If you gathered all of Earth's water into one ball, how big would it be?
― About 1,400 km across. Rivers and lakes alone would make just 56 km
Earth is called the "water planet," with oceans covering 70% of its surface. Yet if you scooped up all that water and rolled it into a single ball, it would sit beside Earth as a tiny speck. The ocean is enormously wide, but astonishingly thin.
Spin a globe and all you see is blue. Many of us learned that the Pacific Ocean alone is bigger than all the continents combined.
Sail out to sea and, whichever way you look, water stretches to the horizon. Some spots are said to be over 10,000 metres deep.
With that much water around, you might expect a ball roughly half the size of Earth. But when you actually do the maths, the answer is far smaller than you'd imagine.
Only two reasons it comes out so small
The ocean's average depth is about 3.7 km. Earth's radius is about 6,400 km. That depth is only about 1/1700th of the radius.
A ball's size is set by the cube root of its volume. Even though the water is only about 1/780th of Earth's volume, its diameter looks like only about a ninth.
Working through these two points in turn reveals the true face of the "water planet," and just how little water we actually have to use.
All of Earth's water makes a ball about 1,400 km across
According to estimates from the U.S. Geological Survey, Earth holds about 1.386 billion cubic kilometres of water in total. That includes seawater, glaciers, groundwater, lakes and rivers, water vapour in the air, and even the water inside living things.
Gathered into a single ball, that comes to a diameter of about 1,385 km. Earth's diameter is about 12,700 km, so the water ball is only about a ninth of that. Compared with the Moon (diameter about 3,500 km), it's only about 40% as wide.
Look at the left side of Figure 1. The large circle is Earth, and the small balls lined up on the right are all of Earth's water. Drawn to the same scale, the difference is striking.
On a globe, the ocean is only as thick as a sheet of paper
Thinking about a globe makes the ocean's thinness easy to grasp. On a globe 30 cm across, the distance from centre to surface is 15 cm. At that scale, the ocean's average depth works out to about 0.09 mm.
That's about the same as the thickness of a single sheet of paper. Even the Mariana Trench, the deepest point, is only about 0.26 mm. It's thin enough that you might not even feel the water on your fingertip if you ran it over the globe's surface (right side of Figure 1).
The ocean looks vast because it wraps all the way around the surface. But volume is "area × depth," so no matter how wide it is, if the depth is this thin, the total amount stays small.
What's more, the size of the resulting ball is set by the cube root of the volume. If the volume is a thousandth, the diameter is only a tenth. Earth's water is about 1/780th of Earth's total volume, which works out to about a ninth in diameter. Because the shrinkage looks so gentle, it's a strange number — you might call it "bigger than expected" or "smaller than expected" depending on how you look at it.
About 97% of all water is salty ocean water. Most of the remaining fresh water is locked up in the ice of Antarctica and Greenland, or sits deep underground. Gather up all the lake and river water, and you get a ball only about 56 km across. The water we use in daily life is constantly replenished from this tiny speck, through the endless cycle of rain and evaporation.
Water is lighter than rock or iron, so the difference widens further when compared by weight. All of Earth's water together is thought to make up only about 0.02% of Earth's total mass. Earth isn't so much a "water planet" as "a ball of rock with a thin film of water."
Summary
Gather all of Earth's water together, and you get a ball only about 1,400 km across. The ocean covers 70% of the surface, but its depth is only about 1/1700th of Earth's radius. And because a ball's size is set by the cube root of its volume, that comes to about a ninth of Earth's diameter. Lake and river water alone makes a ball just 56 km wide.
The ocean isn't so much "wide" as "thinly spread."
Our water is a single thin film draped over Earth's surface.
For why the sea is salty, see "Why is seawater salty?", and for how even this thin ocean can crush living things at depth, see "How do deep-sea creatures survive that water pressure?" For an extreme example of weight packed into tiny volume, see the article on neutron stars too.
- Get one mandarin orange. One about 7 cm across has a volume of roughly 200 millilitres. Let this be "Earth."
- Prepare about 0.25 millilitres of water — about 1/780th of that — roughly 5 drops from a dropper or straw tip. This is "Earth's water."
- Drop the 5 drops together onto a sheet of plastic wrap, and they'll form a near-round bead. Place it beside the orange and compare the sizes.
Even though it's 1/780th by volume, the bead is under a centimetre across — about a ninth of the orange. "Amount" and "length" feel out of step because volume grows with the cube of length.
Want to know more? ― terms, formulas, and textbook linksWe mark clearly which level each part belongs to, from middle-school science to university specialist courses
- MSCovered in middle-school science and maths
- HSCovered in high-school "Earth Science" and "Maths"
- HS+Advanced high-school material, or textbook sidebar content
- Univ.Not covered in high school — university-level hydrology/geochemistry content
- ResearchNot yet settled even at university level — something researchers are actively investigating
MSTerms: this phenomenon has a name
- Water cycle: the repeating process in which ocean and land water evaporates into clouds, falls as rain or snow, and returns to the sea via rivers and underground flow.
- Fresh water: water with almost no salt content. Found in glaciers, groundwater, lakes and rivers, it's estimated to make up about 2.5% of Earth's water.
- Similarity ratio and volume ratio: for solids of the same shape but different size, doubling the length makes the volume eight times bigger. The volume ratio is the cube of the length ratio.
