Just add this as a bookmark to your browser:
javascript: (function() {function bR(a,b,c,d){return function(i){let f=i;let g;while((g=a.exec(i))!=null){let e=g[0].substr(0,g[0].length-b).replace(',','').replace('\u2212','-').trim();e.endsWith('-')&&(e=e.substr(0,e.length-1));let h=Math.round(c(e)*10)/10;isNaN(h)||(f=f.replace(g[0],' '+h+' '+d));}return f;};}function checkString(a){return a=iR1(iR2(iR3(iR4(iR5(iR6(a)))))),a=foR1(foR2(foR3(foR4(foR5(foR6(a)))))),a=yR1(yR2(yR3(yR4(yR5(yR6(a)))))),a=mR1(mR2(mR3(mR4(mR5(mR6(a)))))),a=faR1(faR2(a)),a;}function cC(e){let b=e.childNodes;for(let a=0;a<b.length;a++)if(b[a].nodeType===3){let c=b[a].nodeValue;let d=checkString(c);c!==d&&(b[a].nodeValue=d);}else b[a].tagName!=='SCRIPT'&&cC(b[a]);}let iR1=bR(/[0-9]+( |-|)in\./g,3,function(a){return a*2.54;},'cm');let iR2=bR(/[0-9]+( |-|)inch/g,4,function(a){return a*2.54;},'cm');let iR3=bR(/[0-9]+( |-|)inches/g,6,function(a){return a*2.54;},'cm');let iR4=bR(/[0-9]{1,3},[0-9]{3}( |-|)in\./g,3,function(a){return a*2.54;},'cm');let iR5=bR(/[0-9]{1,3},[0-9]{3}( |-|)inch/g,4,function(a){return a*2.54;},'cm');let iR6=bR(/[0-9]{1,3},[0-9]{3}( |-|)inches/g,6,function(a){return a*2.54;},'cm');let foR1=bR(/[0-9]+( |-|)ft\./g,3,function(a){return a*0.3048;},'m');let foR2=bR(/[0-9]+( |-|)foot/g,4,function(a){return a*0.3048;},'m');let foR3=bR(/[0-9]+( |-|)feet/g,4,function(a){return a*0.3048;},'m');let foR4=bR(/[0-9]{1,3},[0-9]{3}( |-|)ft\./g,3,function(a){return a*0.3048;},'m');let foR5=bR(/[0-9]{1,3},[0-9]{3}( |-|)foot/g,4,function(a){return a*0.3048;},'m');let foR6=bR(/[0-9]{1,3},[0-9]{3}( |-|)feet/g,4,function(a){return a*0.3048;},'m');let yR1=bR(/[0-9]+( |-|)yd\./g,3,function(a){return a*0.9144;},'m');let yR2=bR(/[0-9]+( |-|)yard/g,4,function(a){return a*0.9144;},'m');let yR3=bR(/[0-9]+( |-|)yards/g,5,function(a){return a*0.9144;},'m');let yR4=bR(/[0-9]{1,3},[0-9]{3}( |-|)yd\./g,3,function(a){return a*0.9144;},'m');let yR5=bR(/[0-9]{1,3},[0-9]{3}( |-|)yard/g,4,function(a){return a*0.9144;},'m');let yR6=bR(/[0-9]{1,3},[0-9]{3}( |-|)yards/g,5,function(a){return a*0.9144;},'m');let mR1=bR(/[0-9]+( |-|)mi\./g,3,function(a){return a*1.609344;},'km');let mR2=bR(/[0-9]+( |-|)mile/g,4,function(a){return a*1.609344;},'km');let mR3=bR(/[0-9]+( |-|)miles/g,5,function(a){return a*1.609344;},'km');let mR4=bR(/[0-9]{1,3},[0-9]{3}( |-|)mi\./g,3,function(a){return a*1.609344;},'km');let mR5=bR(/[0-9]{1,3},[0-9]{3}( |-|)mile/g,4,function(a){return a*1.609344;},'km');let mR6=bR(/[0-9]{1,3},[0-9]{3}( |-|)miles/g,5,function(a){return a*1.609344;},'km');let faR1=bR(/([−-]|)[0-9]+( |-|)[Dd]egree [Ff]ahrenheit/g,17,function(a){return(a-32)*(5/9);},'\xB0C');let faR2=bR(/([−-]|)[0-9]+( |-|)[Dd]egrees [Ff]ahrenheit/g,18,function(a){return(a-32)*(5/9);},'\xB0C');cC(document.body);})();Or use the file metricPleaseBookmarklet.js
It also works for feet, inches, miles and degrees Fahrenheit or ft., in. and mi.!