My FIRE date is a straight line, and I left it that way on purpose

Telemetry will tell you roughly when you hit your FI number. It does it with the dumbest arithmetic in the entire product.

It takes your net worth today, subtracts your net worth twelve months ago, divides by twelve to get an average monthly gain, and divides the remaining gap by that. No compounding. No expected return. Nothing about what markets are supposed to do. Whatever the last year did, straight ahead, forever.

Writing the compound-growth version is the easy part, and I keep not doing it. I want to explain why, because the reasoning changed how I think about every projection in the app.

The version that looks better is easier to write

Give me a growth rate and I will give you a much nicer number. Seven percent annually, contributions continuing, compounding monthly, and now the projection curves the way projections are supposed to curve and lands on a specific year that feels earned.

The problem is what that number is made of. It is one assumption, compounded over every year you are away. At twenty years out, dropping the assumed return from 7% to 5% pushes the date back somewhere between three and six years, depending on how much of the pile you have already built. The reader did not supply that assumption. I did, on their behalf, invisibly, and then rendered the result to the year as if it were a fact about their finances.

The straight line has the opposite property. It is obviously rough. Nobody looks at it and thinks the software knows something. A range would be honest too, because its width is the point. A single compounded date is the one version that isn’t.

What the straight line is actually good for

It answers a question you can act on: at the rate I have actually been going, how far away is this?

That is a real question, and the input is real too. Your last twelve months happened. It already contains your actual savings rate, your actual spending, whatever the market actually did to your holdings, the bonus you got, the roof you replaced. None of that is modelled, because none of it needs to be. It is observed.

It is also honest about the direction of travel in a way a growth curve is not. If your last year was bad, the line says so, immediately and rudely. A compound-growth model would quietly reassure you that the long run is fine. It might even be right. But “the long run is fine” is not information you can do anything with this month.

And when the input isn’t real yet, it doesn’t pretend. The line needs most of your accounts to have about a year of history. Before that, an account you connected in the spring would look like a year of savings, because a year ago it reads as zero. So instead of a date, Telemetry tells you it needs a year of history, and shows your progress without guessing when you arrive.

Where the compounding version does belong

We have compound-growth calculators, and I want to be precise about the difference rather than pretend there isn’t one.

The public FIRE calculator and its variants, Coast, Barista, Lean and Fat, all compound a return that you supply. That is the right shape for them, because they are answering a different question: not “where am I going” but “what would it take?” You are deliberately exploring assumptions, so the assumption should be yours, visible, and adjustable.

Coast FIRE is the clearest case. The entire concept is compound growth doing the work after you stop contributing. You cannot express it without a growth rate. A straight line there would not be conservative, it would be wrong.

So the split is: a calculator you point at the future gets a growth assumption, and the date I project from your actual account does not. One is a question, the other is a measurement.

There is exactly one place inside the app where that line bends, and it bends for the reason above. Telemetry tracks CoastFIRE alongside the regular number, and a Coast target cannot be computed without a return. It is the whole idea. So that one figure is discounted at a rate, 7% after inflation unless you change it, which is roughly what US stocks have returned over the last century. It also assumes you stop working at 65. The Net Worth page prints both right beside the number, and the rate is yours to change in the FIRE setup. The projected date is still the straight line. I would rather have assumptions you can see and argue with than a curve with the same assumptions buried in it, which is the entire point of this post and not an exception to it.

The note under the number

There is a line under the FIRE projection in the app that says it extends your recent monthly gain in a straight line, so it does not model investment growth, compounding, or a downturn, and to treat it as a direction of travel.

I went back and forth on that line. It is an admission that the number is crude, sitting directly under the number. Some fraction of people read it and think less of the projection.

They should. That is what it is for. The alternative is a projection that looks authoritative and quietly isn’t, which is worse in exactly the situation where it matters most: someone deciding whether they can leave a job.

That person does not need a nicer curve. They need to know how far away it is at the rate they are actually going, and how sure anyone can honestly be. The straight line tells them both.