I have received a pack of resin and hardener,and the only mixing information is by weight.

Use the data sheet density data to convert from weight mixing ratio to a volume one.  We don't recommend it generally as epoxies are sensitive to accurate mixing ratios and it's introducing another potential area of error.

You can get micro scales that will go down to really tiny weights if needed.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant