FinanceTool.

The method behind the number

Useful estimates.
Visible assumptions.

A financial tool should help you understand the answer, not just display it. Here is how the existing calculators work, what the tests check, and where the limits are.

Deterministic calculations

The same valid inputs and assumptions produce the same result. Existing calculator math runs in your browser, without an AI model calculating the answer.

Explanations alongside results

Calculator pages include worked explanations, assumptions and source references. Look at the relevant tool's notes before interpreting an estimate.

Test cases, not guarantees

The codebase includes automated formula tests. Those cases help catch regressions; they do not establish independent auditing, complete coverage, or accuracy for every situation.

Worked examples

The math, in plain sight.

Future value of savings

Each month, the balance earns the assumed monthly return, then the monthly contribution is added.

next balance = current balance
  × (1 + annual rate ÷ 12)
  + monthly contribution

At 0% return, $100 a month for one year adds up to $1,200. With no contributions, $1,000 at a 12% nominal annual return compounds monthly to about $1,126.83 after a year.

Rates are decimals in the equation (12% = 0.12). The tool can also model an annual contribution increase. Assumed returns are not forecasts.

Open the savings calculator

Coast FIRE target

First estimate the retirement target, then discount it by the assumed real return over the remaining years.

target = annual spending
  ÷ withdrawal rate
coast target = target
  ÷ (1 + real return)^years

$40,000 in annual spending divided by a 4% withdrawal rate gives a $1,000,000 target. At an assumed 7% real return for 30 years, the amount needed today is about $131,367.

A constant real return and withdrawal-rate rule simplify reality. Neither sustainable withdrawals nor investment returns are guaranteed.

Open Coast FIRE

Adding or removing GST

Tax is applied to the tax-exclusive base. Removing tax reverses that relationship, rather than subtracting the percentage of the gross.

gross = net × (1 + rate)
net = gross ÷ (1 + rate)
tax = gross − net

At an illustrative 10% rate, $100 net becomes $110 gross. Removing that tax from $110 gives $100 net and $10 tax.

Select the relevant modeled region and check its rate. Exemptions, registration rules and special tax treatment are not established by a calculator.

Browse tax tools

What the existing test files check

Check the examples.
Check the edge cases.

Formula code is separate from the interface, so numerical behavior can be checked directly. On October 7, 2026, the preview build passed all 252 checks across 61 Vitest test files. Eight stale expected values were corrected to reflect the existing 2026 federal standard deduction; no calculation formula was changed for that correction. These checks validate implemented behavior, not compliance with every current tax rule.Read the test summary.

  • Known-value examples

    Future-value tests compare zero-growth contributions and monthly compounding with expected arithmetic.

  • Relationships between scenarios

    Credit-card tests check that larger payments can clear a modeled balance faster and reduce modeled interest.

  • Cases that do not resolve normally

    Tests cover a card payment below monthly interest, where the modeled debt does not pay off.

  • Precision and rounding

    Floating-point calculations are compared with tolerances. Results shown in the interface may be rounded; a displayed total may differ slightly from a sum of rounded lines.

What an estimate cannot promise

Tax calculators model selected rules and may omit local taxes, credits, deductions or individual circumstances. Growth tools simplify variable returns; debt tools depend on repayment assumptions. Always read the tool's notes and verify material decisions with a qualified professional.

Tests are not a security audit, a tax certification or an endorsement. Published sources and dates help you evaluate an estimate, but do not guarantee a rule is current or applicable to you.

Separate from today's calculators

Planned AI report methodology

Monthly P&L explanations are in development and not implemented. The intended design uses deterministic numeric validation, Claude interpretation of validated figures, and factual output grounded in source lines. Unsupported causes would be framed as questions, not facts.

There is no report intake here, and no claim that these safeguards are already running. Future data-handling details would need to be documented before launch.

See the planned workflow

Found something that does not add up?

Email the calculator name, non-sensitive example inputs, the result you expected, and a source if available. Do not send personal financial records.

hello@financetool.tech