There is 25.4 mm to an inch,
8 inches to a link,
100 links in a chain,
so 800 inches in a chain x by 25.4m =20320mm =20.3 meters.
250 links x 8 inches x 25.4 mm = 50800mm =50.8 meters
20.3 x50.8 =1031.24 SQM.
A chain consisted of 100 links and was used by land surveyors to measure and peg out properties.
Some times after years of use the chain would become longer by wear in the links or stretching and that is why you often find a block that may measure a bit over.