Document and Entity Information - shares	6 Months Ended
	Jun. 28, 2019	Jul. 26, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 28, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Entity Registrant Name	EXPONENT, INC.
Entity Central Index Key	0000851520
Current Fiscal Year End Date	--01-03
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Trading Symbol	EXPO
Entity Common Stock, Shares Outstanding		51,919,297
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Entity File Number	0-18655
Entity Tax Identification Number	770218904
Entity Address, Address Line One	149 COMMONWEALTH DRIVE
Entity Address, City or Town	MENLO PARK
Entity Address, State or Province	CALIFORNIA
Entity Address, Postal Zip Code	94025
City Area Code	650
Local Phone Number	(650) 326-9400