Synoria AI CorporationAutonomous software company
Accounting & Finance

RulesOfThumb: Personal Finance Rules-of-Thumb Calculator

Two rules of thumb that get applied without checking the math behind them. The Rule of 72 estimates years to double your money as 72 divided by the interest rate - accurate near 8%, but it overstates the time at low rates and understates it at high rates, in opposite directions. The 28/36 mortgage rule is really two separate limits: housing costs at or under 28% of gross income, AND total debt payments (housing plus everything else) at or under 36% - a housing cost that clears the first line alone can still fail once your other debts are added to the second. Enter your numbers and see both rules checked properly, with the real formula next to the approximation.

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Accounting & Finance

Anyone applying a personal-finance rule of thumb who wants to see the actual numbers behind it, not just the round fraction.

How it works

  1. Name this check, e.g. "My numbers".
  2. For the Rule of 72, enter your expected annual interest rate.
  3. For the 28/36 rule, enter gross monthly income, monthly housing cost, and other monthly debt payments.
  4. See both rules applied with the real numbers, not just the rounded rule.

What you gain

  • Rule of 72 next to the real formula (ln(2)/ln(1+rate)) - the rule is close near 8% but drifts in OPPOSITE directions above and below it, and this shows exactly how many years off it is at your own rate
  • The 28/36 mortgage affordability rule checked as TWO separate conditions, not one - a housing cost that clears 28% on its own can still fail the 36% total-debt limit once other payments are added
  • Each rule states plainly whether it passes or fails, with the number that decided it
  • Checks saved by name: recalculating updates the same check, not a duplicate

Screenshot

RulesOfThumb: Personal Finance Rules-of-Thumb Calculator

Technical details

Standard12.99 USD · 1500 requests · 31-day license · one-time payment · 31-day access
Pro25.98 USD · 6000 requests · 31-day license · one-time payment · 31-day access
Isolationdedicated instance per license
Usage meteringLLM usage metered per license
Accessweb sign-in with license key

How to set up & use

  1. Buy the license — your key (lic_...) appears on the order page and in your email.
  2. Sign in at app.synoriaai.com with your license key.
  3. No installation — the product runs in your browser, on your own isolated instance.
  4. Open the calculator and give your check a name.
  5. Enter your rate and your monthly gross income, housing cost, and other debt payments.
  6. Press Calculate to run the rules against your numbers.
  7. Read each rule's pass or fail result along with the number that decided it.
  8. Recalculate the same saved check whenever your numbers change.

Frequently asked questions

What does this calculator actually do?

It applies a few common personal finance rules of thumb to the numbers you enter and shows whether each one passes or fails, along with the figure that decided it.

Does it store my financial numbers?

Checks are saved by name so recalculating updates the same check instead of creating a duplicate. The numbers you enter are used to run the rules and are kept with that saved check.

Is this financial advice?

No. It only applies rules of thumb and shows the arithmetic behind each result. It does not recommend products or tell you what to do.

With USDT or USDC stablecoins on one of the payment networks shown at checkout. You scan a QR code with the exact amount pre-filled — no card and no wallet connection is required.
The payment is detected on-chain and matched to your order by its unique amount. After confirmations complete, your license key is generated and your own product instance is prepared; the order page updates by itself.
No. You make a one-time stablecoin payment for a 31-day license with a fixed request quota. There is no auto-renewal — to keep using the product, you simply purchase again.
Each plan includes a fixed number of requests for the license period. A small overage allowance is defined beyond that; once it is used up, requests pause until a new license is purchased.
Yes. Every license runs in its own isolated instance with its own data directory — customers never share an instance or data.
On-chain payments are never lost. Keep your order number and contact support; the payment can always be matched to your order.
Each product is built for its target market and uses that market's language; this storefront is available in 9 languages.

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