Earth’s Water Percentages: The Denominator Makes the Difference

A reproducible calculation from the complete USGS water-distribution table.

The same estimated 24,064,000 km³ of ice caps, glaciers and permanent snow is about 68.7% of freshwater and 1.74% of all Earth water. The numerator stays the same; the denominator changes. Adding the non-overlapping volumes in the USGS table explains the rounded percentages without circularly reconstructing a total from the percentage being checked.

Which water estimate is this?
Add volumes without double-counting the subcategories
Use the same numerator with each denominator
Why 68.7% multiplied by 2.5% gives about 1.72%
Fresh groundwater is not total freshwater
A store is a volume; a flow is a volume per time

Which water estimate is this?

The USGS Water Science School table attributes its estimate to Igor Shiklomanov's chapter in the 1993 book Water in Crisis, edited by Peter H. Gleick. The page is dated June 6, 2018. These are historical estimates of water stores, not a new 2026 global measurement. The page explicitly warns that its percentages are rounded.

Its categories describe where water is held. They do not tell us how much is safe, accessible or affordable to drink. Fresh groundwater, ice, saline groundwater and freshwater lakes have different meanings; a word like “Fresh” inherits the category under which its row is indented.

Add volumes without double-counting the subcategories

Source-reported cubic-kilometer volumes and their role in the sum

Store	All-water sum, km³	Freshwater sum, km³
Oceans, seas and bays	1,338,000,000	—
Ice caps, glaciers and permanent snow	24,064,000	24,064,000
Groundwater	23,400,000	10,530,000 (fresh subset)
Soil moisture	16,500	16,500
Ground ice and permafrost	300,000	300,000
Lakes	176,400	91,000 (fresh subset)
Atmosphere	12,900	12,900
Swamp water	11,470	11,470
Rivers	2,120	2,120
Biological water	1,120	1,120
Original sum of the displayed rows	1,385,984,510	35,029,110

For the all-water total, include the groundwater parent row once, not that parent plus its fresh and saline children. Do the same for lakes. For the freshwater total, select the fresh groundwater and fresh lake children instead. The totals above are our arithmetic sums of the source's displayed volumes; they are not independently measured new totals. The long integer endings reflect addition, not a claim that the source measured every cubic kilometer precisely.

Use the same numerator with each denominator

Ice share of freshwater:
24,064,000 / 35,029,110 × 100 = 68.697149...% → 68.7%

Ice share of all water:
24,064,000 / 1,385,984,510 × 100 = 1.736238...% → 1.74%

Freshwater share of all water:
35,029,110 / 1,385,984,510 × 100 = 2.527381...% → 2.5%

The intermediate digits make the arithmetic auditable. The rounded conclusions, historical source and uncertain water estimates remain the scientific context. The calculation record includes the sums and ratios for reproduction.

Why 68.7% multiplied by 2.5% gives about 1.72%

Using already-rounded values gives 0.687 × 0.025 × 100 = 1.7175%. That is about 1.72%, rather than the separately rounded table value of 1.74%. With consistent unrounded ratios, the freshwater denominator cancels: (ice / freshwater) × (freshwater / all water) = ice / all water. Rounding before multiplying loses information.

The sum-derived freshwater share is approximately 2.5274%, which is conventionally displayed as 2.5% to one decimal place. Multiplying the sum-derived ratios reproduces the sum-derived ice share; it does not require the rounded percentages to behave as exact inputs. Nor should one “fix” the table by pretending 68.7% or 1.74% has unlimited precision.

Fresh groundwater is not total freshwater

The table's 10,530,000 km³ fresh groundwater is part of 23,400,000 km³ total groundwater; the other 12,870,000 km³ is saline. The freshwater percentage of 30.1 belongs to that fresh groundwater subset. Calling 10,530,000 km³ “fresh surface water” or using it as all freshwater would change the category and produce a wrong denominator.

Likewise, 2,120 km³ is the river store, not all surface freshwater. It is approximately 0.006% of the freshwater sum and 0.0002% of the all-water sum when rounded as in the table. The accompanying diagrams use a broader surface-and-other grouping that includes ground ice and permafrost. Keep their groupings with their percentages instead of moving one percentage into another denominator.

A store is a volume; a flow is a volume per time

The USGS pools and fluxes tables distinguish stored volumes in km³ from transfers in km³ per year. A small amount held in rivers at a moment can be replenished and pass through the system. A storage fraction cannot state annual runoff, an annual melting rate or the amount people can sustainably withdraw.

Different estimates and diagrams may use different dates, categories or methods. Do not replace one row of this historical distribution with a newer estimate and leave its old denominator unchanged. Compare source, category, units and time basis first. The same reasoning appears in our articles on rates and denominators and temperature baselines.

Generated with Ithy Pro; AI-assisted editorial and primary-source checks completed October 23, 2026. The three independent model perspectives are retained with corrections. Original examples and test limits are identified.

