How to calculate percentages (with the three formulas)

The three formulas that cover everything

Every percentage problem in daily life is one of three shapes:

Percent of a number — multiply, then divide by 100:

X% of Y = Y × X ÷ 100

12% of 2,500 is 2,500 × 12 ÷ 100 = 300.

Percentage change — the difference divided by the old value:

Change % = (New − Old) ÷ Old × 100

Going from 800 to 1,000 is (1,000 − 800) ÷ 800 × 100 = a 25% increase.

What percent A is of B — divide A by B, multiply by 100:

A as % of B = A ÷ B × 100

Scoring 45 out of 60 is 45 ÷ 60 × 100 = 75%.

Memorize the shape of all three and you will never be lost again — most percentage mistakes come from grabbing the wrong shape, not from bad arithmetic.

The discount trap nobody warns you about

Here is the mistake that costs real money. Percentage decreases do not cancel percentage increases.

A product marked up 25% then discounted 25% is not back to the original price. Take 800, add 25% to get 1,000, then remove 25% — you land on 750, not 800. The second percentage is calculated on the new, larger number.

The same trap works in reverse when a business thinks a 25% margin and a 25% markup are the same thing. They are not. Markup is a percentage of cost; margin is a percentage of the selling price, and the two diverge the moment you do the math. If you price products or services, the profit margin calculator shows both numbers side by side so you never quote one when you mean the other.

Run the numbers before you believe them

Percentage claims are everywhere — "50% off", "interest up 20%", "returns grew 30%". The formulas above take ten seconds to verify, and the percentage calculator does all three shapes instantly. It also shows the increased and decreased totals, so you can check a discount in both directions without doing the reverse math yourself.

Percentages are the language of money

Percentages are how we talk about prices, margins, interest, and growth. The same number can mean very different things depending on which shape you use — which is exactly why the calculator matters. Run the numbers before you set a price, accept a discount, or believe a claim. The three formulas above are all you need to stay honest.