A Chinese airline has calculated that it takes a litre of fuel to flush the toilet at 30,000 feet and is urging passengers to go to the bathroom before they board.

As Chinese airlines come under increasing pressure to cut fuel expenditures, China Southern's latest strategy is to encourage passengers "to spend their pennies before boarding the aircraft," Xinhua news agency reported last week.

"The energy used in one flush is enough for an economical car to run at least 10 kilometres," Captain Liu Zhiyuan, who flies regularly between Hangzhou and Beijing, was quoted as saying.

Citing a survey by the company's logistics department, Liu said carrying one kilogram of items such as blankets and pillows by air for one hour uses 0.2 kg of fuel.

"This means the blankets and pillows on board the aircraft eat up 60 tons of fuel every day. If each seat is loaded with three 450-gram magazines, another 60 tons will be consumed," Liu explained.

Another cost-saving technique will keep 47 million yuan (US$6 million) a year in the airline's coffers, Xinhua said.