How To Quickly Calculate Percentages In The Head?

Easy Way to Calculate a 15% Tip

Rule of thumb — standard service — 15%. The most commonly used shortcut to 15% is to find 10% and then add a half. This is an easy calculation, since all you need to do to find 10% is move the decimal point one space to the left (make the number smaller).

Consider a bill for 47.31. First impressions show us 10% is 4.70 and a half of this amount is 2.35, so a tip of 7.00 is reasonable. This is a simplification as we can do the exact math — 4.70 add 2.35 is 7.05 — but we are looking for an easy method, not an exacting science. Another sound strategy is to work from the highest place value, in other words, if the bill is in the 50s then the tip should be in the 7.50 range. If the bill is 124.00, the logic follows that 12 add 6 =18 so a total of 124 add 18 or 142 is reasonable.


How to find the percentage of a number? – Other examples

While it’s certainly quick and painless just to use our percentage calculator, you don’t always have access to a computer or a smartphone. Also, it’s just plain cool to be able to perform calculations in your head. Maybe not as cool as juggling flaming torches, but close.

The percentage tells you how number A relates to number B. A real-world example could be: there are two girls in a group of five children. What’s the percentage of girls? In other words, we want to know what’s the ratio of girls to all children. It’s 2 out of 5, or 2/5. We call the first number (2) a numerator and the second number (5) a denominator because this is a fraction. To calculate the percentage, multiply this fraction by 100 and add a percent sign. 100 * numerator / denominator = percentage. In our example it’s 100 * 2/5 = 100 * 0.4 = 40. Forty percent of the group are girls. That’s the entire procedure of converting between decimal fractions and percentages. Speaking of decimal fractions, there is a way to write very big or very small numbers concisely. Check it out with our scientific notation calculator!

Do you have problems with simplifying fractions? The best way to solve this is by finding the GCF (Greatest Common Factor) of the numerator and denominator and divide both of them by GCF. You might find our GCF and LCM calculator to be convenient here. It searches all the factors of both numbers and then shows the greatest common one. As the name suggests, it also estimates the LCM which stands for the Least Common Multiple.

Let’s go the other way around and try to find the numerator. Say we know that 70 percent of fruits in the basket are apples, and there are 30 fruits altogether. It could be worse – they could be lemons. So how many apples do we have? Let’s get our percentage formula: 100 * numerator / denominator = percentage. We want to find out the numerator. Let’s move all the other parts of the equation to the other side. Divide both sides by 100 (to get rid of 100 on the left) and then multiply both sides by the denominator. This is what we get: numerator = percentage * denominator / 100. Let’s substitute percentage and denominator with our values: numerator = 70 * 30 / 100. Now it’s easy: numerator = 2100 / 100 = 21, we have 21 apples. Should be enough for lunch or a rather violent food fight.

Now let’s solve a problem with an unknown denominator. We spent 30 percent of our pocket money on bubble gum (we never said we’re great investors). We bought 12 sticks for $1 each. So we know that $12 was 30 percent of our total budget. How much money did we have before we almost literally blew it all away? Let’s start with our formula: 100 * numerator / denominator = percentage or 100 * 12 / denominator = 30. This time we want to find out the denominator. Let’s multiply both sides by the denominator and then divide by percentage. This way, we’ll get 100 * 12 / 30 = denominator. And the other way around, denominator = 100 * 12 / 30 = 1200 / 30 = 40. We had 40 dollars, and we spent 30 percent, or $12, on bubble gum. Totally worth it.

How to Do Percentages In Your Head

So how much is 15 percent of 210? You can use the four key percentages you memorized to figure this out. Consider that 15 percent is 5 percent added to 10 percent. Since 10 percent of 210 is 21, and 5 percent is half that, or 10.5, then 15 percent is 21 added to 10.5, or 31.5.

How about 75 percent of 440? Here you can figure that 75 percent is 50 percent added to 25 percent. Since 50 percent of 440 is half that, or 220, and 25 percent of 440 is one fourth, or 110, then 75 percent is 220 added to 110, or 330.

In this way you can combine 5 percent, 10 percent, 25 percent, and 50 percent to calculate a wide variety of percentages in your head. If you need to calculate a percentage that is not a multiple of 5, you can use this technique to estimate the answer very closely.

More Mental Math Videos

To learn more than just percentages, take a look at the other math videos from tecmath on YouTube. You'll need to know how to multiply when you calculate percentages, and this instructor's videos will help teach you shortcuts to make multiplication quicker. There's also a subtraction video. I like this instructor—his voice is kind, and his math methods are magical! You'll love the time-saving tips for working with numbers that you'll learn here.

© 2014 Susan Deppner

About Percentage Calculator:

An online percentage calculator by calculator-online helps you to calculate percentage and unknown percent values in equations. The tool uses simple formula for percentage to calculate percentages of the given equation. Also, this percent calculator allows you to add or subtract a percentage form a number or solve the equations.

How to Figure Out Percentages With This Percentage Calculator:

The calculator is 100% free and allows you to perform percentage calculation or what percent of (any number) within a fraction of seconds. You can easily and accurately determine % age, or more with the assistance of this calculator. Just stick to these steps to obtain the precise measurements corresponding to the percentages.


  • First of all, you have to choose the equation for which you want to calculate the percentage
  • Then, you have to enter the values into the designated fields corresponding to the selected equation
  • Once done, just hit the calculate button to actual percentage value


The percent calculator shows:

  • Percentage value according to the selected percentage equation
  • Show the step-by-step calculation for the equation

First, Review the Basics of Percentages

In our explanation, we'll be using some of the information found in the video below. It's just over 10 minutes long, but that will be time well spent reviewing or learning the basics of percentages, as well as practicing to become familiar and comfortable with using this method of mental math.


What is the percentage?

The percentage can be defined as the dimensionless ratio of two numbers. It can be used to compare two numbers and find out how much one is more than the other or compare the two numbers against a common scale.


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