Three ways to calculate percentages — pick the one you need.
Divide the part by the whole, then multiply by 100. For example, 30 is what percent of 150? 30 ÷ 150 = 0.2, then 0.2 × 100 = 20%. So 30 is 20% of 150.
Multiply the number by the percentage divided by 100. For example, 20% of 150 = 150 × (20 ÷ 100) = 150 × 0.2 = 30.
Subtract the original value from the new value, divide by the original value, then multiply by 100. For example, from 80 to 100: (100 − 80) ÷ 80 × 100 = 25% increase.
If the new value is higher than the original, it's a percentage increase. If it's lower, it's a percentage decrease. The formula is the same — a negative result means a decrease.