Every number on this site comes from math you can check. Here’s how our calculators work, the assumptions behind them, and how we write and update our guides.
Our principles
- Show the assumptions. Every calculator lists the assumptions it uses under “How the math works,” and every article states the rate, down payment and other inputs behind its examples.
- Use standard, widely accepted formulas. Nothing proprietary, nothing hidden. Anyone with a spreadsheet can reproduce our results.
- Prefer conservative over flattering. When a choice of assumption changes the answer, we pick the one that’s less likely to get you into trouble, and we tell you.
- Your numbers stay with you. The calculators run entirely in your browser. What you type is never sent to us or stored.
- Estimates, not promises. Real results depend on your lender, taxes, location, market returns and life. Use our results to understand a decision, then confirm the details with the people involved.
How we check the math
Our calculators share one set of tested formulas. Before a calculator goes live, its results are checked automatically against known answers: standard mortgage amortization tables (for example, a $200,000 loan at 6% for 30 years is $1,199.10 a month), compound growth, savings yields, debt payoff schedules and the affordability math. The examples in our articles are computed with the same code that runs the calculators, so the article and the calculator always agree.
How each calculator works
Home Affordability Calculator
Shows two prices side by side. What a lender may approve uses the classic 28/36 guideline: total housing cost up to 28% of gross monthly income, and housing plus other debts up to 36%; we use the lower of the two. What you can comfortably afford uses your take-home pay and the share you choose (25% is our default target; one third is the red line). Housing cost includes principal and interest, property tax, homeowners insurance and HOA dues. Real lender approvals often go higher than 28/36.
Mortgage Calculator
Standard fixed-rate amortization: payment = loan × r ÷ (1 − (1 + r)−n), where r is the monthly rate and n the number of payments. Adds property tax, insurance, HOA and, when the down payment is under 20%, PMI as an annual percentage of the loan (0.5% by default; real PMI depends on credit score and down payment).
Rent vs Buy Calculator
Compares your wealth year by year. Buying: home value (growing at your appreciation rate) minus selling costs minus the remaining mortgage. Renting: the down payment and closing costs invested instead. Each month, whichever option costs less invests the difference at your investment return. Owning costs include mortgage, property tax, insurance and maintenance; tax deductions are not included. The break-even is the first year buying comes out ahead.
Mortgage Refinance Calculator
Compares the remaining payments on your current loan with a new loan for the same balance. Break-even = closing costs ÷ monthly savings. Lifetime comparison = remaining interest on the current loan versus interest on the new loan plus closing costs. Principal and interest only, since taxes and insurance don’t change with a refinance.
Savings Interest Calculator
Converts the APY you enter to a monthly rate ((1 + APY)1/12 − 1) because APY already includes compounding. Deposits are added at the end of each month. Interest is shown before taxes.
Compound Interest Calculator
Grows your starting amount and monthly contributions at a monthly rate equivalent to the yearly return you enter. Returns are an average; real markets go up and down. Taxes and fees are not included.
Emergency Fund Calculator
Target = your essential monthly expenses × the number of months you choose (3 for steady income, 6 or more if your income varies or you’re the only earner). Time to goal = the gap ÷ what you can save per month.
Savings Goal Calculator
Solves for the monthly deposit that reaches your goal by your date, after growing what you’ve already saved at your savings APY.
Debt Payoff Calculator
Simulates every month: interest is added to each debt, all minimums are paid, and your extra amount goes to the target debt: highest rate first (avalanche) or smallest balance first (snowball). When a debt is paid off, its minimum rolls into the next one. Assumes no new charges and fixed rates.
Credit Card Payoff Calculator
Interest is charged monthly at APR ÷ 12 on the remaining balance, and your fixed payment is applied each month. If the payment is below the monthly interest, the balance never goes down, and the calculator says so.
Car Loan Calculator
Amount financed = price + sales tax + fees − down payment − trade-in, then standard amortization. Also shows the four-year payment for the 20/4/10 rule of thumb (20% down, no more than 4 years, total car costs under 10% of gross income).
Raise Calculator
Raise after taxes = raise × (1 − your estimated tax rate on the added income), split by the shares you choose. The invested share is grown monthly at your return for 10 years. The tax rate is your estimate; actual taxes depend on your situation.
Retirement Calculator
Grows your savings and monthly contributions to your retirement age, then converts the result to today’s dollars using your inflation rate. Monthly income uses the withdrawal rate you choose (4% by default, a common rule of thumb, not a guarantee). Social Security is not included here.
Retirement Savings Needed Calculator
Target = (income you want − Social Security or pension) ÷ withdrawal rate, in today’s dollars, then grown by inflation to your retirement year. Monthly savings needed accounts for what you’ve already saved and your expected return. Get your own Social Security estimate at ssa.gov/myaccount.
401(k) Calculator
Each year: your contribution (capped at the IRS limit for your age), plus the employer match up to the match cap, grows monthly at your return; salary rises by your raise rate. Uses the current year’s IRS contribution limits (2026: $24,500, plus $8,000 catch-up at 50+ and $11,250 at ages 60 to 63) and keeps them flat in future years. Before taxes and fees.
How we write our guides
- One question per article. The headline is the question, and the short answer comes first.
- Worked examples with real numbers. Our characters (Tyler, Rosa, Mia and Danny) are illustrative examples, not real people. Their numbers are realistic and calculated with our own calculators.
- Sources linked where they matter. We rely on official and widely trusted sources (government agencies, the Federal Reserve, Freddie Mac, the IRS, published research and surveys) and link to them in the text.
- Clearly labeled examples. Rates and returns in examples (like a 7% mortgage rate or a 7% average investment return) are example inputs, not predictions.
Updates and corrections
- Every article shows the date it was last updated.
- We review our guides regularly and update yearly figures, like 401(k) limits, every January.
- Found an error? Email support@middlemoneyguy.com. We correct mistakes promptly.
Independence
Some pages carry clearly labeled affiliate or sponsored links. They never change our math, our calculators or our conclusions. See our disclosure and about us.
Education, not financial advice. For decisions about your specific situation, talk with a licensed professional.