Waving a big chart as a prop in the White House Rose Garden, Donald Trump suggested his new tariff plan was simple: “Reciprocal – that means they do it to us, and we do it to them. Very simple. Can’t get simpler than that.”

Perhaps a bit too simple. The method used to calculate the most important numbers in international trade, politics and economics has left some of the world’s leading experts shocked.

For each country, the White House looked up its trade in goods deficit for 2024, then divided that by the total value of imports. Trump, to be “kind”, said he would, however, offer a discount, so halved that figure. The calculation was even distilled into a formula.