Cars must weigh at least 743 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 because of larger and heavier tyres and wider cars and for 2018 weight is increased because of halo head protection system introduction. For 2019 F1 season minimum weight is increased due to wider wings.
The cars are weighed with dry-weather tyres.
Teams may use ballast to bring their weight up to minimum weight of 743 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 45.5% and rear with at least 53.5% of total car weight.
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 2019 Technical Regulations can be downloaded here.