Primary references

Compare the AI responses

Read each model’s complete response below.

GPT-6 Luna

Editorial source check: October 23, 2026. This independent research perspective is retained with factual corrections; the checked main article adds current primary references and the stated examples.

USGS’s Water Science School table describes estimated water stores at a single point in time, not a new 2026 measurement. It lists 24,064,000 km³ in ice caps, glaciers and permanent snow: 68.7% of freshwater but 1.74% of all water because the percentages use different denominators. As a worked calculation, those rounded shares imply roughly 35.0 million km³ of freshwater and 1.383 billion km³ of total water. The implied freshwater share of the total is about 2.53%; multiplying the separately rounded 68.7% and 2.5% gives 1.7175%, not 1.74%. The table also distinguishes all groundwater from its fresh groundwater subset. Rivers are a tiny store, not the same quantity as total surface water or water moving through the cycle.

The USGS Water Science School says its chart and table represent the presence of Earth’s water at a single point in time. Treat the figures as historical estimates in that table, not as a measurement newly made in 2026. The source volume for ice caps, glaciers and permanent snow is 24,064,000 km³, with table shares of 68.7% of freshwater and 1.74% of total water. [1.1] [1.2]
The percentages use different denominators: 68.7% is the ice volume divided by all freshwater, while 1.74% is that same volume divided by all Earth water. A worked calculation from the source’s rounded figures gives 24,064,000 ÷ 0.687 ≈ 35.0 million km³ of freshwater, and 24,064,000 ÷ 0.0174 ≈ 1.383 billion km³ of total water. These are inferred approximate denominators, not separately quoted totals. [1.2]
The familiar 2.5% freshwater share is itself rounded. Multiplying the displayed rounded shares gives 0.687 × 0.025 = 0.017175, or 1.7175% of total water—not 1.74%. The worked denominators implied by the ice volume and table shares give a freshwater-to-total ratio of about 35.0 million ÷ 1.383 billion ≈ 2.53%. Thus the mismatch is consistent with multiplying rounded percentages; the table does not supply extra precision that would justify treating these calculations as exact. [1.2] [7.3]
The table’s “Groundwater” row is not synonymous with fresh groundwater: it gives 23,400,000 km³ of groundwater overall and leaves its freshwater percentage blank. A separate “Fresh” row gives 10,530,000 km³, 30.1% of freshwater and 0.76% of total water; the table also lists saline groundwater separately. Do not assign the entire groundwater volume to freshwater. [1.2] [1.3] [1.4]
A useful comparison of the USGS table’s stores is: ice caps, glaciers and permanent snow—24,064,000 km³; all groundwater—23,400,000 km³; fresh groundwater—10,530,000 km³. These are volume estimates, not measures of how readily water can be used. The separate USGS summary says surface freshwater is only 1.3% of freshwater, so rivers should not be mistaken for all surface water or for most freshwater. [1.2] [1.3] [1.4] [7.2]
Rivers are a distinct, very small store in the cited distribution table: 509 cubic miles, listed as 0.006% of freshwater and 0.0002% of total water. That table is adapted from Shiklomanov (1993), so those river figures should retain that source’s historical-estimate context rather than be presented as a current measurement. They describe river water held in the store, not the volume flowing through rivers over time. [8.3] [8.4]
Water stores (pools) are volumes held in places such as oceans; water-cycle flows (fluxes) are transfers such as precipitation, evaporation, discharge and recharge, measured as volume per year. A storage percentage therefore does not state how much water moves annually. The USGS describes its distribution table as a snapshot, while its pools-and-fluxes material explicitly distinguishes pool volumes from annual flux rates. [1.1] [5.1]

Claude Haiku 5.5

Editorial source check: October 23, 2026. This independent research perspective is retained with factual corrections; the checked main article adds current primary references and the stated examples.

The USGS Water Science School table (reference 1.2 and 1.3) lists Earth water by source with volume in cubic miles and cubic kilometers, percent of freshwater, and percent of total water. Ice caps, glaciers and permanent snow hold 24,064,000 km3, listed as 68.7% of freshwater and 1.74% of total water. The same table gives groundwater 23,400,000 km3 (1.69% of total) and fresh groundwater 10,530,000 km3 (30.1% of freshwater, 0.76% of total). The table is a single point-in-time estimate from the USGS page and is not a new 2026 measurement. The 68.7% figure is a share of freshwater, while 1.74% is a share of all water, so the two percentages use different denominators. Multiplying the rounded 68.7% by the rounded 2.5% freshwater share gives about 1.72%, not 1.74%, because the rounded inputs and the source's underlying volume figures differ slightly. Groundwater is explicitly split into fresh and saline subcategories; 23,400,000 km3 is their parent total. Rivers appear in a separate bar of surface freshwater, and the table's stores are not water-cycle flows.

