  Recently published space-based observations of main-belt asteroids with the
AKARI telescope provide a full description of the 3 {\mu}m band, related to the
presence of OH bearing minerals. Here, we use laboratory spectra of
carbonaceous chondrites obtained under controlled atmosphere (CI,CM,CO,CV,CR
Tagish Lake) to derive spectral metrics related to the water content in the
samples. After testing several spectral metrics, we use a combination of band
depth at 2.75 {\mu}m and 2.80 {\mu}m that shows a correlation with [H2O] in the
sample determined by TGA, though with a high uncertainty (4 wt% H2O). This
relation is used to determine water content at the surface of large C-complex
main-belt asteroids and discuss the origin of the variability found. On average
C-complex Main-Belt Asteroids (MBA) have water contents of 4.5 wt% (volume
average, (1) Ceres excluded), significantly lower than average CM chondrites.
The estimated water content for the most hydrated asteroids are lower than
those of the most hydrated meteorites, a difference that could be attributed to
space-weathering. An anti-correlation is also present between water content and
overall spectral slope, which is opposite to expectation from laboratory
simulations of space weathering on dark carbonaceous chondrites. This suggests
that part of the variability in the surface hydration among the different
C-complex asteroids is not due to space-weathering, but to the composition of
surface material. When applied to Ceres, the hygrometer presented in this work
enables us to estimate that at least 1.22 wt% of the hydrogen is present in the
form of organics. This richness in organics strengthens the connection between
Ceres and cometary materials.

Earth and Planetary Astrophysics