Weexperimentallydemonstrateextractionofsiliconwaveguidegeometrywithsubnanometeraccuracyusingopticalmeasurements.Effectiveandgroupindicesofsilicon-on-insulator(SOI)waveguidesareextractedfromtheopticalmeasurements.AnaccuratemodellinkingthegeometryofanSOIwaveguidetoitseffectiveandgroupindicesisusedtoextractthelinewidthsandthicknesseswithinrespectiveerrorsof0.37and0.26nmonadiefabricatedbyIMECmultiprojectwaferservices.Adetailedanalysisofthesettingoftheboundsfortheeffectiveandgroupindicesispresentedtogettherightextractionwithimprovedaccuracy.