Dust grains grow in interstellar clouds by accretion and coagulation .
In this paper , we focus on these two grain growth processes and numerically investigate how they interplay to increase the grain radii .
We show that accretion efficiently depletes grains with radii a \la 0.001 ~ { } \micron on a time-scale of \la 10 Myr in solar-metallicity molecular clouds .
Coagulation also occurs on a similar time-scale , but accretion is more efficient in producing a large bump in the grain size distribution .
Coagulation further pushes the grains to larger sizes after a major part of the gas phase metals are used up .
Similar grain sizes are achieved by coagulation regardless of whether accretion takes place or not ; in this sense , accretion and coagulation modify the grain size distribution independently .
The increase of the total dust mass in a cloud is also investigated .
We show that coagulation slightly ‘ suppresses ’ dust mass growth by accretion but that this effect is slight enough to be neglected in considering the grain mass budget in galaxies .
Finally we examine how accretion and coagulation affect the extinction curve : The ultraviolet slope and the carbon bump are enhanced by accretion , while they are flattened by coagulation .