How Every Calculator on This Site Works
Every calculator on SumCrafter runs on a small set of published formulas, figures from official sources like the IRS, and assumptions we state right on the page. There is no black box, no secret sauce, and no server involved: the math happens in your browser, and your numbers never leave your device. Try the Coast FIRE calculator and you can trace everything it does back to a formula on this page. Below you'll find the formula behind each tool, where our numbers come from, what the results deliberately leave out, and how we test them. The full source list lives on our methodology page.
The short answer
Our calculators are spreadsheets with better design. Each one takes your inputs, runs them through a published formula, applies official figures like IRS tax brackets, and shows the result with the assumptions printed beside it. We test every tool against hand-calculated reference cases before it ships, and the computation runs entirely in your browser, so nothing you enter is ever sent anywhere. The results are planning estimates, not predictions, and we'll say exactly that on each tool's page too.
We think of each tool as an open-book test. You should be able to see every assumption, trace every number to a published source, and reproduce the headline result in a spreadsheet if you want to. Any calculator that hides its method is asking for your trust instead of earning it. Ours would rather be checked.
The formulas behind each tool
Here is the actual math, tool by tool. If you've ever wanted to double-check a result with a spreadsheet, this section gives you everything you need.
Coast FIRE calculator. It starts with the full FIRE number, annual spending × 25, then discounts it back to today using your expected return over the years left until retirement:
Here r is your expected annual real return (after inflation) and years is the time between now and retirement. Example: $1,000,000 ÷ 1.0730 ≈ $131,367. The tool also compares your current savings with the target and projects where today's savings land at retirement age if you add nothing more.
FIRE calculator. Simpler still. It multiplies your annual spending by 25, which is the math behind the 4% withdrawal rule. The tool then translates the number into what it means in years of saving at your current income, so you can see the finish line in a language that means something.
Barista FIRE calculator. Your portfolio doesn't have to cover all of your spending, because part-time work covers some of it. The target is (annual spending − part-time income) × 25, discounted back to today the same way Coast FIRE discounts it when there's a gap of years involved.
Why does the income subtract before the ×25 instead of after? Because the portfolio only has to replace the part of your spending the job doesn't cover. If you need $50,000 a year and part-time work brings in $20,000, the portfolio only needs to generate $30,000. At a 4% withdrawal rate, that's $750,000 instead of $1,250,000. Half a million dollars of difference from an honest part-time paycheck.
Debt payoff calculators. The snowball-versus-avalanche tool simulates your debts month by month. Each debt amortizes by its own formula: with balance B, monthly rate r, and monthly payment P, the payoff length in months is n = −ln(1 − rB/P) / ln(1 + r). The method layer then decides the payment order: smallest balance first for the snowball, highest APR first for the avalanche. When one debt dies, its whole payment rolls into the next target. A warning: if a monthly payment is smaller than the interest charged that month, the debt never shrinks, and the calculator flags that case.
Paycheck calculator. This one stacks the layers: gross pay, minus pre-tax deductions, then federal income tax applied through the published brackets, then FICA at 7.65% on gross wages, then state tax and any state extras like California's disability tax. Every figure comes from the sources in the next section.
One subtlety: supplemental wages like bonuses are withheld at a flat 22% federal by default, which usually overshoots the actual tax. The calculator shows the withholding estimate your employer would apply, and your real tax bill is settled when you file. That's why the tool distinguishes between withheld and owed, a distinction most paycheck calculators gloss over.
Growth projections across all the tools use annual compounding at the real return rate you choose. We keep the return assumption visible next to every result because it is the single most powerful input in every tool on this site.
One thing the tools intentionally don't do is compound more than once a year. The difference between annual and monthly compounding at 7% is about a third of a percentage point over a decade, far smaller than the uncertainty in the return assumption itself. Modeling it would add false precision. We keep the mechanics simple and put the honesty budget where it matters: the return rate and the assumptions box.
Now that you know the formula, run your numbers
The Coast FIRE calculator shows the formula applied to your age, spending, and return assumption.
Open the Coast FIRE calculator →Where our numbers come from
A formula is only as honest as the figures you feed it. Our tax numbers come from IRS Revenue Procedure 2025-32, which sets the 2026 federal brackets and the standard deduction, $16,100 for single filers in our 2026 estimates. When the IRS publishes new figures each fall, we update ours and note the change on the methodology page, so you can always see which year's rules a result reflects.
State tax data is the messiest input we handle. States change rates, brackets, and payroll taxes on their own schedules, and some publish tables in formats that defy copying. We re-verify the figures we use against state revenue publications and flag any state where the situation changed mid-year. It's the least glamorous work on the site and some of the most important, because a stale state rate bends every paycheck result for that state.
FICA rates come from the statute itself: 6.2% for Social Security and 1.45% for Medicare, the rates every paycheck in America has paid for decades. State tax figures come from each state's published tax tables and revenue department guidance. We simplify the quirky ones: local taxes, credits, and special deductions vary too much to model exactly, and we'd rather show a clean estimate than a fake precision.
