How to Calculate Percentage: Formulas and Step-by-Step Examples

Anyone who has stared at a sale tag and tried to work out the real price knows the feeling: the percentage is everywhere, but the math can feel slippery. The good news is that every percentage problem — a 50% discount, a 75% exam score, a year-over-year growth column — runs on the same small set of formulas. The following walks through each one with worked examples, mental shortcuts, and Excel formulas, so the next time a number asks for a percentage, you can answer in seconds.

20% of 70: 14 · 10 → 15 increase: 50% · 50 vs 30 difference: 50% · 45 of 60 exam: 75%

Six formulas, one pattern: every percentage question is a division — or, in Excel, a formatting decision — followed by ×100.

The pattern: pick the formula by the shape of the question, not by the numbers in front of you.

What is a formula of percentage?

The basic percentage formula

• Percentage = (Part ÷ Whole) × 100 — GoSkills (Excel skills training) describes a percentage as a fraction of a whole multiplied by 100 (GoSkills (Excel skills training)).
• “Percent” means “per hundred,” so the ×100 multiplier is where the result gets its name.
• In Excel, the same formula is =part/total, with the cell formatted as a percentage.

The formula runs in two directions. To turn a part into a percentage, divide the part by the whole, then multiply by 100. To pull a percentage out of a number, reverse the move: write the percentage as a decimal (20% → 0.20) and multiply.

Converting a fraction to a percentage

• Divide the numerator by the denominator.
• Multiply the result by 100 to express it as a percent.
• In Excel, =13/17 formatted as a percentage displays as 76% (GoSkills (Excel skills training)).

This is the same math behind exam scores and recipes scaled for guests: 45 correct answers out of 60 questions is 45 ÷ 60 = 0.75 = 75%.

Example: calculating 20% of 70

20% of 70: 14

Write 20% as 0.20 and multiply: 0.20 × 70 = 14. The quick check: 10% of 70 is 7, so 20% is double that — 14.

The pattern: if you can convert the percentage to a decimal, the multiplication is the easy half — the real skill is knowing which number is the whole.

How do I calculate a percentage of a number?

The percentage-of-a-number method

• Convert the percentage to a decimal by moving the decimal point two places left: 20% → 0.20, 5% → 0.05.
• Multiply the decimal by the whole: 0.20 × 70 = 14.
• This is the same method Microsoft Support uses in its Excel example for 8.9% of 800: =800*0.089 (Microsoft Support (Excel documentation)).

Example: what is 20% of 70?

• Step 1: 20% = 0.20.
• Step 2: 0.20 × 70 = 14.
• Check: 10% of 70 is 7, so 20% is double that — 14.

The same numbers appear in the first section on purpose: formula, decimal multiplication, and mental math should all land on the same answer.

Finding 2% of a number

2% of 250: 5

Convert 2% to 0.02 and multiply: 0.02 × 250 = 5. The same decimal move works for single-digit percentages, which is handy for small rate changes and interest-like calculations.

Calculating what percent one number is of another

• Divide the part by the whole: 45 ÷ 60 = 0.75.
• Multiply by 100: 0.75 × 100 = 75%.
• So 45 correct answers out of 60 questions is a 75% score.

This is the formula running in its “share” direction — the version to use for marks, quotas, and any “what percent of X is Y?” question.

Calculating a percentage of a total (sales tax example)

• To add a percentage to a total, multiply the total by the decimal rate: $120 at 8% sales tax adds 0.08 × 120 = $9.60.
• To subtract a percentage, multiply by (1 − rate): $120 with a 30% discount is 120 × 0.70 = $84.

Tax and discount questions both reduce to one decision: is the percentage being added to the base or removed from it?

Calculating percentage of marks and money

• Marks: (marks obtained ÷ total marks) × 100. A student with 45 out of 60 is at 75%.
• Money: same formula with a currency unit. A $50 shirt at 20% off costs 50 × 0.80 = $40.
• Caution: mixing percentages from different-sized totals needs weights, not a simple average.

How to calculate overall percentage from multiple percentages

• If the totals are equal, average the percentages: (70% + 90%) ÷ 2 = 80%.
• If the totals differ, add the parts and divide by the sum of the totals: (30 + 50) ÷ (40 + 60) = 80%.
• The unweighted average would give (75% + 83.3%) ÷ 2 = 79.2% — close, but misleading when the totals behind the percentages are different sizes.

The catch: the formula never changes, but the base does. Name the whole before reaching for a calculator, and most percentage errors disappear.

How do I calculate the percentage difference between two numbers?

Percentage difference formula

Difference between 50 and 30: 50%

• Formula: |V1 − V2| ÷ ((V1 + V2) ÷ 2) × 100 — the same formula sourced from Ablebits in the quick-reference table above.
• It is symmetric: 30 vs 50 gives the same 50%, because the absolute value removes direction.
• Use it when neither value is the natural starting point — comparing two products, two teams, or two time periods without naming one as the base.

Percentage change (increase and decrease)

• Percentage change = (New Value − Old Value) ÷ Old Value × 100 (Corporate Finance Institute (finance education site)).
• Positive result = increase; negative result = decrease.
• Corporate Finance Institute also writes it as New Value ÷ Old Value − 1 — the same math in a shorter form.

