your assumptions, in the open.
every fire calculator online gives you a number and hides how it got there. the return, the inflation figure, the withdrawal rate, whether fees were counted — all buried, and all of them change the answer by years.
the ones that show their working usually show it in a footnote while the big number sits at the top looking like a fact.
build your own and the assumptions are inputs on the screen, changeable, with the answer moving as you argue with them. which is the only honest way to look at a projection.
arithmetic you can argue with.
01 / what it isthis page compounds numbers you supply. that is all it does, and saying so plainly is the reason it is more useful than the calculators that present the same arithmetic as insight. the value is not the answer — it is watching the answer move when you change the return from seven percent to five.
it is not advice and cannot be. it does not know your tax position, your pension rules, your job security, or what markets will do. what it can do is be honest about which of your assumptions the conclusion is actually resting on, which is usually one or two of them.
- every assumption as a labelled, adjustable input
- the arithmetic shown year by year, not just the final number
- a range rather than a single answer, by varying the return
- the effect of one change visible immediately
- a plain statement of what the model does and does not include
- no advice, no recommendation, no projected outcome dressed as a fact
- what you supply
- current savings, contributions, expected return, inflation, spending, withdrawal rate
- what it computes
- a year-by-year projection and the year a target is reached, under those inputs
- what it stores
- nothing by default. optionally your inputs, in this browser only
- what it is not
- financial advice, a forecast, or anything that accounts for tax rules or real markets
see one in action.
02 / examplea projection with seven visible assumptions, each adjustable, and the year the number is reached
the live example is still being built — the prompt below is the one it was made with.
copy this, then make it yours.
03 / starting prompta starting point, not a template — the point of an agent is that the result is yours. change the sections to match what you want, and remove anything you don’t need.
Build me a retirement arithmetic page — a FIRE calculator — as a single web page.
Ask me first:
- My currency.
- Which assumptions I want as inputs. At minimum: current invested savings, annual contributions, expected annual return, inflation, annual spending in retirement, and a withdrawal rate.
- Whether I want to model anything extra: a lump sum at a known date, a change in contributions, a pension starting at a certain age.
- Whether the page should remember my inputs in this browser, or forget them when I close it.
Do not supply default values for return, inflation or withdrawal rate as though they were facts. If you offer a starting value, label it clearly as an arbitrary placeholder I should replace.
WHAT IT DOES
- Every assumption is a visible, labelled, editable control. Nothing is hardcoded and nothing is hidden in the JavaScript.
- Results update as I change an input, so I can see which assumptions the conclusion actually depends on.
- Show the year-by-year table, not only the final number: year, contributions, growth, balance. The working is the product.
- Show a range rather than a single answer — for example the same projection at the return I gave, and at two percentage points either side — so the answer reads as a spread, not a date.
- State in plain language on the page: these are my numbers, compounded; it excludes tax, fees, market sequence, and everything else it does not model.
- Let me change the target — a number, an age, or a spending level — and see it recalculate.
WHAT IT MUST NOT DO
- No advice, no recommendations, no "you should save more", no risk profiling.
- Do not present the projection as a prediction. Language like "you will retire in 2041" is wrong; "under these assumptions, the target is reached in 2041" is right.
- Do not fetch market data, inflation figures or anything else. There are no network requests.
- Do not invent my financial details.
DESIGN
- Inputs and results on one screen on a laptop, so cause and effect are visible together.
- Numbers formatted consistently, monospace, with the units and currency stated.
- Calm and instrument-like. This is a slide rule, not a sales page.
BE HONEST ON THE PAGE
- A clear line stating this is arithmetic on assumptions I supplied and is not financial advice.
- If inputs are remembered, say they are stored in this browser only.
DELIVERY
- Produce one self-contained HTML file. Inline the CSS and JavaScript.
- No build step, no server, no framework, no package install, no external
requests at runtime.
- Mobile-first and responsive; it has to be usable one-handed on a phone and
still look right on a laptop.
- Accessible: semantic HTML, real contrast, visible keyboard focus, labelled
controls, touch targets big enough to hit, and respect reduced-motion.
