How We Calculate

Every FinanceMaxing calculator uses a standard, published formula. This page documents the math, the assumptions, and how we keep results accurate — so you can verify any answer yourself.

Mortgages & loans

Monthly principal-and-interest payments use the standard fixed-rate amortization formula:

M = P · r · (1 + r)^n / ((1 + r)^n − 1)

where P is the loan amount, r is the monthly rate (annual rate ÷ 12 ÷ 100), and n is the number of monthly payments (years × 12). Our mortgage tool adds property tax, homeowners insurance, PMI, and HOA to show a full PITI payment, and builds a month-by-month amortization schedule. The loan calculator applies the same engine to auto, personal, and student loans.

Home affordability & DTI

Affordability follows the debt-to-income guidelines lenders actually use: the 28/36 rule — housing costs up to ~28% of gross monthly income (front-end ratio) and total debt up to ~36% (back-end ratio). Our affordability and DTI calculators use these thresholds; lender limits vary, so we treat them as planning guidelines rather than guarantees.

Investing & compound interest

Growth uses the compound-interest formula:

A = P · (1 + r/n)^(n·t)

where n is the number of compounding periods per year. Our compound interest, 401(k), and Roth IRA tools layer in regular contributions and (where relevant) employer match and contribution growth. Projections assume a constant annual return for illustration; real markets vary year to year.

Paycheck, salary & income

Pay conversions use consistent period factors (52 weeks, 26 biweekly periods, 12 months per year, and your stated hours and weeks worked). Take-home estimates subtract the federal, state, and FICA amounts you enter; because withholding depends on your W-4, filing status, and locality, we present these as estimates and label every assumption on the page.

Taxes

Tax tools apply current federal brackets and standard rules for the relevant calculation (for example, the self-employment tax combines the 12.4% Social Security and 2.9% Medicare rates on net earnings, with the deductible-half adjustment). Tax law changes and individual situations vary, so our estimates are a starting point — always confirm with the IRS or a tax professional before filing.

How we keep it accurate

• Worked examples. Each calculator is checked against a hand-computed example, and we publish that example on the page so you can confirm the result.
• Standard formulas only. We implement published, widely-accepted formulas — no proprietary "secret sauce" that can't be verified.
• Reader reports. If a number looks wrong, readers can tell us via the contact page; corrections are prioritized.
• Clear assumptions. Where a result depends on an assumption (a rate, a rule, a rounding choice), we state it rather than hiding it.

FinanceMaxing's calculators provide good-faith estimates for education and planning. They are not financial, tax, legal, or investment advice. See About for more, and always verify important figures with your lender, the IRS, or a qualified professional.