Giant Galapagos tortoises Dolly, Polly and Priscilla had their carapaces measured ahead of their move across the zoo to their new Giants of the Galapagos home.
Even the growth of corals, rescued from the illegal wildlife trade, in the new coral tank had to be measured as part of the event.
Asiatic lioness Arya, who arrived in April as a mate for male Bhanu, stretched out against her first giant ruler.
Getting the 20,000 animals at the zoo to cooperate is not always easy; keepers had to coax the troop of Bolivian black-capped squirrel monkeys onto the scales one by one by offering snacks.
Zookeepers had to use novel tactics to entice the animals to stand up and be measured, including tricking penguins into walking over scales as they line up for their morning feed.
ZSL London Zoo’s animal manager Angela Ryan says: “We have to know the vital statistics of every animal at the Zoo - from the tallest giraffe to the tiniest ant.
“It helps to ensure that every animal we look after is healthy, eating well, and growing at the rate they should - weight is a particularly important indicator of health and wellbeing.
“A growing waistline can also help us to detect and monitor pregnancies, which is so important as many of the species at ZSL London Zoo are threatened and part of international breeding programmes, including today’s Asiatic lions and big-headed turtles.
“By sharing information with other zoos and conservationists worldwide, we can all use this knowledge to better care for the species we’re striving to protect.”
The statistics are used to help inform the animals’ health and wellbeing over the next year.