a 2 by 4 would be a 5 by 10 in metric wouldn't it? you're not calling it a "2 by four inches" just the same as you wouldn't call it a "5 by 10 centimeters". In

a 2 by 4 would be a 5 by 10 in metric wouldn't it? you're not calling it a "2 by four inches" just the same as you wouldn't call it a "5 by 10 centimeters". In fact I find in practice I hardly ever mention units. Being off by a factor 10 is usually obvious enough that people know what unit you're using.