ATV Johannes Kepler approaching ISS

Based on the current Ariane 5 launcher performance, the ATV can carry to the ISS slightly more than 6.6 tonnes of wet and dry cargo. In addition, for a typical mission profile (such as for the ATV-2 Johannes Kepler) ATV carries about 2.5 tonnes of fuel for its own use, i.e. for rendezvous and docking with the ISS, performing different manoeuvres on the way, and eventually deorbiting itself on conclusion of its mission.

The weights are calculated for each mission depending on the cargo need, but in general the total ATV net cargo capacity can be allocated as follows (of course not all the maximum quantities presented here can be carried at once on the same flight):

Wet cargo

Propulsive support propellant (up to 4.0 tonnes)
This takes up by far the largest proportion of the ATV cargo. This propellant is used by the ATV to reboost the ISS to a higher orbiting altitude in order to counter the effects of atmospheric drag, which cause the ISS to very slowly lose altitude, for ISS attitude control, and for ISS Debris Avoidance manoeuvres, whenever needed. The propellant consists of two different fluids: monomethylhydrazine (MMH) and mixed oxides of nitrogen (MON3).

ATV-2 being fuelled

Refuelling propellant (up to 860 kg)
Once attached to the Station, up to 860 kg of refuelling propellant can be transferred from the ATV to the ISS. This consists of two different fluids: the fuel unsymmetrical dimethylhydrazine (UDMH) and the oxidiser, nitrogen tetroxide (N2O4), which provides a source of oxygen so the fuel can ignite and burn in orbit. This is used by the ISS for orbit and attitude control with its own engines.

Water (up to 855 kg)
This is what is known as potable water for use by the crew for drinking, food rehydration and oral hygiene.

Gases (up to 102 kg)
Two out of three types of gases (oxygen, nitrogen and air) can be transported in order to resupply the atmosphere inside the ISS, which is similar to that on Earth.