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Quick Calculation Answers

Common calculation formulas, examples, and quick answers. Click any question to use our free calculator.

How do I calculate a percentage?

To find X% of a number, multiply the number by X and divide by 100. To find what percent A is of B, divide A by B and multiply by 100.

Formula: Percentage = (Part / Whole) × 100
Example: What is 20% of 150? Answer: 150 × 0.20 = 30
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How do I calculate tip?

Multiply the bill amount by the tip percentage (as a decimal). For 20% tip on $50: $50 × 0.20 = $10 tip.

Formula: Tip = Bill Amount × (Tip Percentage / 100)
Example: 15% tip on $85.50: $85.50 × 0.15 = $12.83
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How do I calculate BMI?

BMI equals weight in kilograms divided by height in meters squared. For imperial: multiply weight (lbs) by 703, then divide by height (inches) squared.

Formula: BMI = weight(kg) / height(m)²
Example: 180 lbs, 5'10" (70 in): (180 × 703) / (70)² = 25.8 BMI
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How do I calculate mortgage payment?

Monthly payment depends on loan amount, interest rate, and term. The formula uses compound interest to determine fixed monthly payments.

Formula: M = P × [r(1+r)ⁿ] / [(1+r)ⁿ-1]
Example: $300,000 loan at 6.5% for 30 years: ~$1,896/month
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How much concrete do I need?

Calculate volume in cubic feet (L × W × D), then divide by 27 to get cubic yards. Add 10% for waste.

Formula: Cubic Yards = (Length × Width × Depth in ft) / 27
Example: 10ft × 10ft slab, 4in deep: (10 × 10 × 0.33) / 27 = 1.22 cubic yards
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How do I calculate calories burned?

BMR (Basal Metabolic Rate) estimates calories burned at rest. Multiply by activity factor for TDEE (total daily expenditure).

Formula: Men BMR = 10×weight(kg) + 6.25×height(cm) - 5×age + 5
Example: 30-year-old man, 180 lbs, 5'10": BMR ≈ 1,800 calories/day
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How do I add fractions?

Find a common denominator, convert both fractions, then add the numerators. Simplify the result if possible.

Formula: a/b + c/d = (ad + bc) / bd
Example: 1/4 + 1/3 = 3/12 + 4/12 = 7/12
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How do I calculate compound interest?

Compound interest grows exponentially because interest earns interest. Use the compound interest formula with principal, rate, time, and compounding frequency.

Formula: A = P(1 + r/n)^(nt)
Example: $10,000 at 7% for 20 years (annual): $10,000 × 1.07²⁰ = $38,697
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How do I calculate sleep cycles?

Each sleep cycle is approximately 90 minutes. Count backward from wake time in 90-minute increments, adding 15 minutes to fall asleep.

Formula: Bedtime = Wake Time - (90 minutes × cycles) - 15 minutes
Example: Wake at 7:00 AM with 5 cycles: 7:00 - 7:30 - 0:15 = 11:15 PM bedtime
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How do I calculate body fat percentage?

The US Navy method uses circumference measurements (neck, waist, and hip for women) along with height to estimate body fat.

Formula: Men: 86.010×log10(waist-neck) - 70.041×log10(height) + 36.76
Example: Man with 34" waist, 15" neck, 70" height: approximately 18% body fat
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How do I calculate macros?

Calculate TDEE first, then distribute calories among protein (4 cal/g), carbs (4 cal/g), and fat (9 cal/g) based on your goal.

Formula: Protein calories = grams × 4, Carb calories = grams × 4, Fat calories = grams × 9
Example: 2000 cal diet (30/40/30): 150g protein, 200g carbs, 67g fat
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How do I calculate pregnancy due date?

Add 280 days (40 weeks) to the first day of your last menstrual period. This uses Naegele's Rule.

Formula: Due Date = LMP + 280 days
Example: LMP January 1: Due date = October 8
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