literally: “Milkmaid’s calculation” / meaning: a naive conclusion that turns out to be wrong
There are people out there who will drive 120 kilometers from Berlin across the Polish border because Pjotr gives them a trendy bob haircut for half the price they’d pay in Germany. They generously ignore the fact that the fuel alone eats up the saving. So they have made a classic Milchmädchenrechnung.
Germans use this term for an assumption or calculation that seems logical at first glance but turns out to be a fallacy on closer inspection. The expression goes back to the tale of “The Milkmaid and the Milk Jug” by French fabulist Jean de La Fontaine. It is about a young farmer’s maid who went to the market in the morning to sell milk. On the long walk, she started imagining all the things she could afford to do with the earnings—especially if she reinvested them. She got so caught up in her dreams that she tripped and spilled all the milk on the floor. So, all the poor girl’s plans became null and void.
At least Pjotr is happy because he continues to make good money from German customers with obvious arithmetic difficulties.