The above image shows how big a sphere would be compared to Earth if it were made of all the Earth's water. It would have a diameter of about 860 miles and a volume around 332,500,000 cubic miles. The calculations are based on estimations for all of the Earth's water including not just oceans, lakes, rivers, seas, and ice caps, but groundwater, atmospheric water, and even the water inside living creatures such as you, your cat, and all of the world's plants.
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Image: Illustration by Jack Cook, Woods Hole Oceanographic Institution; USGS. Via Peter Edmonds.