You're at checkout. The sign says "30% off." The original price is $47. The cashier asks if you want it. You stand there, trying to do the math while the line grows behind you. Is it a good deal? Should you get two? Your brain goes blank. You say yes just to end the awkwardness—and never know if you got a good deal or got played.
Simple math shouldn't freeze your brain in public. The formulas aren't hard—you just need them at the right moment.
Some calculations come up over and over—percentages at stores, tips at restaurants, unit conversions for recipes. We created this reference because we were constantly looking up the same formulas. Bookmark this page for quick access, or use our tools for instant answers.
Percentage Calculations
Finding X% of a Number
Formula: Number × (Percentage ÷ 100)
Examples:
| Find | Calculation | Result |
|---|---|---|
| 15% of $80 | 80 × 0.15 | $12 |
| 20% of $45 | 45 × 0.20 | $9 |
| 7.5% of $120 | 120 × 0.075 | $9 |
| 33% of $60 | 60 × 0.33 | $19.80 |
Quick trick for 10%: Move the decimal one place left.
- 10% of $85 = $8.50
- 10% of $127 = $12.70
From 10%, derive others:
- 5% = half of 10%
- 15% = 10% + 5%
- 20% = 10% × 2
Use our Percentage Calculator for instant results.
Finding What Percent X Is of Y
Formula: (X ÷ Y) × 100
Examples:
| Question | Calculation | Result |
|---|---|---|
| 25 is what % of 100? | (25 ÷ 100) × 100 | 25% |
| 15 is what % of 60? | (15 ÷ 60) × 100 | 25% |
| 7 is what % of 28? | (7 ÷ 28) × 100 | 25% |
| 42 is what % of 200? | (42 ÷ 200) × 100 | 21% |
Percentage Change
Formula: ((New - Old) ÷ Old) × 100
Examples:
| Scenario | Calculation | Result |
|---|---|---|
| Price $50 → $60 | ((60-50) ÷ 50) × 100 | +20% |
| Price $80 → $64 | ((64-80) ÷ 80) × 100 | -20% |
| Salary $45K → $50K | ((50-45) ÷ 45) × 100 | +11.1% |
Discount Calculations
Sale price: Original × (1 - Discount%)
| Original | Discount | Calculation | Sale Price |
|---|---|---|---|
| $100 | 20% off | 100 × 0.80 | $80 |
| $75 | 15% off | 75 × 0.85 | $63.75 |
| $250 | 30% off | 250 × 0.70 | $175 |
Stacked discounts: Multiply, don't add.
- 20% off then 10% off ≠ 30% off
- 20% off then 10% off = $100 × 0.80 × 0.90 = $72 (28% total)
Tip Calculations
Use our Tip Calculator or these quick methods:
Standard Tips
| Tip Level | How to Calculate |
|---|---|
| 10% | Move decimal left |
| 15% | 10% + half of 10% |
| 18% | 20% - 10% of that |
| 20% | Move decimal, double |
| 25% | 20% + 5% (or ÷ 4) |
Quick Examples
$45 bill:
- 10% = $4.50
- 15% = $4.50 + $2.25 = $6.75
- 20% = $4.50 × 2 = $9.00
$78 bill:
- 10% = $7.80
- 15% = $7.80 + $3.90 = $11.70
- 20% = $7.80 × 2 = $15.60
Bill Splitting
Formula: (Bill + Tip) ÷ Number of people
Example: $120 bill, 20% tip, 4 people
- Tip: $24
- Total: $144
- Per person: $36
Unit Conversions
Use our Unit Converter or these common conversions:
Length
| From | To | Multiply by |
|---|---|---|
| Inches | Centimeters | 2.54 |
| Feet | Meters | 0.3048 |
| Miles | Kilometers | 1.609 |
| Centimeters | Inches | 0.394 |
| Meters | Feet | 3.281 |
| Kilometers | Miles | 0.621 |
Common references:
- 1 inch ≈ 2.5 cm
- 1 foot ≈ 30 cm
- 1 meter ≈ 3.3 feet
- 5 miles ≈ 8 km
Weight
| From | To | Multiply by |
|---|---|---|
| Pounds | Kilograms | 0.454 |
| Ounces | Grams | 28.35 |
| Kilograms | Pounds | 2.205 |
| Grams | Ounces | 0.035 |
Common references:
- 1 kg ≈ 2.2 lbs
- 1 lb ≈ 450 g
- 1 oz ≈ 28 g
Temperature
Celsius to Fahrenheit: (°C × 9/5) + 32
Fahrenheit to Celsius: (°F - 32) × 5/9
| Celsius | Fahrenheit |
|---|---|
| 0°C | 32°F (freezing) |
| 10°C | 50°F |
| 20°C | 68°F (room temp) |
| 30°C | 86°F |
| 37°C | 98.6°F (body temp) |
| 100°C | 212°F (boiling) |
Quick estimate: Double Celsius, add 30.
