I Built a Retirement Calculator, and the Hardest Part Wasn't the Math

chuck zhang — Wed, 21 Oct 2026 01:29:05 +0000

I've been working on a side project called Freedom Calcs, a collection of calculators for exploring financial independence and retirement planning.

It started with a question that sounds pretty simple:

How much money do you need invested today so that compound growth can carry you toward retirement?

That's the idea behind Coast FIRE.

At first glance, building a calculator for this seems straightforward. Take a few inputs, apply a formula, and display the result.

But while working on it, I realized something interesting: getting the math right is only part of building a trustworthy calculator.

Small assumptions can make a big difference

One thing I had to pay attention to was the difference between nominal returns and real returns.

For example, if you assume a 7% annual investment return and 3% inflation, the real return isn't exactly 4%.

It's calculated as:

(1 + 0.07) / (1 + 0.03) - 1

That gives approximately 3.88%.

The difference looks tiny, but when you're projecting results over 20 or 30 years, details like this start to matter.

There's also the question of how monthly contributions are handled. Are they fixed nominal amounts, or do they increase with inflation?

Both approaches can be modeled, but mixing them without explaining the assumptions creates misleading results.

Edge cases are more interesting than happy paths

Another interesting part was thinking about unusual inputs.

What happens when:

The real return is negative?

The expected return is exactly zero?

Someone is already at their retirement target?

Someone stops contributing entirely?

The retirement age equals the current age?

One detail I found especially interesting is that, under a constant-return Coast FIRE model, someone who is below their Coast target won't eventually reach it simply by waiting without contributing.

Their investments grow, but the Coast threshold also changes as the time remaining until retirement gets shorter.

This is the kind of behavior that's easy to overlook if you only test typical examples.

For Freedom Calcs, I've tried to account for these cases and compare the calculations against independent mathematical checks.

A correct result can still be misleading

This was probably the most important product-design consideration.

A calculator might display something like:

"You could reach your Coast FIRE target at age 45."

But that result depends on assumptions about returns, inflation, contributions, and future spending.

It's an estimate, not a prediction.

I decided to make assumptions editable and include different contribution scenarios and return sensitivities, rather than presenting a single number as the answer.

I also wanted the calculations to be inspectable, so the site includes methodology pages explaining the formulas and their limitations.

The calculations run in the browser, without requiring users to create accounts or upload their financial information.

Keeping things simple is surprisingly difficult

The project has expanded beyond Coast FIRE to include Barista FIRE, Lean FIRE, Fat FIRE, and FIRE Number calculators.

They cover related questions, but they don't all use the same financial model.

One challenge is deciding how much explanation to show.

Too little, and users may misunderstand the results.

Too much, and a simple calculator starts feeling like a textbook.

I'm still thinking about how to find the right balance.

I'd love some feedback from other developers

The project is available at Freedom Calcs, if anyone is curious about the implementation or wants to explore the calculators.

But I'm especially interested in the engineering and UX side of this problem.

How do you approach testing tools where the calculations might be technically correct, but the results could still be misunderstood?

And how do you decide which assumptions should be visible to users without making the interface overwhelming?

I'd love to hear how other developers approach this kind of problem.

DEV Community: chuck zhang

I Built a Retirement Calculator, and the Hardest Part Wasn't the Math