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 has a mass of about 5.3 × 10^{18} kg = 5.3 × 10^{6} Gt = 5.3 million Gt,
so one ppm (part-per-million by mass) weighs 5.3 Gt.

However, atmospheric gas concentrations are customarily expressed in ppmv (parts-per-million by volume),
so to calculate the mass of one ppmv requires scaling according to the molecular weight of the gas in question.
The average molecular weight of the Earth's atmosphere is 28.966 g/mole (~29). So, for example: ↑

**Carbon Dioxide:**

1 ppmv CO2 (molecular wt 44.01) has mass ~(44/29) × 5.3 Gt = 8.053 Gt, of which 12/44-ths or 2.196 Gt is carbon.

400 ppmv CO2^{†} has mass 400 × 8.053 Gt/ppmv = 3221 Gt

**Methane:**

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

1.8 ppmv CH4^{†} has mass 1.8 × 2.9356 Gt/ppmv = 5.284 Gt

**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

^{†}which are the approximate current average atmospheric concentrations of the two gasses: 400 ppmv CO_{2} and 1.8 ppmv CH_{4}