The distance of all races, from the start signal to the chequered flag, shall be equal to the least number of complete laps which exceed a distance of 305 km.
For example, if circuit is 4,444 km long, number of laps is calculated by dividing 305 with 4,444 (68,63) and taking the next whole number (69).
However, should two hours’ elapse before the scheduled race distance is completed, the leader will be shown the chequered flag when he crosses the control line (the Line) at the end of the lap following the lap during which the two hour period ended, provided this does not result in the scheduled number of laps being exceeded.
Only under the circumstances below will any exception be made to the above:
a) The distance of the race in Monaco shall be equal to the least number of complete laps which exceed a distance of 260km.
b) Should the race be suspended (see Article 41) the length of the suspension will be added to this period up to a maximum total race time of four hours.
c) If the formation lap is started behind the safety car (see Article 39.16) the number of race laps will be reduced by the number of laps carried out by the safety car minus one.
FIA F1 2017. sporting regulations can be downloaded here.