Method and sources
How the numbers on this site work.
Last reviewed: November 5, 2026. Everything here is for learning. Nothing is personalized financial, tax, legal or investment advice.
Our rules
- Standard formulas. Each calculator uses a textbook or published formula, listed below, so you can check the math yourself.
- Labeled assumptions. When a tool assumes a return, inflation rate or tax rate, you can change it, and the result says what it assumed.
- Your data stays with you. Calculators run in your browser. Saved progress, quiz scores and net worth entries are stored on your own device and are never uploaded.
- No paid placements. No sponsored rankings, no affiliate links, and no fake reviews. If that ever changes, it will be disclosed clearly on the page where it appears.
How each tool is calculated
| Tool | Formula or method | Main assumptions |
|---|---|---|
| Compound Growth | Future value of a lump sum plus monthly contributions, compounded monthly. | Constant return; no taxes or fees. |
| Take-Home Pay | 2026 federal brackets and standard deduction by filing status, plus Social Security (6.2% up to $184,500) and Medicare (1.45%), minus pre-tax 401(k) and an approximate state rate. | One job, no other income, standard deduction, no credits, health premiums or local tax. State rates are rounded typical rates and some states changed in 2026. |
| Roth vs. Traditional | Same take-home cost either way. Traditional invests the pre-tax amount and is taxed at withdrawal; Roth invests the after-tax amount and is not taxed. | Constant return and tax rates; qualified withdrawals only. |
| Student Loan, Debt Payoff | Standard amortizing payment, then a month-by-month simulation with any extra payment. | Fixed rate; payments made on time; no fees. |
| Savings Goal | Month-by-month growth at the given APY plus monthly deposits until the goal is reached. | Constant APY; no taxes on interest. |
| Inflation and Rule of 72 | Present value = amount / (1 + inflation)^years. Doubling time ≈ 72 / return. | Constant inflation and return. |
| Car Affordability | Loan payment plus running costs as a share of take-home pay, compared with a common 15–20% guideline. | The guideline is a rule of thumb, not a rule. |
| Retirement Number | Future annual spending (today's dollars grown by inflation) divided by 4%, then the monthly saving needed to close the gap. | The 4% guideline is debated; Social Security, taxes and health costs are not included. |
| Credit Utilization | Balance divided by limit; targets at 30% and 10%. | Scoring models differ; these are common guidelines. |
| Budget, Net Worth, Subscriptions, Emergency Fund | Simple sums and ratios of the numbers you enter. | Only as accurate as your inputs. |
Data sources
- Federal income tax rates and brackets: IRS. The 2026 figures come from the IRS annual inflation adjustments (Rev. Proc. 2025-32): standard deduction $16,100 single and $32,200 married filing jointly.
- Social Security wage base ($184,500 for 2026): Social Security Administration.
- State income tax rates: Tax Foundation and each state's revenue department. Our picker uses rounded typical rates, not exact calculations.
- Consumer finance basics (credit, debt, saving): Consumer Financial Protection Bureau.
Limits you should know about
These are estimates for learning. Real taxes depend on your full situation. Investment returns are not guaranteed and can be negative. Past results do not predict the future. For decisions about your own money, check the numbers with your payroll or tax documents, and consider a licensed professional.
Found a mistake?
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