Albert Barnes Commentary 1 Samuel 17:5

"And he had a helmet of brass upon his head, and he was clad with a coat of mail; and the weight of the coat was five thousand shekels of brass." — 1 Samuel 17:5 (ASV)

Coat of mail—Also known as a “breastplate of scales,” this was a type of metal shirt that protected the back as well as the breast. It was made of scales like those of a fish, similar to the corselet of Rameses III, which is now in the British Museum. The terms for helmet, coat, and “clothed” (rendered as “armed” in the King James Version) are the same as those used in Isaiah 59:17.

Five thousand shekels—This weight is probably about 157 pounds avoirdupois . It is very probable that Goliath’s brass coat was preserved for a long time as a trophy, just as we know his sword was, which is how its weight was ascertained.