ks ▁system ▁based ▁on ▁the ▁met re , ▁kil ogram ▁and ▁second ▁as ▁well ▁as ▁the ▁k el vin , ▁amp ere , ▁cand ela , ▁and ▁mole . ▁ ▁The ▁SI ▁includes ▁two ▁classes ▁of ▁units ▁which ▁are ▁defined ▁and ▁agreed ▁intern ation ally . ▁The ▁first ▁of ▁these ▁classes ▁includes ▁the ▁seven ▁SI ▁base ▁units ▁for ▁length , ▁mass , ▁time , ▁temperature , ▁electric ▁current , ▁l umin ous ▁intensity ▁and ▁amount ▁of ▁subst ance . ▁The ▁second ▁class ▁consists ▁of ▁the ▁SI ▁derived ▁units . ▁These ▁derived ▁units ▁are ▁defined ▁in ▁terms ▁of ▁the ▁seven ▁base ▁units . ▁All ▁other ▁quantities ▁( e . g . ▁work , ▁force , ▁power ) ▁are ▁expressed ▁in ▁terms ▁of ▁SI ▁derived ▁units . ▁ ▁Imperial ▁and ▁US ▁custom ary ▁units ▁ ▁Both ▁imperial ▁units ▁and ▁US ▁custom ary ▁units ▁derive ▁from ▁earlier ▁English ▁units . ▁Imperial ▁units ▁were ▁mostly ▁used ▁in ▁the ▁former ▁British ▁Empire ▁and ▁the ▁British ▁Commonwealth , ▁but ▁in ▁all ▁these ▁countries ▁they ▁have ▁been ▁largely ▁supp l anted ▁by ▁the ▁metric ▁system . ▁They ▁are ▁still ▁used ▁for ▁some ▁applications ▁in ▁the ▁United ▁Kingdom ▁but ▁have ▁been ▁mostly ▁replaced ▁by ▁the ▁metric ▁system ▁in ▁commercial , ▁scientific , ▁and ▁industrial ▁applications . ▁US ▁custom ary ▁units , ▁however , ▁are ▁still ▁the ▁main ▁system ▁of ▁measurement ▁in ▁the ▁United ▁States . ▁While ▁some ▁steps ▁towards ▁metric ation ▁have ▁been ▁made ▁( main ly ▁in ▁the ▁late ▁ 1 9 6 0 s ▁and ▁early ▁ 1 9