The return assumption deserves a special note because no government publishes it. The 7% default on our retirement tools is the approximate long-run real return of the US stock market, nominal returns minus inflation. It's a historical observation, not a promise, which is why we let you lower it and why the results page shows what happens at 5% or 6%. Conservative inputs make better plans.
How do the updates reach the site? When the IRS publishes its fall revenue procedure, we plug the new brackets and standard deduction into the tools, update the reference cases in the test suite, and re-run everything. The methodology page records which year's figures are live, so a result you got last year and one you get today can be traced to the right rule set. We'd rather show the changelog than silently move the numbers.
What the results don't tell you
A calculator that fits in a web page can't model everything, and pretending it could would be worse than saying what it skips. Here are the main limits.
Returns are assumed smooth. Every growth projection uses one flat annual rate. Real markets crash and surge. That means the tools don't model sequence-of-returns risk, the damage done when a big drop lands early in your plan. Treat the result as the midpoint of a wide range, not a promise.
Taxes are simplified. The retirement tools don't tax your withdrawals. In reality, traditional 401(k) withdrawals are taxed as income in retirement, and Roth accounts aren't. That changes how much you need to save. Our tools give you the headline number; a detailed plan needs a tax-aware layer on top.
Some income is ignored. None of the retirement tools model Social Security benefits, pensions, or part-time income after retirement, except Barista FIRE, which asks for it directly. If you expect meaningful pension income, you'll need less saved than the calculator suggests. Count it as margin, not as an error.
Inflation is handled only if you use real returns. Enter spending in today's dollars and a real return like 7%, and inflation cancels out neatly. But if your spending habits will change with age, healthcare costs are the classic riser, no input on the page can capture that. The plan is a framework, not a forecast.
You'll notice we put these limits right next to the results, in an assumptions box on each tool's page. We could bury them in a disclaimer at the bottom, and most sites do, but a planning number without its limits is a lie wearing a suit. If a result looks too good to be true, read the assumptions first. That's where the fine print of every estimate lives.
Rounding hides tiny amounts. We round displayed results to whole dollars and show months, not weeks. On a 40-month payoff, the rounding error is a rounding error. We mention it only because a spreadsheet reproduction will differ by a few dollars, and that difference is expected, not a bug.
Planning numbers beat guessing
An estimate with known limits is more useful than no number at all. Start with Coast FIRE.
Try the Coast FIRE calculator →How we test accuracy
Before any tool ships, it has to survive our test suite. That suite is a list of hand-calculated reference cases, each with an expected answer we worked out independently, by hand or in a spreadsheet. The tool passes only if it matches every case exactly. When we change the code, we re-run the whole suite before publishing.
The reference cases cover the obvious ones and the tricky ones. The Coast FIRE example of $1,000,000 discounted at 7% over 30 years must produce $131,367. The debt examples from our guides must reproduce the same month counts and interest totals we published. And the edge cases get special attention: a zero percent return, a debt payment too small to beat the monthly interest, a paycheck with no state tax. Those cases are where naive code breaks quietly, so they're where we test loudly.
We also cross-check tax figures against the IRS tables line by line after each annual update. If a bracket boundary moved, the test suite has to show the new numbers. That process is tedious, and it is exactly why a calculator you found here and a calculator you found on a random blog can give different answers. Different assumptions, different precision, different results.
Testing has caught real mistakes. More than once, a refactor that looked harmless broke a rounding rule and shifted a published example by a few dollars. The suite flagged it before it shipped, because the reference cases don't care about our code. They only care about the math. That feedback loop is the whole reason the suite exists: to make sure the answer you get today is the same answer you'd get next year.
Your privacy: nothing leaves your browser
All of our calculators run as JavaScript in your browser tab. When you type your salary or your debts, those numbers go into a calculation on your device and nowhere else. There is no sign-up, no account, and no form submission to a server. Close the tab and everything you entered is gone, because it was never anywhere except your device's memory.
That's a design choice, not an accident. A calculator that never sees your numbers can't leak them, sell them, or lose them in a breach. It also means the tools work offline once the page loads, and they work the same whether you're in California or Texas. Your financial details stay yours. Full stop.
You don't have to take our word for it. Everything runs in standard JavaScript in the page itself, and anyone with a browser's developer tools can watch the computation happen locally, with no network calls carrying your inputs away. Trust, but verify: the mechanism is right there in the open.
The bottom line
Every SumCrafter calculator is a published formula, official figures, and a stated set of assumptions, tested against hand-worked reference cases and run entirely in your browser. The math is inspectable, the data sources are documented on the methodology page, and the limits are printed alongside the results instead of hidden in fine print. If you want to see the formulas in action, start with the Coast FIRE calculator. And if you ever spot a number that looks wrong, tell us. We'd rather fix a test than defend a bug.
Use every result the way it was designed: as the first draft of a plan, checked against your own spreadsheet, revisited once a year as the rules and your life change. A calculator's job isn't to decide for you. It's to give you a number honest enough to argue with. Check it, challenge it, and make it yours.