The USGS table lists ice caps, glaciers and permanent snow at 24,064,000 cubic kilometers, 68.7% of freshwater and 1.74% of total water. [1.2] [1.3]
The same table lists groundwater at 23,400,000 cubic kilometers and 1.69% of total water, with no freshwater percentage given. [1.2]
The 10,530,000 km³ row is fresh groundwater, not fresh surface water. Its 2,526,000 cubic miles and 30.1% freshwater share belong to that groundwater subset. The whole freshwater denominator is much larger.
The 68.7% figure uses the freshwater total as its denominator, while the 1.74% figure uses total water on Earth, which explains why the two percentages differ so much.
Multiplying the rounded 68.7% by the rounded 2.5% gives about 1.72%, not 1.74%, because the inputs are rounded and the source's volumes are the more precise basis.
The USGS Water Science School table is an estimate of water stores at one point in time, and the page notes that these numbers will change over a million years. [1.1]
The 2.5% freshwater share and the statement that most freshwater is frozen appear in USGS water-distribution pages, supporting the freshwater denominator used in the calculation. [7.3] [7.4]
Groundwater and surface water are distinct stores: the USGS pages describe groundwater as a large store of freshwater and rivers as only a small part of surface freshwater, so storage figures should not be used as water-cycle flow rates. [4.4] [7.2]

The complete USGS table explicitly splits 23,400,000 km³ groundwater into 10,530,000 km³ fresh and 12,870,000 km³ saline water. Count the parent once in an all-water sum and only its fresh child in a freshwater sum.

The USGS Water Science School table and the Water Pools and Fluxes data tables use different sources and categories, and their values are not directly interchangeable without the full dataset.

Claude Sonnet 5.5

Editorial source check: October 23, 2026. This independent research perspective is retained with factual corrections; the checked main article adds current primary references and the stated examples.

The USGS Water Science School table (a historical estimate, not a new 2026 measurement) gives ice caps, glaciers and permanent snow as 24,064,000 km3. That is 68.7% of freshwater (a 10,530,000 km3 denominator would not fit; the freshwater total is much larger) and 1.74% of all water (a much larger total, with oceans at 1,338,000,000 km3). The two percentages have different denominators. The rounded 2.5% freshwater share times 68.7% gives about 1.72%, close to but not exactly 1.74%, because 2.5% is rounded. Groundwater, fresh groundwater, rivers and surface water are separate lines, and the table shows stores, not water-cycle flows.

The USGS table is a single-point-in-time estimate of stores. It says the chart and table show where water is at one moment, not the movement described by the water cycle. The figures are historical estimates and should not be read as new measurements. [1.1]
The ice row lists 5,773,000 cubic miles, or 24,064,000 km3, with 68.7% of freshwater and 1.74% of total water. One store therefore has two percentages because each uses a different denominator: freshwater only, or all water. [1.2]
Original arithmetic from the table's volumes: 24,064,000 divided by 0.0174 gives about 1.383 billion km3 of total water. Dividing 24,064,000 by 0.687 gives about 35.0 million km3 of freshwater. The freshwater total divided by total water is then about 2.53%. This is my own calculation from rounded inputs, so it is only approximate. The oceans are listed at 1,338,000,000 km3 and 96.5% of total water, which is consistent with a total near 1.39 billion km3. [1.2] [7.3]
USGS says only 2.5% of all water is freshwater. Multiplying 0.687 by 0.025 gives 1.7175%, or about 1.72%, rather than 1.74%. The gap comes from rounding the 2.5%. A freshwater share near 2.53%, as another source states, gives about 1.74%. A shown percentage is not the underlying unrounded ratio. [7.3] [9.1]
Groundwater is 23,400,000 km3, or 1.69% of total water. This is all groundwater, fresh and saline together, and it has no freshwater percentage in the table. The fresh groundwater row is 10,530,000 km3, 30.1% of freshwater and 0.76% of total water. A shorter excerpt of the same chart gives the same fresh-groundwater figures and lists a separate saline row of 12,870 thousand km3. [1.2] [1.4] [6.3] [6.4]
Rivers are a separate 2,120 km³ store in the complete historical table. Its reported 0.006% is a share of freshwater and 0.0002% a share of all water. Diagram percentages can instead use a surface-and-other grouping; keep each denominator and diagram together.
Stores and flows are different things. Pools are places where water is stored, while fluxes such as evaporation, precipitation, recharge and discharge are movements measured per year. The USGS table lists stores, not flows. Water also moves between groundwater and rivers, for example by seepage. [5.1] [4.4] [4.5]
A freshwater store is not automatically available drinking water. Ice, groundwater, rivers and lakes have different accessibility, quality and replenishment constraints. Compare source dates, categories and denominators before combining numbers from different global water estimates.

The sources differ on groundwater and ice volumes, and one USGS copy lists 68.6% rather than 68.7% for ice.