Pure (fresh) liquid water has a density of about 1.000 kg/liter, or equivalently 1.000 g/cm3, or 1000 kg/m3 (1.000 metric tonnes), at 4.3°C and 1 atm pressure.
The units are often omitted, so the density of liquid H2O 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 km3 (cubic kilometer, or cu-km) = 0.239913 mi3
1 GT = 1 gigaton = one billion tons = 109 tons (U.S. tons or "short tons," each 907.185 kg or 2000 lbs) 1 Gt = 1 gigatonne = 1 Pg = 1 petagram = 1015 grams = 1000 Tg = 1000 teragrams = 109 tonnes (metric tons, each 1000 kg or 2204.62 lbs) = 1012 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 × 1018 kg = 5.3 × 106 Gt = 5.3 million Gt, so one ppmm (part-per-million by mass) weighs 5.3 Gt.
However, atmospheric gas concentrations are customarily expressed in ppmv (parts-per-million by volume,
or molar fraction),
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 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.
408 ppmv CO2† has mass 408 × 8.053 Gt/ppmv = 3286 Gt
1 ppmv CH4 (molecular wt 16.044) has mass ~(16/29) × 5.3 Gt = 2.9356 Gt.
1.85 ppmv CH4† has mass 1.85 × 2.9356 Gt/ppmv = 5.431 Gt
Meltwater & sea-level:
The oceans cover about 3.618 × 108 km2 (sq-km) = 3.618 × 1014 m2. 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 m3 × (3.618 × 1014) = 3.618 × 1011 m3 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 × 1014 kg = 361.8 Gt of meltwater.
Ice has a density of about 0.9167, so 361.8 Gt = ~394.7 km3, which is 94.7 cubic miles.
Calculated another way, 361.8 Gt/mm-SLR × 0.262 mi3/Gt = 94.8 cubic miles per millimeter of sea-level rise.
Melting ~95 cubic miles of grounded ice (= 362 Gt = 395 km3) 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: 408 ppmv CO2 and 1.85 ppmv CH4