Unicef said the difference in time spent working amounted to 160m extra hours a day worldwide.
Two out of three girls cook and clean in the home, and almost half collect water or firewood.
They also perform more "less visible" domestic work like childcare or looking after the elderly, the report says.
It also found that the extra workload increased with time: between ages five and nine, girls spend 30% more time on chores - by 14, it rises to 50%.
