Answer:
0.00012 of an inch or double that if they are completely under.
Explanation:
Currently, there are almost 8 billion people on Earth. Human beings come in all sizes, from tiny babies to large adults. Let’s assume the average size is 5 feet tall – a bit bigger than a child – with an average volume of 10 cubic feet. Only half of each person’s body would be submerged when they sit down, so only 5 cubic feet adds to the water level. With 8 billion people total, you can calculate 5 x 8 billion which gives a whopping 40 billion cubic feet that would be added to the oceans.
But remember, this volume would be spread over the vast area of the oceans. Using the same bathtub math as before, we divide the 40 billion cubic feet of volume over the 140 million square miles of ocean.
The answer? The total rise in sea level would be about 0.00012 of an inch, or less than 1/1000th of an inch. If everyone completely submerged themselves, this would double the answer to 0.00024 inches, which is still only about the width of a human hair.