A water molecule is the smallest unit of water: one oxygen atom joined to two hydrogen atoms, written H₂O. It is about 0.28 nanometers (280 picometers) across, measured as the room one molecule takes up among its neighbors. A nanometer is a billionth of a meter, and a picometer is a thousandth of a nanometer.

Size

A molecule has no hard outer surface. Its electrons form a cloud that thins out gradually, so its size depends on how it is measured.

The centers of its three atoms sit close together. Each hydrogen atom is about 0.096 nanometers from the oxygen atom, and the two bonds meet at an angle of about 104.5 degrees. That angle gives the molecule a bent shape, with the two hydrogen atoms about 0.15 nanometers apart.

The electron cloud reaches beyond the centers of the atoms, so each molecule takes up more room than that. Scientists often estimate its size from how closely water molecules pack together. In liquid water, X-ray measurements put the centers of neighboring molecules 0.280 nanometers apart. A textbook estimate, based on how many molecules fit in a liter of water, gives roughly 0.3 nanometers.

Size comparisons

About 290,000–360,000 water molecules side by side would span a human hair, which is 0.08–0.1 millimeters wide.
If a water molecule were enlarged to the size of a tennis ball, a human hair 0.1 millimeters wide would be about 24 kilometers (15 miles) wide.
A cesium atom, one of the largest atoms, is about 0.5 nanometers across, not quite twice the width of a water molecule.

Why water molecules cling together

Oxygen pulls on shared electrons more strongly than hydrogen does. So the electrons in a water molecule spend more time near the oxygen atom. That gives the oxygen end a slight negative charge and the hydrogen ends a slight positive charge. A molecule with charges separated like this is called polar.

Opposite charges attract, so each water molecule pulls on its neighbors. These attractions are called hydrogen bonds. They hold water molecules together at the surface of a liquid, which gives water its surface tension. That is why water forms droplets on a dry surface instead of spreading flat.

How water gets into cells

Water crosses the membranes around many cells quickly through protein channels called aquaporins. Peter Agre and his colleagues identified the first one in the membranes of human red blood cells, and each red blood cell carries about 200,000 of them. Agre shared the 2003 Nobel Prize in Chemistry for the discovery of water channels.

At its narrowest, the channel is 0.28 nanometers wide, about the size of a single water molecule. Water molecules therefore pass through in single file, about 3 billion of them every second through each channel. The narrow opening and an electric charge inside it turn back protons, the charged particles that make a liquid acidic. In the kidneys, aquaporins help take back 99 percent of the water filtered out of the blood.

Sources

Experimental data for H2O (Water). NIST Computational Chemistry Comparison and Benchmark Database.
Benchmark oxygen-oxygen pair-distribution function of ambient water from x-ray diffraction measurements with a wide Q-range. Skinner et al., Journal of Chemical Physics, 2013.
How big are biochemical nuts and bolts?. In Cell Biology by the Numbers, by Ron Milo and Rob Phillips. Garland Science, 2015.
Inhibitors of Mammalian Aquaporin Water Channels. Abir-Awan et al., International Journal of Molecular Sciences, 2019.
Aquaporin Water Channels. Nobel Lecture by Peter Agre, Nobel Foundation, 2003.
The Nobel Prize in Chemistry 2003. Nobel Prize Outreach.
Cesium. PubChem, National Library of Medicine.
Water. In Biology 2e. OpenStax, 2018.
Just How Small Is "Nano"?. National Nanotechnology Initiative.
Rules of Tennis 2026. International Tennis Federation, 2026.

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