- 20°C → 40 + 30 = 70°F (actual: 68°F)
Volume (Cooking)
| Measure | Milliliters |
|---|---|
| 1 teaspoon | 5 ml |
| 1 tablespoon | 15 ml |
| 1 cup (US) | 237 ml |
| 1 pint (US) | 473 ml |
| 1 quart (US) | 946 ml |
| 1 gallon (US) | 3,785 ml |
Cup conversions:
- 1 cup = 16 tablespoons
- 1 cup = 48 teaspoons
- 1 cup ≈ 8 fluid ounces
Time Calculations
Time Zone Conversions
From UTC:
| Time Zone | Offset |
|---|---|
| EST/EDT | -5/-4 |
| CST/CDT | -6/-5 |
| MST/MDT | -7/-6 |
| PST/PDT | -8/-7 |
| CET/CEST | +1/+2 |
| JST | +9 |
Hours and Minutes
To decimal hours: Minutes ÷ 60
- 30 minutes = 0.5 hours
- 45 minutes = 0.75 hours
- 15 minutes = 0.25 hours
From decimal hours: Decimal × 60
- 1.5 hours = 1 hour 30 minutes
- 2.25 hours = 2 hours 15 minutes
Duration Calculations
Between times: End - Start (accounting for AM/PM)
- 9:30 AM to 2:15 PM = 4 hours 45 minutes
- Convert to same day if crossing midnight
Health Calculations
BMI (Body Mass Index)
Use our BMI Calculator or:
Metric: Weight (kg) ÷ Height (m)²
Imperial: (Weight (lbs) × 703) ÷ Height (in)²
| BMI | Category |
|---|---|
| Under 18.5 | Underweight |
| 18.5-24.9 | Normal |
| 25-29.9 | Overweight |
| 30+ | Obese |
Heart Rate Zones
Maximum heart rate: 220 - Age
| Zone | % of Max | Purpose |
|---|---|---|
| 1 | 50-60% | Recovery |
| 2 | 60-70% | Fat burning |
| 3 | 70-80% | Aerobic |
| 4 | 80-90% | Anaerobic |
| 5 | 90-100% | Maximum |
Example (age 30):
- Max HR: 190
- Zone 2: 114-133 bpm
- Zone 3: 133-152 bpm
Financial Quick Math
Hourly to Annual
Formula: Hourly rate × 2,080 (40 hrs × 52 weeks)
Quick estimate: Hourly × 2,000
| Hourly | Annual |
|---|---|
| $15 | $31,200 |
| $20 | $41,600 |
| $25 | $52,000 |
| $30 | $62,400 |
| $50 | $104,000 |
Monthly to Annual
Formula: Monthly × 12
Rule of 72 (Doubling Time)
Years to double: 72 ÷ Interest Rate
| Rate | Years to Double |
|---|---|
| 4% | 18 years |
| 6% | 12 years |
| 8% | 9 years |
| 10% | 7.2 years |
Cooking Math
Scaling Recipes
Formula: New servings ÷ Original servings = Multiplier
Example: Recipe for 4, making for 6
- Multiplier: 6 ÷ 4 = 1.5
- 2 cups flour × 1.5 = 3 cups
Common Fractions
| Fraction | Decimal | Percent |
|---|---|---|
| 1/8 | 0.125 | 12.5% |
| 1/4 | 0.25 | 25% |
| 1/3 | 0.333 | 33.3% |
| 1/2 | 0.5 | 50% |
| 2/3 | 0.667 | 66.7% |
| 3/4 | 0.75 | 75% |
Measurement Substitutions
Butter:
- 1 stick = 1/2 cup = 8 tablespoons = 113g
Flour:
- 1 cup ≈ 125g (spooned) or 140g (scooped)
Quick Mental Math Tricks
Multiplying by 5
Divide by 2, then multiply by 10.
- 48 × 5 = 24 × 10 = 240
Multiplying by 9
Multiply by 10, subtract the original.
- 7 × 9 = 70 - 7 = 63
Multiplying by 11
Multiply by 10, add the original.
- 6 × 11 = 60 + 6 = 66
Squaring Numbers Ending in 5
Multiply first digit by (first digit + 1), append 25.
- 35² = 3 × 4 = 12, append 25 = 1,225
- 75² = 7 × 8 = 56, append 25 = 5,625
Conclusion
Math anxiety isn't about being bad at math. It's about not having the formula when you need it.
The calculations in this guide aren't hard—they're just easy to forget under pressure. Bookmark this page, or better yet, use our tools when you need instant answers:
- Percentage Calculator - All percentage math
- Tip Calculator - Tips and bill splitting
- Unit Converter - Any unit conversion
- BMI Calculator - Health calculations
The best calculator is the one you actually use. The second best is the formula you can look up in three seconds.
Keep Reading
- Personal Finance Calculations Guide - Money math that actually matters
- Travel Planning Calculations Guide - All the math you need for trips
- Tip Calculator Tipping Guide - Never freeze at the restaurant again