Guides & Tips
In-depth guides on personal finance, health, and math — with real numbers and free calculators.
How to Pay Off Your Mortgage Early with Extra Payments
Adding $200/month to a $300k mortgage saves $73,000 in interest and 5.5 years. Here's exactly how it works.
How Much House Can I Afford? The 28/36 Rule Explained
On a $90k income, the 28/36 rule limits your mortgage to $2,100/month — supporting a home price of ~$330,000–$380,000 at today's rates.
When Does PMI Go Away? How to Remove Private Mortgage Insurance
PMI automatically cancels at 78% LTV. You can request removal at 80%. Here's exactly when it ends and how to speed it up.
Should You Refinance Your Mortgage? How to Calculate Break-Even
Break-even = closing costs ÷ monthly savings. If it takes more than 3–5 years and you might move, don't refinance.
What Is a Good Debt-to-Income Ratio? DTI Explained
A DTI below 36% is excellent. Most mortgages require below 43%. Here's how to calculate yours and lower it fast.
How Much Should You Contribute to Your 401(k)?
At minimum, contribute enough to get your full employer match — that's a guaranteed 50–100% instant return.
Auto Loan Calculator: Calculate Your Car Payment Before the Dealer Does
On a $35k car with 10% down at 7% APR over 60 months you pay $623/month. Know your number before the dealer quotes you.
Student Loan Payoff Strategies: Pay Off Faster and Save Thousands
Adding $200/month extra to a $40k loan at 6% saves $8,400 in interest and pays it off 4 years early.
How to Calculate Stock Profit and Loss (With Fees)
Real returns depend on your cost basis, brokerage fees, and how long you held. Here's the full calculation.
How to Calculate ROI: Return on Investment Explained
ROI = (Net Profit ÷ Cost) × 100. How to use it for stocks, real estate, business, and any spending decision.
Salary to Hourly: How Much Is Your Annual Salary Per Hour?
Divide your annual salary by 2,080. A $60,000 salary = $28.85/hr. See the full chart and how taxes change your take-home.
What Is a Good BMI? BMI Chart by Age and Gender Explained
Standard BMI ranges, how age and gender affect interpretation, and the real limits of BMI as a health metric.
How Many Calories Should You Eat to Lose Weight?
A 500-calorie daily deficit produces ~1 lb of fat loss per week. Here's how to calculate your exact target from your TDEE.
What Is a Healthy Body Fat Percentage? (By Age and Gender)
For men, 10–20% is fit. For women, 18–28%. Full breakdown by category and how to measure yours accurately.
How to Calculate Your Macros for Weight Loss or Muscle Gain
Macros determine body composition more than calorie counting alone. Here's your ideal split for fat loss or muscle gain.
How Much Water Should You Drink Per Day?
The "8 glasses" rule is outdated. Your needs depend on body weight, activity, and climate. Here's the real formula.
TDEE vs BMR: What's the Difference and How to Use Them
BMR is calories at rest. TDEE adds activity. Here's how to calculate both and set smart calorie targets.
How Compound Interest Works with Monthly Contributions
$300/month at 8% from age 25 becomes $1,006,000 by 65. The math behind exponential wealth growth.
How Much Should You Have in Your Emergency Fund?
The 3–6 month rule explained — and how to calculate your exact target based on your real situation.
How to Calculate Percentage Change (Increase and Decrease)
% Change = ((New − Old) ÷ Old) × 100. Learn the formula, examples, and the common mistake that trips everyone up.
How to Calculate GPA: Semester and Cumulative GPA Explained
Step-by-step GPA calculation with the full grade point scale, cumulative GPA formula, and what grades you need to hit your target.
How Much to Tip: Tipping Guide for Every Service
18–20% for restaurants. But what about bars, hotels, delivery, and salons? The complete tipping guide.
How to Calculate Your Net Worth (Step-by-Step Formula)
Net worth = assets minus liabilities. What counts, what doesn't, and what a good net worth looks like by age.
How to Calculate Overtime Pay (FLSA Rules + Examples)
Overtime = 1.5× your regular rate for hours over 40/week. Exact formula, salaried worker rules, and state exceptions.
How to Calculate Square Footage of a Room or House
Square footage = length × width. How to measure every room shape and get accurate totals for flooring and paint.
Ohm's Law Explained: V = IR Formula, Examples, and Calculator
V = I × R. How to use all three forms of Ohm's Law with real-world examples and power calculations.
Dog Years to Human Years: The Real Conversion (Not ×7)
The 1:7 rule is a myth. Scientists use a logarithmic formula. A 2-year-old dog equals a ~42-year-old human.
What Is Standard Deviation? Plain-English Explanation with Examples
Standard deviation measures data spread from the average. Low SD = consistent. High SD = unpredictable. Step-by-step calculation.
How Much Protein Do You Need Per Day? (By Goal and Weight)
0.36 g/lb for sedentary. 1.0 g/lb for muscle gain. Calculate your exact daily target by weight and goal.
How to Calculate Your Electricity Cost (Formula + Examples)
Cost = (Watts ÷ 1,000) × Hours × kWh rate. A 1,500W space heater costs ~$57/month at national average rates.
How to Read a Mortgage Amortization Schedule
In year 1, 87% of each payment is interest. Here's how to read your amortization table and use it to save money.
How to Calculate Roof Pitch (Rise Over Run, Degrees, and Multiplier)
A 6:12 pitch means 6 inches of rise per 12 of run = 26.6°. How to measure, convert to degrees, and calculate roof area.
Speed vs. Velocity: Difference, Formulas, and Examples
Speed = distance ÷ time (scalar). Velocity = displacement ÷ time (vector with direction). Formulas and worked examples.
How to Calculate Sales Tax: Formula, Examples, and State Rates
Tax = Price × Rate. A $200 item at 9% = $18 tax. All 50 state rates plus how to reverse-calculate from a total.
How Much Should You Save for Retirement? (By Age Benchmarks)
By 30: 1× salary. By 40: 3×. By 67: 10×. The 4% rule, monthly savings by start age, and what to do if you're behind.
How to Calculate Probability: Basic Formulas and Examples
P(A) = favorable ÷ total outcomes. AND, OR, complement, and conditional probability explained with worked examples.
How to Calculate Miles Per Gallon (MPG): Formula and Guide
MPG = miles ÷ gallons. How to measure real-world fuel economy, annual cost by MPG, and how driving habits affect it.
How to Calculate BMI by Hand (Formula in Imperial and Metric)
BMI = (lbs × 703) ÷ in². A 5'10" person at 185 lbs has BMI 26.5. Complete formula, chart, and limitations.
What Is a Good Credit Score? FICO Score Ranges Explained
670+ is good. 740+ is very good — that's where you get the best rates. What each range costs you on a mortgage.
How to Calculate Car Depreciation (and Why It Costs You Thousands)
New cars lose 20% the moment you drive off the lot. By year 3, you've lost 40%. The formula and cars that depreciate slowest.
What Is Inflation? How It Erodes Your Money (With Examples)
At 3% inflation, $1,000 today is worth $740 in 10 years. How inflation is measured, its history, and how to protect your wealth.
What Is Compound Interest? How It Works (With Examples)
$10,000 at 8% for 30 years grows to $100,627 with compound interest — vs. $34,000 with simple interest. The formula, Rule of 72, and how to use it.