Find the total number of respondents. For example, suppose the results of your survey about income are:
Under $20,000 - 5 people
$20,000 to $39,999 - 10 people
$40,000 to $59,999 - 15 people
$60,000 to $99,999 - 11 people
$100,000 or more - 9 people
The total number of people is 5 + 10 + 15 + 11 + 9 = 50.
Divide that number in half. In the example, 50/2 = 25. If the number in Step 1 is odd, you can round it down (it will make little difference).
Add the number of people in each category until you exceed the number in Step 2. In the example, 5 people are in the lowest category, 5 + 10 = 15 people are in one of the lowest two categories and 5 + 10 + 15 = 30 people are in one of the lowest three categories. Thirty is more than 25, so stop here.
Subtract the number in Step 2 from the final sum in Step 3. In the example, 30-25 = 5.
Divide the number in Step 4 by the number of people in the highest category included in Step 3. In the example, 5/15 = 0.333.
Subtract the lower limit from the upper limit in the highest category in Step 3. In the example, $59,999 - $40,000 = $19,999.
Multiply the result in Step 5 by the result in Step 6. In the example, 0.333*$19,999 = $6,659.67.
Add the result in Step 7 to the lower limit of the highest category in Step 3. This is the midpoint. In the example, $40,000 + $6,659.67 = $46,659.67.