Every guide here works through a real money decision with actual numbers, charts, sources and a link to the free calculator that runs the same math. A new guide is added every day. Browse by topic, or go straight to the full list of calculators.
Mortgage Discount Points: When Is Paying for a Lower Rate Worth It?
One point costs $3,500 on a $350,000 loan and trims the payment by about $59. Whether that is worth it depends on how long you keep the loan.
Car Depreciation: How Fast Cars Lose Value, Year by Year
Depreciation is usually the biggest cost of owning a new car. A $40,000 car can lose $8,000 in the first year. Here is the curve and what to do about it.
Zero-Based Budgeting: Give Every Dollar a Job (With a Worked Example)
Income minus every planned expense, including savings, equals zero. Here is a worked $4,500 budget, the steps to build one and how to handle surprises.
Personal Loan vs Credit Card: Which Costs Less for a Big Expense?
At 12% a personal loan on $8,000 costs $1,566 in interest over three years; a 24% card paid at the same amount costs $4,247.
Successive Discounts: Why 20% Off Plus 10% Off Is Not 30% Off
Successive discounts multiply: 0.80 × 0.90 = 0.72, so 20% plus 10% off is 28% off. Here is the formula and a table of common combinations.
High-Yield Savings Accounts: APY, Insurance Limits and What You Really Earn
The same $20,000 can earn about $816 or about $80 a year, depending on the account. Here is how APY works, what FDIC insurance covers and how the interest is taxed.
Roth Conversion Explained: When Paying Tax Now Beats Paying Later
A $50,000 Roth conversion costs about $10,350 in federal tax in this example. Whether that is a good deal depends on the tax rate you would otherwise pay later.
Capital Gains Tax Rates 2026: 0%, 15% and 20% Brackets Explained
Hold more than a year and a $30,000 gain costs about $3,082 in this example instead of $5,560. Here are the 2026 thresholds and the stacking rule.
CPI Explained: How Inflation Is Measured and What the Number Misses
CPI measures the price of a basket of goods and services over time. Headline CPI was 3.7% in September 2026 and core CPI 2.6%. Here is how it is built and why your inflation can differ.
What Is PMI and How Do You Remove It? Costs and Timeline
PMI protects the lender, not you, and often runs $100 to $300 a month. Here is what drives the price and the exact rules for getting rid of it.
Lease vs Buy a Car: The Real Cost Over 3 and 6 Years
Leasing has the lower payment, but over six years it cost about $18,857 more than buying and keeping in this example. Here is the full comparison.
How Much Rent Can You Afford? The 30% Rule, the 40× Rule and a Better Test
Spend no more than 30% of gross income on rent, or $1,500 on a $60,000 salary. Landlords often want income of 40 times the monthly rent. Here is a more personal test.
Debt-to-Income Ratio: What Lenders Count and What They Ignore
Add up monthly debt payments, divide by gross income: $2,700 ÷ $6,000 = 45.0%. Here is what counts, what does not and typical lender limits.
Percentage Increase and Decrease: Formulas, Examples and the 20% Trap
Percent change = (new − old) ÷ old × 100. A 20% loss needs a 25% gain to get back to even. Here are the formulas and the mistakes to avoid.
How Much Emergency Fund Do You Need? A Month-by-Month Plan
Three to six months of essential expenses is the standard answer. Here is how to size yours, build it in steps and decide where the money should live.
The 4% Rule: Safe Withdrawal Rates From 3% to 5%, With Real Numbers
A $1 million portfolio supports about $40,000 of first-year income under the 4% rule. Here is where the rule came from, how long different rates last and when to use a lower number.
Standard vs Itemized Deduction 2026: Which Saves You More?
With $19,000 of itemizable expenses against a $16,100 standard deduction, itemizing saves $638 in this example. Here is how to decide.
15-Year vs 30-Year Mortgage: Which Costs Less Overall?
A 15-year loan saves roughly $310,744 of interest on $350,000 but raises the payment by $661 a month. Here is how to decide.
How Much House Can I Afford on a $100K Salary?
A $100,000 salary does not come with one answer. Here is the 28/36 math, what rates near 7% do to it, and how $400 of monthly debt quietly removes $30,000 of buying power.
Mortgage Payment on a $400,000 House: Real Numbers
What a $400,000 house really costs each month once taxes and insurance join principal and interest, how rates from 6% to 7.5% change it, and what you pay over 30 years.
Debt-to-Income Ratio for a Mortgage: What Lenders Allow
Your DTI can decide your mortgage approval before your credit score does. Here is how both ratios are calculated, what each loan type tolerates, and how to move yours.
Pay Off Your Mortgage Early: What Extra Payments Really Save
Extra mortgage payments save real money, but the best use of the cash depends on your rate. Here are the actual savings for $100, $250 and $500 a month, and the cases where you should not bother.
Mortgage Refinance Break-Even Point: Is It Worth It?
The only refinance math that matters: closing costs divided by monthly savings. Here is a worked example, why resetting to 30 years can fool you, and a simple checklist.
FHA vs Conventional Loan: Which Costs Less?
FHA wins for some borrowers and loses badly for others. The deciding factors are your credit score and how long you will keep the loan. Here are both cases with real numbers.