How to Calculate Percentage Increase (Prices, Pay Rises, and Inflation)
Open any news app and you will see percentages everywhere. Inflation is 3.7%. Gas is up 34%. Your pay rise is 5%. Most people nod along, but if you ask them to work out what those numbers actually mean for their own wallet, they stall. That gap is expensive, because percentages are how prices, pay, and savings get described to you, and the people quoting them are not always careful.
This guide teaches the one formula behind all of it, then applies it to the real numbers making headlines right now: US inflation, gas prices, and the pay rise that looks like a win until you adjust it. You will also learn the two mistakes that trip up almost everyone, including the percentage-points trap that catches newsreaders every month.
The one formula you need
Percentage increase has a single formula, and it never changes no matter what you are measuring:
Percentage increase = (new value − old value) ÷ old value × 100
Work it in three steps every time:
- Subtract the old value from the new value. This is the change in plain money, points, or whatever unit you are using.
- Divide that change by the old value. This is the step people skip, and it is the whole point: the change only means something compared with where you started.
- Multiply by 100 to turn the decimal into a percentage.
Try it on the headline example. A grocery item cost $100 last year and costs $103.70 now. Step one: 103.70 − 100 = 3.70. Step two: 3.70 ÷ 100 = 0.037. Step three: 0.037 × 100 = 3.7%. That is exactly the US inflation rate reported for September 2026, and now you know where a number like that comes from instead of just repeating it.
If you want to skip the hand math, our free Percentage Calculator runs the same formula in seconds: enter the old value and the new value, and it gives you the percentage change.
Gasoline: the 34% number everyone felt
No price rise in 2026 was felt more directly than gasoline. US gas prices rose 9.1% in September alone, and 34.2% over the previous twelve months. Those two numbers describe the same price, just measured over different windows, which is why headlines can sound contradictory while both being true.
Take a gallon that cost $3.00 a year ago. A 34.2% increase means multiplying by 1.342: $3.00 × 1.342 = $4.026, which rounds to $4.03 at the pump. Work it backwards from the rounded price and you get (4.03 − 3.00) ÷ 3.00 = 34.3%, a tenth of a point higher, because the pump price is rounded. That tiny mismatch is normal. Official percentages are calculated from unrounded figures, so your back-of-envelope check will sometimes land a tenth out. It does not mean either number is wrong.
The monthly figure works the same way. A gallon at $3.70 at the start of September, up 9.1% in the month, becomes $3.70 × 1.091 = $4.04. One month, one multiplication, and suddenly the weekly fuel bill makes sense.
Your pay rise is not what it looks like
Here is where the formula turns personal. Say your employer gives you a 5% pay rise, and inflation is running at 3.7%. It feels like you are 1.3% better off. You are not quite. Subtracting percentages like that is one of the classic errors, and it overstates your gain.
The correct way is to divide, not subtract. A 5% rise multiplies your pay by 1.05. Inflation of 3.7% multiplies prices by 1.037. Your real, inflation-adjusted gain is (1.05 ÷ 1.037) − 1 = 0.0125, or about 1.25%. The gap between 1.3% and 1.25% looks small, but on a $60,000 salary it is the difference between feeling $780 better off and actually being about $750 better off in real terms, and the error grows as the numbers get bigger.
The same logic explains a grim headline from the same data. US real average hourly earnings fell 0.5% between August and September 2026. That means paychecks grew, but not fast enough to keep up with prices in that month, so what an hour of work actually buys went backwards. A pay rise below inflation is a pay cut in disguise, whatever the announcement says.
Percentage points vs percent: the trap
US inflation was 3.4% in August 2026 and 3.7% in September. News reports described this two ways, and both were correct: inflation rose 0.3 percentage points, and inflation rose about 8.8% in relative terms. If that sounds contradictory, it is because two different measurements share the word "percent."
Percentage points measure the plain difference between two percentages: 3.7 − 3.4 = 0.3 percentage points. Percent change measures that difference relative to the starting point: 0.3 ÷ 3.4 = 0.088, or about 8.8%. The first answers "how many points did it move?" The second answers "how big is that move compared with where it started?"
