is a tool that I use, that I maintain because I use it, and that other people happen to find useful. If it stops being useful to me, I would probably stop updating it. But considering I check my own plan on it roughly once a week, that is not happening anytime soon.
What is Next
The main calculator is pretty feature-complete at this point. The areas I am still working on are better mobile layouts (financial calculators and small screens do not mix well), more educational content explaining the strategies behind the numbers, and continued tax bracket updates as the IRS publishes new thresholds each year.
If you are planning early retirement and the existing calculators are not cutting it, give BridgeToFI a try. It is free, it is private, and it was built by someone who is actually using it to plan the same thing you are.