PGR vs. IPR: Differences in Timing, Grounds and Estoppel

Post-Grant Review vs. Inter Partes Review

Updated October 24, 2026 By Wysebridge

Post-grant review (PGR) must be filed within nine months of grant and allows almost any invalidity ground, but only for first-inventor-to-file patents. Inter partes review (IPR) opens after the nine-month window and is limited to anticipation and obviousness based on patents and printed publications. Both are decided by the PTAB in about 18 months and carry estoppel.

The short comparison

	Inter partes review	Post-grant review
Statute	35 U.S.C. §§ 311–319	35 U.S.C. §§ 321–329
Opens	Nine months after grant (or end of PGR)	Grant
Closes	One year after service of an infringement complaint, if sued	Nine months after grant
Eligible patents	Any patent	First-inventor-to-file patents (effective filing date on or after March 16, 2013)
Grounds	§§ 102 and 103, on patents and printed publications	Any § 282(b)(2)/(3) defense, such as §§ 101, 102, 103, 112
Institution standard	Reasonable likelihood of prevailing on at least one claim	More likely than not that at least one claim is unpatentable
Trial length	One year from institution, extendable six months	Same
Burden of proof	Preponderance of the evidence	Preponderance of the evidence
Estoppel	Grounds raised or reasonably could have been raised	Same wording, broader practical scope

How to choose

Is the window still open? If the patent was granted less than nine months ago and is eligible, PGR is available and is the only USPTO trial that can reach § 101 or § 112 issues.
What is your best argument? Strong prior art in patents or printed publications fits IPR. A written-description or eligibility argument needs PGR (or court).
How much risk of estoppel can you accept? Both bar later invalidity arguments. Think about what you would lose if you lost.
Is there litigation? If you were served, the one-year IPR bar is running, and parallel litigation can affect whether the Office institutes.

Read the full guides: inter partes review and post-grant review. For options beyond these two, see the comparison chart.

Frequently Asked Questions

Can I file IPR and PGR against the same patent?

Not at the same time. IPR may not be filed until the PGR window has closed or any PGR has ended, so the two are sequential rather than overlapping (35 U.S.C. § 311(c)).

Which has broader estoppel?

Both bar later assertions of grounds the petitioner raised or reasonably could have raised. Because PGR petitions can raise more kinds of grounds, the practical reach of PGR estoppel is wider.