Why it matters: a rise from 3.4% to 3.7% sounds small in points and large in percent, and commentators pick whichever version suits their argument. When someone says "inflation jumped 8.8%," they mean the rate itself rose by that proportion, not that prices rose 8.8%. Prices rose 3.7% over the year. Keep the two ideas separate and a whole category of misleading headline loses its power over you.
Percentage decrease works the same way
The formula handles falls too: a negative answer just means a decrease. If that $4.03 gallon falls back to $3.00, the change is (3.00 − 4.03) ÷ 4.03 = −0.2556, or about −25.6%. Notice the asymmetry, which surprises people: a 34.2% rise followed by a return to the original price is only a 25.6% fall, because the fall is measured from the higher starting point. A percentage is always relative to where you start, never to where you wish you had started.
The same rule applies to sale prices, portfolio values, and bills. A $200 grocery bill that drops to $195 is (195 − 200) ÷ 200 = −2.5%. Small, honest, and easy to check.
Three mistakes almost everyone makes
Dividing by the new value. The old value always goes on the bottom. Divide by the new value and a $100-to-$130 rise comes out as 23% instead of 30%. If your answer looks too small, check the denominator first.
Adding percentages together. Two 10% rises do not make a 20% rise. The second rise applies to the already-raised price: $100 becomes $110, then $121, which is a 21% total rise. The same applies to stacked discounts and compound interest, and it is the reason mental shortcuts drift.
Confusing points with percent. As above, a move from 3.4% to 3.7% is 0.3 points, not 0.3%, and not 8.8% in the price sense. When a figure sounds dramatic, ask which of the three it is before reacting.
The rest of the inflation picture
Headline inflation is an average of many prices moving at different speeds, which is why your personal experience can differ from the official number. In September 2026, shelter costs were up 3.2% over the year, so a $1,000 monthly rent at the old rate would be about $1,032 at the new one. Food was up 2.5%, turning a $200 weekly shop into about $205. Core CPI, which strips out food and energy, was 2.6%, lower than the headline 3.7%, because energy was doing most of the damage. If your spending leans heavily on driving, inflation felt worse than 3.7% to you. If it leans on groceries, it felt milder. Both experiences are real; the headline is just the average.
One date to keep in mind: the next official US CPI report lands on November 10, 2026. When it arrives, you will know exactly how to read it, and you will be able to check any claim made about it with the same three-step formula.
Frequently asked questions
What is the formula for percentage increase?
(New value − old value) ÷ old value × 100. Always divide by the old value, the starting point you are measuring from.
How do I calculate a 3.7% increase on $100?
Multiply $100 by 1.037 to get $103.70. The 1 in front keeps your original amount; the 0.037 adds the increase.
What is the difference between percent and percentage points?
Percentage points are the plain difference between two percentages (3.7% − 3.4% = 0.3 points). Percent change is that difference relative to the starting figure (0.3 ÷ 3.4 = about 8.8%).
Why is a 5% pay rise not a 5% real gain?
Because prices rose too. Divide the pay factor by the price factor: 1.05 ÷ 1.037 = 1.0125, so the real gain is about 1.25%, not 5% and not 1.3%.
Can a percentage increase be more than 100%?
Yes. If a price doubles, from $50 to $100, the increase is (100 − 50) ÷ 50 = 100%. If it triples to $150, the increase is 200%. There is no ceiling.
How is percentage decrease different?
It uses the same formula and gives a negative answer. A fall from $4.03 to $3.00 is about −25.6%, measured from the higher starting price, which is why it is smaller than the 34.2% rise that got it there.
Percentages stop being intimidating once you see them as one division problem wearing different clothes. Next time a headline quotes a rise, a fall, or a pay deal, run the three steps on your own numbers before deciding what it means for you. It takes ten seconds, and it changes the answer more often than you would expect.
Run your own figures with the Percentage Calculator whenever a number in the news deserves a second look.
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