To find the average of two numbers, add the two values and divide their sum by two. The result is also the midpoint between the numbers on a number line. For example, the average of 10 and 20 is 15 because 10 plus 20 equals 30, and 30 divided by two equals 15.
Enter a list of numbers to calculate the mean, sum, count, median, mode, range, minimum, and maximum.
Adding the two endpoints and dividing by two finds their midpoint on a number line.
The Two-Number Average Formula
The average of two numbers is their sum divided by two.
Because there are exactly two observations, the denominator is always two.
The Average Calculator can calculate this result and display the complete statistical summary.
Worked Example: 10 and 20
Add the two values: 10 plus 20 equals 30.
Divide the sum by two: 30 divided by two equals 15.
The average of 10 and 20 is 15.
The Average Is the Midpoint
For two numbers, the arithmetic mean is also the midpoint between them.
The value 15 is five units greater than 10 and five units less than 20.
This equal distance is why the two-number average can be visualised at the centre of a number line.
A Faster Midpoint Method
When the two numbers are close, find the difference, divide that difference by two, and add it to the smaller number.
For 40 and 50, the difference is 10. Half of 10 is five, and 40 plus five equals 45.
This method gives the same result as adding the numbers and dividing by two.
Average of Two Decimal Numbers
Decimals use the same formula. Consider 2.5 and 7.5.
Their sum is 10. Dividing 10 by two gives five.
The average lies exactly halfway between the two decimals.
Average of a Positive and Negative Number
Suppose the values are minus 10 and 6. Their sum is minus four.
Dividing minus four by two gives minus two.
On a number line, minus two is equally distant from minus 10 and 6.
Average of Two Negative Numbers
Negative values can be averaged without changing the method.
For minus eight and minus four, the sum is minus 12. Dividing by two gives minus six.
The average minus six lies halfway between the two values.
When the Two Numbers Are Equal
When both values are the same, their average is that same value.
For 12 and 12, the sum is 24 and 24 divided by two is 12.
The midpoint does not move because both endpoints are identical.
Finding One Number from the Average
When the average and one number are known, multiply the average by two to find the required total.
Subtract the known number from that total to find the missing value.
For an average of 20 and a known number of 15, the total must be 40, so the missing number is 25.
Average vs Difference
The average identifies the midpoint, while the difference measures the distance between the two numbers.
For 10 and 20, the average is 15 and the difference is 10.
These results answer different questions and should not be confused.
Common Mistakes
A common mistake is dividing each number by two and then forgetting to add the results. Although that can work, adding first is clearer.
Another mistake is dividing by the difference between the numbers instead of dividing their sum by two.
Parentheses should be used when entering the formula so the addition occurs before the division.
Conclusion
To find the average of two numbers, add them and divide the result by two.
The answer is also the midpoint between the two values.
Use the Average Calculator for two numbers or a larger list.
FAQs
How do I find the average of two numbers?
Add the two values and divide their sum by two.
Is the average always halfway between two numbers?
Yes. For two values, the arithmetic mean is their midpoint.
Can the average be a decimal?
Yes. The sum may not divide evenly by two.
How do I find a missing number from a known average?
Multiply the average by two and subtract the known number.
What is the average of two equal numbers?
It is the same value as both original numbers.