Document and Entity Information Document - USD ($)	12 Months Ended
	Dec. 28, 2019	Feb. 14, 2020	Jun. 29, 2019
Cover page.
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 28, 2019
Document Transition Report	false
Entity File Number	000-50307
Entity Registrant Name	FormFactor, Inc.
Entity Central Index Key	0001039399
Amendment Flag	false
Current Fiscal Year End Date	--12-28
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	13-3711155
Entity Address, Address Line One	7005 Southfront Road
Entity Address, City or Town	Livermore
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94551
City Area Code	925
Local Phone Number	290-4000
Title of 12(b) Security	Common stock, $0.001 par value
Trading Symbol	FORM
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 546,284,970
Entity Common Stock, Shares Outstanding (in shares)		76,148,088
Documents Incorporated by Reference	Portions of the registrant's definitive Proxy Statement for the 2020 Annual Meeting of Stockholders, which will be filed within 120 days of the end of the registrant's fiscal year ended December 28, 2019, are incorporated by reference in Part III hereof. Except with respect to information specifically incorporated by reference in this Annual Report on Form 10-K, the Proxy Statement is not deemed to be filed as a part of this Annual Report on Form 10-K.