A curious thought: this is an instance in which English uses a singular noun with a plural number, just as Welsh does, although in English it is a fairly rare exception whereas in Welsh (as I understand) it is the rule.
This is pretty much the rule in English for a number of measurements used as a description (I can't remember the correct term). eg. a five-mile road is five miles long. or someone who is five foot is five feet tall. A ten-tonne boulder weighs ten tonnes. etc.
But we talk about those Imperial weight and height units without a plural marker even as nouns. The road is five miles long, he weighs 100 kilos, he is 2 metres tall, but she weights eight stone and she is 5 foot 8 (inches) tall.