John Gill Commentary Deuteronomy 25:15

"A perfect and just weight shalt thou have; a perfect and just measure shalt thou have: that thy days may be long in the land which Jehovah thy God giveth thee." — Deuteronomy 25:15 (ASV)

[But] you shall have a perfect and just weight, a perfect and just measure shall you have That is, full weights, and full measures; and such as are alike, and everywhere used, according to the standard of the country; (See Gill on Leviticus 19:36);

that your days may be lengthened in the land which the Lord your God gives you ; long life was always reckoned a blessing, and is frequently promised to obedience, and particularly long life in the land of Canaan; which was a most delightful and fruitful land, and which a man might wish to live long in; deceitful men, are threatened with not living half their days, and such may they be said to be that use false weights and measures, (Psalms 55:23) .