Pure (fresh) liquid water has a density of about 1.000 kg/liter, or equivalently
1.000 g/cm^{3}, or 1000 kg/m^{3} (1.000 metric tonnes), at 4.3°C and 1 atm pressure.

The units are often omitted, so the density of liquid H_{2}O may be stated as simply 1.000

Ice has a density of about 0.9167

Seawater has a density of about 1.027

The Dead Sea is nearly 10x as salty as the ocean, and has a density of about 1.240

The density of water varies only slightly with temperature & pressure, but if you need more precision various online tables and calculators can give you nearly exact water densities for specific temperatures, pressures & salinities.

1 km = 0.621371 mile,

so 1 km^{3} (cubic kilometer, or cu-km) = 0.239913 mi^{3}

1 GT = 1 gigaton = one billion tons = 10^{9}tons (U.S. tons or "short tons," each 907.185 kg or 2000 lbs) 1 Gt = 1 gigatonne = 1 Pg = 1 petagram = 10^{15}grams = 1000 Tg = 1000 teragrams = 10^{9}tonnes (metric tons, each 1000 kg or 2204.62 lbs) = 10^{12}kg = 1.1023 GT = the mass of 1 cubic kilometer of fresh water = the mass of 1.091 cubic km of ice = the mass of 0.240 cubic miles of fresh water = the mass of 0.262 cubic miles of ice 1 cubic mile of ice weighs 1/0.262 = 3.82 Gt

The Earth's atmosphere is variously estimated to have a mass of 5.1 to ^{18} kg^{6} Gt = 5.3 million Gt,
so (using the “5.3” estimate) one ppmm (part-per-million by mass) weighs about 5.3 Gt.

However, atmospheric gas concentrations are customarily expressed in ppmv (parts-per-million by volume,
a/k/a molar fraction, µ mol/mol),
so to calculate the mass of one ppmv requires scaling according to the molecular weight of the gas in question.
(Note: if water vapor is ignored this is properly called the ** dry** molar fraction.)

The average molecular weight of the Earth's atmosphere is 28.966 g/mole (~29). So, for example: ↑

1 ppmv CO

1 Pg = 1 Gt, so 1 PgC (“petagrams carbon”) is contained in (44/12) = 3.667 Gt CO

408 ppmv CO

That much CO

1 ppmv CH4 (molecular wt 16.044) has mass ~(16/29) × 5.3 Gt = 2.9356 Gt.

1.85 ppmv CH

**Meltwater & sea-level:**

The oceans cover about 3.618 × 10^{8} km^{2} (sq-km) = 3.618 × 10^{14} m^{2}.
A one mm global average increase in sea-level requires 1/1000-th of a cubic meter of water for each square meter of
ocean surface: 10^{-3} m^{3} × (3.618 × 10^{14}) =
3.618 × 10^{11} m^{3} of water.

(Note: sea ice is frozen nearly-fresh water, not saltwater, because most of the salt is expelled when seawater freezes.)

A cubic meter of fresh water weighs 1000 kg, so (disregarding the minor salinity/density effects of mixing fresh meltwater
with seawater) a one mm increase in sea-level requires about 3.618 × 10^{14} kg = 361.8 Gt of meltwater.

Ice has a density of about 0.9167, so 361.8 Gt = ~394.7 km^{3}, which is 94.7 cubic miles.

Calculated another way, 361.8 Gt/mm-SLR × 0.262 mi^{3}/Gt = 94.8 cubic miles per millimeter of sea-level rise.

**Melting ~95 cubic miles of grounded ice (= 362 Gt = 395 km ^{3})
into ~87 cubic miles of fresh water and adding it to the oceans would raise
globally averaged sea-level by 1 mm.**

-Dave Burton 3/28/2014, 8/18/2014, 5/10/2015, 12/9/2015, 12/13/2016, 2/3/2017, 6/25/2018, 12/23/2018

^{†}which are the approximate current average atmospheric concentrations of the two gasses: 408 ppmv CO_{2} and 1.85 ppmv CH_{4}