The difference between a jury and a grand jury comes down to timing and purpose: a grand jury meets early in a criminal case to decide whether a prosecutor has enough evidence to formally charge someone, while a trial jury (sometimes called a petit jury) meets later, after charges are filed, to decide whether the defendant is guilty. One screens; the other judges. They also work under different rules, with different sizes, different standards of proof, and very different levels of secrecy.
What a Trial Jury Decides
A trial jury is the group of citizens who sit through a trial and decide the facts. In a criminal trial, their job is to determine whether the prosecution has proven the defendant’s guilt beyond a reasonable doubt, the highest standard of proof in the legal system.1United States Courts. Types of Juries That standard means the evidence must leave jurors firmly convinced of guilt, not just thinking it’s likely.2Cornell Law School / Legal Information Institute. Beyond a Reasonable Doubt
In a civil case, the bar is lower. A civil jury decides whether the plaintiff has shown, by a preponderance of the evidence, that the defendant caused harm. That standard essentially means “more likely than not.”1United States Courts. Types of Juries If the plaintiff meets that burden, the jury determines liability and often sets the amount of monetary damages.
The proceeding itself is adversarial. A prosecutor or plaintiff’s attorney and a defense attorney each present evidence, call witnesses, cross-examine the other side’s witnesses, and argue before a judge. The jury’s deliberation happens in private, but the trial is almost always open to the public. At the end, the jury delivers a verdict: guilty or not guilty in a criminal case, or for the plaintiff or defendant in a civil one.
What a Grand Jury Decides
A grand jury operates much earlier in the criminal process. Its job is not to decide guilt but to determine whether enough evidence exists to formally charge someone with a serious crime. It stands between a prosecutor and a citizen, making sure nobody faces a public trial based on weak or unfounded accusations.3Legal Information Institute. Grand Jury
The standard is probable cause, which is far lower than beyond a reasonable doubt. The grand jury only needs a reasonable basis to believe a crime was committed and that the suspect committed it. If it reaches that conclusion, it issues a formal charging document called an indictment. If the evidence falls short, it returns what’s known as a “no bill,” and the case typically stalls there.
Who’s in the Room
The two bodies feel nothing alike from the inside. Grand jury proceedings are entirely one-sided. Only the prosecutor presents evidence and calls witnesses. There is no defense attorney making objections, no judge presiding over the evidence, and no cross-examination. The prosecutor’s role is to advise the grand jury on the law and present evidence for its consideration.4United States Department of Justice. JM 9-11.000 – Grand Jury
Witnesses do retain the Fifth Amendment right to refuse to answer questions that would incriminate them. The target of an investigation has no legal right to testify before the grand jury, though federal prosecutors will generally allow it if the person requests it and waives the privilege against self-incrimination. Even then, the target’s attorney cannot come into the grand jury room; the witness can only step outside periodically to consult with their lawyer in the hallway.4United States Department of Justice. JM 9-11.000 – Grand Jury
Everything that happens inside a grand jury room is secret. Grand jurors, court reporters, interpreters, and prosecutors are all forbidden from disclosing what occurred during the proceedings.5Cornell Law School. Federal Rules of Criminal Procedure Rule 6 Records and subpoenas are kept sealed. The secrecy protects the reputation of someone who is investigated but never charged, encourages witnesses to speak freely without fear of retaliation, and prevents targets from learning about the investigation and destroying evidence or fleeing.
Trial proceedings are the opposite. The Sixth Amendment guarantees criminal defendants the right to a public trial, and courtrooms are generally open to the press and public.6Legal Information Institute. Sixth Amendment Only the jury’s private deliberations are shielded from view.
Size, Voting Rules, and Length of Service
The mechanical differences follow from the different roles.
A trial jury is small. Federal civil and criminal juries have between 6 and 12 members, with 12 being standard for serious criminal cases.1United States Courts. Types of Juries A federal grand jury is larger, with between 16 and 23 members.7Office of the Law Revision Counsel. 18 USC 3321 – Number of Grand Jurors; Summoning Additional Jurors That larger size makes sense because a grand jury reviews many cases over its term rather than sitting for a single trial.
Voting rules differ sharply. In criminal trials, the jury’s verdict must be unanimous. The Supreme Court confirmed in Ramos v. Louisiana (2020) that this unanimity requirement applies to all criminal trials in both federal and state courts.8Constitution Annotated. Amdt6.4.4.3 Unanimity of the Jury A grand jury does not need unanimity. In federal court, an indictment requires at least 12 grand jurors to agree, regardless of how many are seated.5Cornell Law School. Federal Rules of Criminal Procedure Rule 6
Length of service also diverges. Trial jurors hear one case, which might take a day or several weeks, and are then dismissed.1United States Courts. Types of Juries A federal grand jury serves until the court discharges it, up to a maximum of 18 months. Courts can extend that term by up to six additional months if the public interest requires it.5Cornell Law School. Federal Rules of Criminal Procedure Rule 6
Investigative Powers a Trial Jury Doesn’t Have
Grand juries have investigative tools that trial juries lack entirely. A grand jury can issue subpoenas to compel witnesses to testify and to force the production of documents, business records, and other evidence. These subpoenas can be served anywhere in the United States. A witness who refuses to testify can be held in contempt and jailed until they comply, with sanctions lasting for the life of the grand jury.4United States Department of Justice. JM 9-11.000 – Grand Jury
Trial juries have no such powers. They receive the evidence the parties present in court, and that’s it. They don’t call witnesses, demand records, or direct an investigation.
When Each One Applies
The Fifth Amendment requires federal prosecutors to obtain a grand jury indictment before bringing felony charges. That requirement has never been extended to the states. The Supreme Court held more than a century ago that the grand jury clause is one of the few Bill of Rights provisions not incorporated against state governments through the Fourteenth Amendment.9Library of Congress. Grand Jury Clause Doctrine and Practice
As a result, roughly half the states require grand jury indictments for felonies, while the other half allow prosecutors to file charges through a document called an “information,” often after a preliminary hearing. In a preliminary hearing, a judge rather than a group of citizens reviews the prosecution’s evidence and decides whether probable cause exists to send the case to trial. Unlike a grand jury proceeding, a preliminary hearing is open to the public, the defendant is present, and the defense attorney can cross-examine witnesses.
So the grand jury experience varies dramatically depending on where a case is prosecuted. In a federal case, a grand jury is virtually guaranteed. In many state courts, charges may move forward without one. The right to a trial jury, by contrast, applies in both federal and state courts under the Sixth Amendment and covers all non-petty criminal offenses.6Legal Information Institute. Sixth Amendment
What Happens When the Jury Can’t Agree
If a trial jury cannot reach a unanimous verdict after deliberation, the result is a hung jury. The judge typically declares a mistrial, and the case does not end with either a conviction or an acquittal.10LII / Legal Information Institute. Hung Jury
Double jeopardy, the constitutional protection against being tried twice for the same crime, does not apply when a mistrial results from a hung jury. The prosecution can choose to retry the case with a new jury, negotiate a plea, or drop the charges entirely. Retrials after hung juries are common in serious cases.10LII / Legal Information Institute. Hung Jury
Grand juries don’t deadlock in the same way. Because they don’t need unanimity, they rarely fail to reach a result. If a grand jury declines to indict, the prosecutor can sometimes present the case to a different grand jury later.