← FersMathHigh-3 Calculator

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.

Everything here is free. No account, no email, no ads. If it's useful,buying me a coffeehelps 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.