“What is X% of Y?”
Y × (X ÷ 100)
15% of $200 → $30
Everyday math, made legible
Find the percent. Keep the clarity.
Calculate proportions, compare changes, and check discounts without wrestling with formulas. Every answer includes the math behind it.
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Percentage desk
Live calculation
Know a percent and a number, and want the result? You're in the right place.
%
#
Answer
44.55
18 ÷ 100 × 247.5 = 44.55
Try a common percentage
FIRST TIME HERE?
Match your question to a tab.
Five tabs on the calculator above cover most percentage questions. Find the one that matches what you're actually trying to figure out.
“X is what percent of Y?”
(X ÷ Y) × 100
50 out of 200 → 25%
“A value moved from old to new — what's the % change?”
((New − Old) ÷ Old) × 100
$40 → $50 → 25% increase
“How far apart are two values, with neither one 'first'?”
|A − B| ÷ ((A + B) ÷ 2) × 100
100 vs 150 → 40% apart
“What's the new value if I raise or lower a number by X%?”
Old × (1 + X ÷ 100)
$80 up 10% → $88
Change vs. Difference — the easy mix-up
Change has a direction: an old value, then a new one. 100 → 150 is a 50% increase, but 150 → 100 is a 33.3% decrease — same two numbers, different answer.
Difference treats both values equally by dividing by their average. 100 vs 150 is a 40% difference no matter which one you type first.
GRADES & BLENDED SCORES
When each part counts differently.
A plain average treats every number equally, which is wrong for a course grade, a GPA across classes with different credit hours, or a blended interest rate. A weighted average multiplies each score by its own weight first, so the parts that matter more actually count for more.
Weighted average
e.g. a 20% quiz, a 30% midterm, a 50% final
Add one row per part. Weight is how much that part counts, score is what it earned — the weights don't need to add up to 100.
Weight
Score
%
%
%
Weighted average
Add at least two rows
Σ(score × weight) ÷ Σ(weight)
Weights add up to —
01 / THE METHOD
Four formulas cover most percentage questions.
A percentage is simply a fraction measured out of 100. Pick the question that matches what you know, then substitute your numbers.
Find a percentage of a number
Convert the percentage to a decimal, then multiply it by the number.
(percentage ÷ 100) × number
Find what percentage one number is of another
Divide the part by the whole, then multiply the result by 100.
(part ÷ whole) × 100
Measure percentage increase or decrease
Find the difference, divide it by the original value, then multiply by 100.
((new − original) ÷ original) × 100
Combine several scores into a weighted average
Multiply each score by its own weight, add those together, then divide by the total weight.
Σ(score × weight) ÷ Σ(weight)
Find the percentage difference between two numbers
Divide the gap between two values by their average, then multiply by 100. Unlike percentage change, neither value is treated as the "original."
|A − B| ÷ ((A + B) ÷ 2) × 100
Increase or decrease a value by a percentage
Multiply the value by 1 plus the percent change, written as a decimal. Use a negative percentage to decrease instead of increase.
value × (1 + percent ÷ 100)
02 / IN PRACTICE
Percentages in the wild.
The same small set of formulas handles shopping, restaurant bills, school results, business reports, and everyday comparisons.
A jacket costs $86.40 after a 20% discount.
The discount is $21.60 because 20% of $108 is $21.60. Subtract that amount from the original price.
18% of a $47.35 bill is $8.52.
Multiply 47.35 by 0.18, then round the result to the nearest cent.
Percentage change
Monthly sign-ups rose from 184 to 231.
Subtract the old value from the new value, divide by the old value, and convert the decimal to a percentage.
(231 − 184) ÷ 184 × 100
25.54%
increase
A 78 quiz, an 85 midterm, and a 91 final average to 87.1.
With weights of 20%, 30%, and 50%, the final exam pulls the result up because it counts for the most.