there were approx 7.442 billion people in 2016 according to google

the average male shoulder-to-shoulder width is about 0.465m, while adult females have an average shoulder-to-shoulder width of about .395m

in 2016, it was estimated that 23% of the population was below 18

if we assume that 25% of the population is below 18, then 75% of the population is adult and the average stats can be used for this portion of the population. assuming that there are 7.50 billion people in the world, there are an equal number of males and females, and all the adults hold hands in a straight line and in as little space as possible, then the line would be

(1/2)*(3/4)*(7.50*10^9)*(.465 +.395) = 2.42*10^9m

children have a continuous shoulder-to shoulder length distribution.

some may have only a .2m shoulder length (infants) or less, while others may have .35m

regardless, they most likely have a shoulder length below .40m, so if we were to assume the non-adults to have at most .400m, then they would form a line of at most

(1/4)*(7.50*10^9)*(.400) = .75*10*9m

put this together, if everyone were to hold hands in a line, the total length would be

<= (2.42 + .75)*10^9 = 3.17*10^9m = 3,170*10^6m

for comparison, the circumference of earth is estimated at 40 x 10^6m

that means, if everyone were to hold hands, they would theoretically circle the earth

floor(3,170/40) = up to 79 times!

http://www.firstinarchitecture.co.uk/average-male-and-female-dimensions/
http://datacenter.kidscount.org/data/tables/99-total-population-by-child-and-adult#detailed/1/any/false/870,573,869,36,868/39,40,41/416,417