Jobi Cheriyan

Why I built this

It started because I wanted the answer to a fairly ordinary question: if I keep saving what I am saving now, when could I stop working? I did not want advice. I did not want a product. I wanted arithmetic.

So I did what anyone would do and searched for a FIRE calculator, and then a net worth tracker. What I found was disappointing in a very consistent way. The tools that looked capable wanted a subscription before they would show me the projection. The ones that were free gave me a single headline number, no formula, no assumptions, and a prominent button inviting me to book a call with an adviser. Several asked me to connect my actual bank account before doing any maths at all. A few were genuinely good for about ninety seconds, until the part I actually needed turned out to be the paid tier.

What struck me was that none of this was hard maths. A FIRE projection is compound growth against a withdrawal rate. A net worth tracker is a list of numbers and a subtraction. These are not proprietary models worth locking behind a paywall; they are formulas you could work through on paper if someone would just tell you which formula they used. Nobody would tell me which formula they used.

I am a developer. At some point it became faster to write the thing than to keep evaluating tools that would not show their working. So I wrote it, mostly for myself, and then kept going because the version I wanted did not seem to exist anywhere for free.

The number I could not fake

The FIRE calculator worked, and then it embarrassed me. Every projection it produces rests on one input above all others: what you actually spend in a year. Change the growth rate by half a percent and the answer moves a little. Change annual spending by two hundred a month and the answer moves by years.

I did not know my number. I had a confident guess, the way everyone does, and when I finally added up real transactions the guess was wrong: not by a rounding error, by a meaningful amount, and in the direction you would expect. I had built a precise machine and was feeding it a number I had essentially invented.

So I went looking for that too, and ran into the same wall. My bank's own categories were close to useless: every card payment filed under something generic, half the useful detail in the merchant name it had already thrown away. The budgeting apps wanted to connect to my bank account, which is the one thing I did not want, and most of them wanted a subscription on top.

The Expense Diary is my answer to that. You type entries in yourself (amount, category, done in about five seconds) and it totals the month and breaks it down by category. No bank connection, so there is nothing to leak and nothing to authorise. Manual entry sounds like the worse option until you notice that it is the only version where you see every transaction as it happens, which turns out to be the entire point.

I will be honest about the timeline, because most tools in this category are not. The first week tells you nothing you did not already know. The value arrives around the second and third month, when the shape of a normal month emerges and you can finally tell an ordinary month from an expensive one. That is also the point most people never reach, which is why the diary is built to take seconds rather than to be clever. With a free account it grows into a proper ledger: multiple accounts with running balances, recurring bills that post themselves for you to confirm, and a safe-to-spend figure that already accounts for what is due.

Then it closes the loop: the spending figure it produces is the one the FIRE calculator needed all along. That is really what this site is: a set of tools that stopped being separate once I noticed each one was answering a question the last one raised.

What I decided it had to do

Once I was building it rather than shopping for it, the frustrations turned into rules. Each of these is a direct reaction to something that annoyed me while I was looking.

• Show the formula. Every calculator names the arithmetic it used and walks a worked example. If you want to check it by hand, you should be able to.
• Say where the model breaks. A projection assuming an identical return every year is not a forecast. Each tool states the conditions under which it stops being reliable, and the ones that need it offer a thousand randomised runs and a century of market history to sanity-check against.
• No paywall on the maths. Every calculator on this site is free and works without an account. The account exists so your figures persist between visits, not to gate the answer.
• No bank connection. You type your numbers in. It takes a few minutes a month, and it means there are no banking credentials here to leak.
• No recommendations. Nothing here will suggest a fund, a platform or a product, and there are no affiliate links inside the calculators. I am not qualified to advise you and I am not going to pretend otherwise.

What I actually do here

My background is software rather than finance: web applications in JavaScript and React, automation in Python, and cloud infrastructure. The site runs on Cloudflare's edge network, so pages load quickly wherever you happen to be, and the projections are computed in your own browser rather than on a server, which is also why your figures never have to leave your account to produce a result.

The finance side I have learned the way most people in the FIRE community learn it: reading the underlying research, working through the models, and being careful about the difference between a number and a prediction. That is precisely why every page here shows its working and refuses to give advice. I am confident about the arithmetic. The decision is yours, and for anything significant it is worth taking to someone qualified and regulated.

I also run FinToolSuite, a larger library of general financial calculators built on the same principle. FireMathLab is the focused version: fewer tools, aimed squarely at financial independence, debt and net worth, with planning you can keep coming back to.

Corrections

If a formula on this site is wrong, I want to know; that is the whole premise, and a maths error here is a real failure rather than a typo. Email hello@firemathlab.com or use the contact page, and see the editorial policy for how corrections get handled. You can also find me on LinkedIn.