← FersMathCalculators

About this site

I'm a federal employee. When I started trying to understand my own retirement, I couldn't find a single calculator that did the math correctly.

Most tools either ask you to enter a single salary — ignoring that your pay changes every year — or they round off the date arithmetic in ways that don't match what OPM actually does. The High-3 calculation in particular has real nuance: a 360-day year, 30-day months, a sliding window that finds your highest 36 months (not your last 36), and a pay table that rolls over every January 1st. Get any of that wrong and the number is off.

So I built the calculator I wanted to exist. It uses published OPM pay tables, splits your career history at year boundaries automatically, and shows you exactly which months contributed to your High-3 and by how much.

The other half of a federal retirement is the TSP, and most tools there have a similar blind spot: they ask for one assumed rate of return and hand back one confident-looking number. Real markets don't behave that way, so the investment tools here start from what the funds actually did.

You can compare any TSP funds over any date range using TSP's published share prices, refreshed daily. The Monte Carlo projection resamples real historical months — each fund's history goes back to when it began, as early as 1987 for the G Fund — and shows a range of outcomes instead of a single line. The TSP Projection Calculator builds on that: enter your balances, salary, and Traditional and Roth contributions, and it applies the IRS contribution limits, the age-based catch-up rules, and the FERS agency match year by year, then projects below-average, average, and above-average outcomes, in today's dollars if you prefer.

Everything here is free. No account, no email, no ads. If it's useful, buying me a coffee helps keep it running.

This is also a learning project

I'm using this site as a hands-on way to get better at modern software practices. Every calculator ships through a CI/CD pipeline — push to GitHub, automated build, deployed to production in under a minute. No manual steps, no FTP, no crossing fingers.

The code is written with the help of AI — Claude for architecture and logic, a local model for autocomplete. I find it genuinely changes how fast you can move on a solo project, but it still requires knowing what you're building and why. The AI doesn't know OPM's 360-day rule. You have to.

The stack is Astro, React, TypeScript, Tailwind v4, and GitHub Actions. Nothing exotic — just tools that work well together and are worth knowing.

What's here now

What's coming

A note on accuracy

I've tested the High-3 math extensively against OPM's published rules. That said, I'm not a financial advisor or an HR specialist. Use this to understand your situation — then verify the final numbers with your agency HR office before making any retirement decisions.

The TSP projections are a different kind of tool. They show a range of outcomes built from historical returns — not a forecast of what your account will do. Past performance doesn't guarantee future results, so treat them as a way to see how wide the range of possibilities is, not as a prediction.