Percentage Calculator
Use this quick calculator for the three most common percentage questions.
Tip: You can enter decimals and negative numbers as needed.
Percentages are everywhere: discounts, taxes, exam scores, financial growth, and even battery health on your phone. If you have ever asked, “how do we calculate the percentage?”, the answer is simpler than it seems once you know the basic formulas.
The core percentage formula
At its heart, percentage means “per hundred.” So 25% literally means 25 out of 100.
- Percentage = (Part ÷ Whole) × 100
- Part = (Percentage ÷ 100) × Whole
- Whole = Part ÷ (Percentage ÷ 100)
Which one you use depends on what you are trying to find.
Method 1: How to find X% of a number
This is useful for tips, discounts, commissions, and taxes.
Formula
X% of Y = (X ÷ 100) × Y
Example
Find 15% of 240:
- 15 ÷ 100 = 0.15
- 0.15 × 240 = 36
So, 15% of 240 is 36.
Method 2: How to find what percent one number is of another
This is the one most people use for grades, completion rates, and comparisons.
Formula
Percentage = (Part ÷ Whole) × 100
Example
You scored 42 out of 50 on a quiz:
- 42 ÷ 50 = 0.84
- 0.84 × 100 = 84
Your score is 84%.
Method 3: How to calculate percentage increase or decrease
Use this for salary changes, price movements, business growth, and analytics.
Formula
Percentage change = ((New − Old) ÷ Old) × 100
Example
A product price went from 80 to 100:
- Change = 100 − 80 = 20
- 20 ÷ 80 = 0.25
- 0.25 × 100 = 25
That is a 25% increase.
Increase vs decrease
If your result is positive, it is an increase. If it is negative, it is a decrease. For example, a result of -12% means a 12% drop.
Common percentage mistakes to avoid
- Forgetting to divide by 100 before multiplying.
- Using the wrong denominator (especially in percentage change).
- Mixing up part and whole in the formula.
- Assuming percentage points and percent are the same (they are not).
Real-world percentage examples
Shopping discount
A $120 item is 30% off:
- Discount = 30% of 120 = 36
- Final price = 120 − 36 = 84
Restaurant tip
18% tip on a $55 bill:
- 0.18 × 55 = 9.9
- Tip = $9.90
Exam performance
72 correct out of 90:
- 72 ÷ 90 × 100 = 80%
Quick mental math percentage tips
- 10% is easy: move the decimal one place left.
- 5% is half of 10%.
- 1% is one-tenth of 10%.
- 20% is double 10%.
- 15% is 10% + 5%.
Final takeaway
If you remember one thing, remember this: percentage is just a ratio compared to 100. Start with the right formula, identify your part and whole clearly, and the rest is straightforward arithmetic. Use the calculator above any time you need fast, accurate answers.