Cars must weigh at least 734 kg at all times during an event (including the driver and safety equipment, but not fuel). The minimum weight for 2017 is increased to 728 kg becaufe of larger and heavier tyres and wider cars and for 2018 weight is increased because of halo head protection system introduction. The cars are weighed with dry-weather tyres.
Teams may use ballast to bring their weight up to minimum weight of 734 kg, but it must be firmly secured to the cars and it cannot be removed or added during a race.
Rules dictate minimum weight on each axle to minimize weight avalaible to play with weight distribution. In past, some teams got their weight distribution right and had the advantage for the rest of the year so FIA wanted to eliminate that possibility by narrowing weight window to play with.
Front axe must always be loaded with at least 334 kg and rear with at least 393 kg. That means teams have only 7 kg to play with weight distribution. Maximum rear-biased weight distribution is 334 : 400 kg (45.50 : 54.50%) and maximum forward-biased distrubution is 341 : 393 kg (46.46 : 53.54%).
After each of three practice sessions cars can randomly be weighed, just like in Q1 and Q2 qualifying parts. Each car that takes part in final part of qualifying (Q3) must be weighed after the end of the session.
FIA F1 2018 Technical Regulations can be downloaded here.