Worked examples: 10 to 15, and 80 to 60

• 10 → 15: (15 − 10) ÷ 10 × 100 = 50% increase.
• 80 → 60: (60 − 80) ÷ 80 × 100 = −25% decrease.
• Difference check: 50 and 30 → (50 − 30) ÷ 40 × 100 = 50% difference.

Increase and difference are easy to confuse because both can produce “50%.” The tell is the base: change divides by the old value; difference divides by the average.

The trade-off: percentage change is directional and time-aware; percentage difference is neutral. Pick the one that matches the question you are actually answering.

What’s the easiest way to calculate percentages in my head?

Start from 10%

• 10% of any number: move the decimal one place left — 10% of $48 is $4.80.
• 20% = double 10%; 5% = half of 10%.
• 15% = 10% + half of 10%: $4.80 + $2.40 = $7.20.

Use fractions you already know

• 25% = 1/4, 50% = 1/2, 75% = 3/4 — because a percentage is a fraction of a whole (GoSkills (Excel skills training)).
• So 25% of $120 is $120 ÷ 4 = $30, and 75% is $90.
• 10% and its multiples handle most tipping, sale, and rounding situations without a calculator.

Example: 15% tip on $48

15% tip on $48: $7.20

Break the tip into 10% + 5%: $4.80 + $2.40. When the bill is uneven, round to the nearest dollar first — on a $47.80 meal, the same shortcut lands within a few cents of perfect.

Why this matters: the fastest mental math is not speed arithmetic; it is knowing which shortcut unlocks the answer, and 10% is the key to nearly all of them.

How to calculate percentage in Excel?

The two-step Excel pattern

• Step 1: enter a division formula, e.g., =B2/C2.
• Step 2: format the cell as a percentage. Excel shows 0.75 as 75% — the format handles the ×100 for display.
• As the snapshot notes, there is no =PERCENT() function; every percentage in Excel is ordinary division plus formatting (Spreadsheeto (Excel tutorial site)).

Percentage of a total in Excel

• Part ÷ total pattern: =part/total.
• Percentage of a number: =number × rate, e.g., 8.9% of 800 → =800*0.089.

Microsoft Support’s worked examples cover the common cases: =42/50 for correct answers, =800*0.089 for 8.9% of 800, =(2500-2342)/2342 for change, =B2*C2 for sales tax, =B2*(1−C2) to reduce a price by a percentage, and =15/0.75 to recover the original number when you know the percentage and the result (Microsoft Support (Excel documentation)).

Percentage change in Excel

• Put the old value in the denominator: =(B2−A2)/A2.
• Format the result as % and Excel displays the increase or decrease without extra work.
• This mirrors the (New − Old) ÷ Old formula from the percentage change section above.

Formatting cells as percentages

• Select the cells and press Ctrl+Shift+%, or choose Percentage from the Number group in the ribbon.
• The stored value remains a decimal (0.75); only the display changes to 75%.
• Use fewer decimal places for clean reports, or more when precision matters.

Common Excel mistakes

• Fix: keep the formula as =part/total and let the cell format add the % sign.
• Another trap: type 20 when you mean 20%, and Excel reads it as the number 20 — 20 × 50 = 1,000 instead of 10.
• A result above 100% means the part is bigger than the whole — valid for change and difference, but not for a share of a total.

The pattern: percentages in Excel are a formatting feature, not a math feature — keep the division raw and let the cell do the display work.

Step-by-step: a repeatable workflow for any percentage

The five-step process

1. Name the whole — the base value you are comparing against (original price, total marks, starting revenue).
2. Name the part — the amount you want to express as a percentage (the discount, the marks you got, the change in revenue).
3. Pick the matching formula: Part ÷ Whole × 100 for a share; (New − Old) ÷ Old × 100 for change; |A − B| ÷ Average × 100 for a symmetric difference.
4. Divide first, then multiply by 100 — or leave the division as a decimal and let Excel’s Percentage format finish the job.
5. Sanity-check with a mental shortcut: 10% of the number is the number divided by 10. If your answer looks implausible against that benchmark, re-check the base.

The pattern: if the answer feels wrong, the denominator — the base — is almost always where the error slipped in.

What’s confirmed — and what still trips people up

The implication: the math is stable; the confusion lives in choosing the right base and the right format.

What the experts say

Parliamentary researchers and data-analytics educators describe the same core maneuver: divide, then scale to 100.

In general, to calculate what e is as a percentage of f, we first divide e by f and then multiply by 100.

UK Parliament research briefing

You can calculate a percentage in Excel by entering =(B2/C2)*100.

upGrad (data analytics tutorials)

The implication: when a parliamentary research briefing and a data-analytics course describe the same division-then-multiply move, you can trust the pattern to travel across contexts.

The takeaway

Percentages look like a thousand different problems, but they all answer one question: what share of the base is this number? The fastest way to get comfortable is to stop memorizing tools and start naming the base first. For students calculating marks, shoppers decoding discounts, and analysts building Excel reports, the choice is clear: use Part ÷ Whole × 100 for shares, (New − Old) ÷ Old for change, and format the cell as % in Excel — or accept results that are quietly 100× too big.

Related reading

• How to Calculate CAGR in Excel: Formula & Examples
• How to Calculate Annual Value of Property | LPT Bands 2026 Guide

Frequently asked questions

If a percentage question still feels stuck, name the base first — that single habit resolves most of the confusion.