- Use placeholders where information is missing. Do not invent facts, numbers,
names, dates, prices, or results I have not given you.
- Before you call it finished, use it yourself: enter real-looking data, reload
the page, and confirm nothing is lost or mangled.
PUBLISHING
Publish the finished page to HTMLShare and give me the link.
Use exactly one of the two paths below, preferring the first. Make the call
directly. Do not route it through a code sandbox, container, or computer-use
session — that is where the request gets blocked, and it is never necessary.
1. An HTMLShare integration or connector, if you have one configured.
The page is permanent and owned by my account. Report the url and the
siteId. There is no claimUrl or manageToken on this path — do not go
looking for one, and do not switch paths in order to produce one.
2. Otherwise, publish anonymously over REST. No account, key, or signup is
needed:
POST https://api.htmlshare.net/v1/publish
content-type: application/json
{"html": "<!doctype html> ... "}
The reference is https://htmlshare.net/docs and, for the full schema,
https://api.htmlshare.net/openapi.json
Report these from the response, in your reply and never inside the page:
url the public page
claimUrl a private link that is mine alone. opening it is how I keep
the page past its 24-hour expiry, or set an email reminder
before it goes. it carries the manage token, so it must
never appear in the published HTML or anywhere a visitor
could see it.
manageToken the only thing that can edit or delete this page later. it is
returned once and never again, so tell me what it is.
Tell me which of the two you used. If the path you reach for first is
unavailable in your environment, say so and tell me what you are switching to
before you publish — a permanent page on my account and a 24-hour anonymous
page are different things, and which one I get is my decision, not a detail.
If I ask for a change afterwards, update the existing page instead of
publishing again, so the URL I have already shared keeps working — through the
same integration on path 1, or with the manage token as an X-Manage-Token
header on path 2.
RECIPE MARKER
Include this tag in the <head>, exactly as written:
<meta name="htmlshare-recipe" content="fire-calculator">
It tells htmlshare which recipe the page came from, so the library can count
that this one was actually made. It carries nothing else.give it a url.
04 / publishing ithow you publish depends on how you’re working.
- with an integration
- if your assistant already has htmlshare connected — mcp or another supported integration — just ask it to publish the finished page and hand back the url. the page belongs to your account and is permanent. when you want a change, ask for an update to that same page rather than a new one.
- with no integration
- the prompt tells the assistant to post the html to the rest api instead, which needs no account and no key. it comes back with a live url you can open immediately, a private claim link, and a manage token. the claim link is how the page stops being temporary — keep it.
- by hand
- nothing stops you doing it yourself: have the assistant give you the html file and drop it on the htmlshare home page. same url, same page, one more step.
- what matters here
- this is a page you come back to when something changes — a raise, a move, a market year. claim it so it is there, and keep any real figures out of the html itself.
you will want to rerun this in a year.
05 / keeping it aroundgenuinely fine for a single session: the page stores nothing, so an anonymous publish gives you the calculator for a day. it stops serving after 24 hours, with a 7-day recovery window.
claim it if you want it to be the page you reopen every time your circumstances change, which is the way this one actually gets used.
- keep it around
- this is a page you reopen whenever something changes. anonymous gives you one day.
- custom url
- a calculator you can get back to without rebuilding it from scratch each time.
- stable updates
- you will want another assumption modelled eventually. add it at the same address.
questions.
06 / questionsis this financial advice?
no. it compounds numbers you type in and prints the assumptions beside the result. it knows nothing about your tax position, your pension rules or the markets, and the page says so rather than implying otherwise.
what return should i use?
that is exactly the question the page refuses to answer for you, and the reason to build it this way. use a few and look at the spread — the honest output of this arithmetic is a range, not a date.
does it account for tax?
only if you model it yourself in the inputs. tax treatment depends on jurisdiction, account type and your circumstances, none of which a static page knows. the prompt makes it state what it excludes.
are my numbers saved anywhere?
not by default — it calculates and forgets. if you asked for inputs to be remembered, they are in your browser only and the page says so. nothing is ever sent to us.