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Use the formula: M = P[r(1+r)^n]/[(1+r)^n-1], where P is the loan principal (home price minus down payment), r is the monthly interest rate (annual rate / 12 / 100), and n is the total number of monthly payments. For a $300,000 home with 20% down at 6.5% for 30 years, the monthly payment is approximately $1,517.

BMI = weight (kg) / height (m)². For example, if you weigh 70 kg and are 1.75 m tall: BMI = 70 / (1.75 × 1.75) = 22.9 (Normal). BMI categories: Underweight (<18.5), Normal (18.5–24.9), Overweight (25–29.9), Obese (30+). In imperial units: BMI = (weight in lbs × 703) / (height in inches)².

Three common percentage calculations: 1) X% of Y = Y × X/100. Example: 15% of 200 = 30. 2) What % is X of Y = (X/Y) × 100. Example: 30 out of 200 = 15%. 3) Percentage change = (New − Old) / |Old| × 100. Example: 80 to 100 = +25% increase.

Compound interest earns interest on both the principal and previously accumulated interest. Formula: A = P(1 + r/n)^(nt), where P = principal, r = annual rate, n = compounding frequency per year, t = years. For $10,000 at 7% compounded monthly for 10 years: A = $20,096 — you earned $10,096 in interest alone.

Multiply your bill by the tip percentage divided by 100. For a $50 bill with 18% tip: $50 × 0.18 = $9 tip, total = $59. Common tip rates: 15% (adequate service), 18% (good service), 20%+ (excellent service). To split between people, divide the total by the number of diners.

GPA = total grade points / total credits. Grade points: A=4.0, A-=3.7, B+=3.3, B=3.0, C=2.0, D=1.0, F=0. Multiply each course grade by its credits, sum all, then divide by total credits. Example: A in 3-credit course + B in 4-credit course = (4×3 + 3×4) / 7 = 3.43 GPA.

First calculate your BMR (Basal Metabolic Rate), then multiply by your activity level. Mifflin-St Jeor formula — Male: 10×weight(kg) + 6.25×height(cm) − 5×age + 5. Female: same but −161. Multiply by 1.2 (sedentary) to 1.9 (very active). To lose weight, eat 500 calories below maintenance for ~1 lb/week loss.

Multiply the item price by the tax rate divided by 100. For a $100 item with 8.25% sales tax: $100 × 0.0825 = $8.25 tax, total = $108.25. US sales tax varies by state: 0% in Oregon/Montana, up to 7.25% in California (plus local taxes that can push it over 10%).