MSHSChecking with formulas: the water ball's diameter, and how thin the ocean is
Working the sphere-volume formula backwards lets us get the ball's diameter from the volume of water. We calculate the very thing at the heart of this article — the diameter of the water ball — and compare it with Earth's diameter.
| In symbols | V ＝ (π ÷ 6) × d³ so d ＝ ∛( 6 × V ÷ π ) |
| In words | Ball's diameter ＝ the cube root of "6 × volume of water ÷ pi" |
| Where it comes from | The sphere-volume formula (volume ＝ 4/3 × pi × radius cubed), rewritten in terms of diameter and solved in reverse. Since the volume of water is conserved regardless of shape, it's the same whether spread across the ocean or gathered into a ball |
| Symbol | Meaning and unit |
| V | Volume of water (cubic kilometres) |
| d | Diameter of the ball of gathered water (kilometres) |
| π | Pi (about 3.14) |
| Total water on Earth (USGS) | ~1.386 billion km³ |
| Fresh water in lakes and rivers (same) | ~93,113 km³ |
| Earth's average radius | ~6,371 km |
| Ocean's average depth | ~3.7 km |
| 6 × volume of water (billion km³) | 6 × 13.86 ＝ 83.16 |
| Divide by pi | 83.16 ÷ 3.14 ≒ 26.48 |
| Cube root gives the diameter | The cube root of 2.648 billion is about 1,384 km |
| Earth's diameter | 6,371 × 2 ＝ 12,742 |
| Ratio of Earth's diameter to the water ball's | 12,742 ÷ 1,384 ≒ 9.2 |
| Lake and river water: 6 × volume | 6 × 93,113 ＝ 558,678 |
| Divide by pi | 558,678 ÷ 3.14 ≒ 177,924 |
| Cube root gives the diameter | The cube root of 177,924 is about 56.2 km |
| Ocean depth ÷ Earth's radius | 3.7 ÷ 6,371 ≒ 0.00058 |
| Ocean thickness on a globe (radius 150 mm) | 150 × 0.00058 ≒ 0.087 |
The ball of all water comes to about 1,384 km across, about a ninth of Earth. Lake and river water alone is about 56 km. On a globe, the ocean works out to about 0.09 mm — about the thickness of a single sheet of paper.
HSHS+Where the water is, and how much
HSIn Earth science, we learn about the distribution of Earth's water as the "hydrosphere." Seawater accounts for about 97%, fresh water about 2.5%; of that fresh water, about 70% sits in the ice sheets and glaciers of Antarctica and Greenland, and about 30% is groundwater. Surface water — lakes, rivers, marshes — makes up less than 1% of all fresh water.
HS+The smaller the reservoir, the faster its water turns over. Water vapour in the atmosphere is thought to fall as rain and be replaced on average every 10 days or so. Ocean water, by contrast, is thought to take thousands of years to fully cycle through. Rivers and lakes, despite holding so little water, never run dry — thanks to this rapid turnover.
Univ.Hydrology: water budgets and residence time
The field that studies the amount and flow of water on Earth is called hydrology. Its basic tools are the water budget, which balances inflow and outflow for each reservoir (ocean, ice sheets, groundwater, atmosphere, and so on), and residence time, found by dividing "amount stored" by "rate of flow in and out." The widely used table of global water distribution comes from estimates compiled in the 1990s by Soviet hydrologist Igor Shiklomanov and colleagues, and the USGS figures are based on this work too.
📖 For the derivation of the formula and further reading: Water cycle (Japanese Wikipedia) ／ U.S. Geological Survey, "How Much Water is There on Earth?"
ResearchWhat's still not fully understood
- Water inside the mantle: minerals in the mantle, hundreds of kilometres down, hold water locked inside their crystal structure. Some estimates put the amount at several times the volume of the ocean, but the range is wide and unsettled.
- Where Earth's water came from: was it delivered by comets, brought by fragments of asteroids, or present from the moment Earth formed? Debate continues, using the ratio of hydrogen isotopes as a clue.
- Total groundwater: deep groundwater can't be measured directly, so estimates vary widely. How fast the shallow groundwater people actually use gets replenished is also still being studied.
In other words, even this article describes things only "as far as we currently understand them." Surface water has been estimated fairly precisely, but include the water deep inside Earth, and the answer might change.
Links to the curriculum (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Maths "similar figures," Science "weather changes (water cycle)" | Why 1/780th of the volume becomes about a ninth of the diameter |
| HS | Basic Earth Science, "Earth's hydrosphere" | Proportions of seawater, ice, and groundwater |
| HS+ | Earth Science, "water cycling and residence" | Atmospheric water turning over roughly every 10 days |
| Univ. | Hydrology / geochemistry | Water budgets and residence time |
| Research | Planetary science / deep Earth science | Water in the mantle, the origin of Earth's water |
| ― | Connection to daily life | The water we use is constantly replenished from a 56 km speck |
- U.S. Geological Survey, "How Much Water is There on Earth?"（Water Science School）
- NASA Earth Fact Sheet (mass, average density, radius)
- Shiklomanov, I. A. (1993). World fresh water resources. In P. H. Gleick (Ed.), Water in Crisis: A Guide to the World's Fresh Water Resources. Oxford University Press.
- National Astronomical Observatory of Japan (ed.), Rika Nenpyō (Chronological Scientific Tables), Maruzen Publishing (国立天文台編『理科年表』丸善出版) (Earth's size, ocean depth)
※This article is a general-audience science explainer. The figures given are rough estimates meant to aid understanding. Figures for the total amount and distribution of water are estimates and may vary somewhat between sources.