Percentage Calculator for Everyday Money
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Which Tab Do You Need?
Most percentage questions in daily life are one of three things. The tabs match them.
- Find Percentage answers "how much is this cut worth?" Use it for a sale tag, a tip, a deposit, a commission.
- What Percent answers "how big a share is this?" Use it when you know two amounts and want the ratio, like how much of a paycheck went to rent.
- Percentage Change answers "how much did this move?" Use it for a rent increase, a raise, a price that crept up since last year.
Percent in Plain Terms
A percent is just a count out of a hundred. When a sign says 35% off, it means 35 dollars come off every hundred you were going to spend.
That single idea makes the mental shortcuts obvious. Ten percent is the number with the decimal point shoved one place left. Five percent is half of that. Twenty percent is ten percent doubled. Most tipping and most sale math can be done in your head with just those three, and the calculator is for when the numbers get untidy.
The Formulas Behind Each Tab
| The question | Formula | A real one |
|---|---|---|
| How much is the discount? | (P / 100) x N | 35% of $80 = 0.35 x 80 = $28 |
| What do I actually pay? | N x (1 - P/100) | $80 x 0.65 = $52 |
| What share is this? | (Part / Whole) x 100 | 340 / 2000 x 100 = 17% |
| How much did it go up? | (New - Old) / Old x 100 | (1240 - 1150) / 1150 x 100 = 7.83% |
| Add a percentage on top | N x (1 + P/100) | $1,200 x 1.07 = $1,284 |
Three Situations, Worked Through
A jacket marked $80, 35% off
The discount is 0.35 x 80 = $28, so you pay 80 - 28 = $52. Faster route: you are paying 65% of the tag, and 80 x 0.65 = $52.
An $86.40 dinner bill, 18% tip
0.18 x 86.40 = $15.55. The total comes to $101.95. If you are splitting it four ways, that is $25.49 each.
Rent going from $1,150 to $1,240
The increase is $90. Divide by the old rent, not the new one: 90 / 1150 = 0.0783, so 7.83%. Over a year that is $1,080 more.
Where People Slip Up
- A 20% drop then a 20% rise does not get you back. $100 falls to $80, then 20% of $80 is only $16, so you land at $96. The second percentage is taken from a smaller number.
- Percentage points are not percent. A savings rate moving from 4% to 5% is up one percentage point, but that is a 25% increase in what you earn. Adverts lean on this difference.
- Dividing by the wrong number. For any change, the bottom of the fraction is always the original value. Using the new one quietly shrinks the answer.
- Stacking discounts. An extra 10% off an already 30% off price is not 40% off. It is 0.70 x 0.90 = 0.63, so 37% off.