A trial jury and a grand jury do two completely different jobs. A trial jury hears both sides of a case and decides guilt or liability. A grand jury meets in secret before any trial, hears only the prosecutor’s evidence, and decides whether there is enough probable cause to charge someone with a crime at all. The comparison between a trial jury vs a grand jury comes down to that split: one decides whether charges stick, the other decides whether charges get filed.
Everything else — size, voting rules, secrecy, who gets a lawyer in the room — follows from that single difference in purpose.
What Each One Decides
A trial jury, sometimes called a petit jury, sits through a case from opening arguments to closing. Jurors hear witnesses, review documents, weigh credibility, and receive instructions from the judge on the law they must apply. In a criminal trial, they decide guilt or innocence. In a civil case, they decide whether one party is liable to the other and how much money to award.
A grand jury never reaches those questions. It is an investigative body. Its job is to decide whether there is probable cause to believe a crime was committed and that a specific person committed it. If the grand jury finds probable cause, it returns an indictment known as a “true bill,” and the case moves toward trial. If it doesn’t, it returns a “no bill,” and the charges don’t go forward.
The standards of proof are nowhere close to each other. A criminal trial jury must be convinced beyond a reasonable doubt to convict. A grand jury only needs to find probable cause, which is a far lower bar and one of the reasons federal grand juries indict in the overwhelming majority of cases presented to them.
Who Is in the Room
Trial proceedings are public. The Sixth Amendment guarantees the right to a public trial, and the courtroom typically holds the judge, jury, prosecutor, defense attorney, defendant, witnesses, and often spectators and press.
Grand jury proceedings are the opposite. Only the jurors, the prosecutor, the witness currently testifying, an interpreter if needed, and a court reporter may be present while the grand jury is in session. No judge sits in the room. No defense attorney is allowed inside. When the grand jury deliberates or votes, everyone leaves except the jurors themselves.
Secrecy is deliberate. It encourages witnesses to speak freely, keeps potential defendants from fleeing or tampering with evidence, and protects the reputation of anyone the grand jury investigates but ultimately declines to charge.
Rights of the Accused
This is where the two systems diverge most sharply.
At trial, the Sixth Amendment gives the defendant a full set of protections: the right to be present, to have an attorney, to confront and cross-examine prosecution witnesses, to compel favorable witnesses to testify, and to know the charges and evidence. These rights exist because the defendant’s liberty is on the line and the proceeding is adversarial.
Grand jury targets get almost none of that. The person under investigation generally cannot be present in the grand jury room unless subpoenaed as a witness. Even then, their attorney must wait outside and cannot cross-examine anyone or present evidence. The grand jury hears only the prosecution’s case. That lopsided structure is by design: the grand jury’s role is not to adjudicate guilt but to filter out cases that lack enough evidence to justify a trial. It also explains why grand juries so rarely reject a prosecutor’s request for an indictment.
A witness called before a grand jury does keep the Fifth Amendment right against self-incrimination. If a truthful answer might incriminate the witness, they can refuse to answer. That protection applies whether the witness is the target of the investigation or a bystander.
Grand juries also have broad authority to compel cooperation. They can issue subpoenas requiring witnesses to testify or produce documents, including financial records held by third parties like banks. A witness who refuses to comply without a valid legal excuse can be held in contempt and jailed for the rest of the grand jury’s term.
Size, Votes, and How Long Each Sits
The mechanics of each body reflect what it is being asked to do.
- Size. A federal trial jury has 12 members by default, though the parties can agree in writing to fewer. State courts commonly use juries as small as six. A federal grand jury is much larger, with 16 to 23 members.
- Voting. A criminal trial jury must reach a unanimous verdict to convict, matching the beyond-a-reasonable-doubt standard. A federal grand jury needs only 12 of its members to agree that probable cause exists.
- Duration. Trial jury service lasts as long as the trial, from a single day to several weeks or, in complex cases, months. A federal grand jury serves for up to 18 months, with the possibility of a six-month extension if the court finds it is in the public interest. Grand jurors don’t sit every day; they typically convene on a set schedule, such as once or twice a week.
Trial jurors are also under strict rules about outside contact. They cannot discuss the case with anyone, including family. They cannot research it online, consult reference materials, or communicate about it through social media or text. If anyone tries to discuss the case with a juror, the juror must refuse to listen and report the contact to the judge.
How Jurors Are Chosen
Trial jurors are selected through voir dire, where the judge and attorneys question prospective jurors about potential biases. Both sides can remove jurors for cause when someone appears unable to be impartial, and each side also has a limited number of peremptory challenges that can be used without stating a reason, subject to the Supreme Court’s prohibition on removing jurors based on race or sex.
Grand jurors are drawn from the same community pool, but because the role is investigative rather than adversarial, there is no voir dire in the traditional sense. The defense plays no part in choosing who sits on a grand jury.
When a Grand Jury Is Required
The Fifth Amendment requires a grand jury indictment for anyone facing a “capital, or otherwise infamous crime” in the federal system. In practice, that covers virtually all federal felonies.
This requirement applies only in federal court. The Supreme Court has never extended it to the states through the Fourteenth Amendment, so each state sets its own rule. Roughly half of states require a grand jury indictment for at least some serious crimes, typically felonies. The remaining states allow prosecutors to bring felony charges by filing a document called an “information,” usually after a judge reviews the evidence at a preliminary hearing. A preliminary hearing is adversarial: the defense can be present, see the prosecution’s witnesses, and cross-examine them, which makes it a very different screening mechanism than the one-sided grand jury process.
In states where prosecutors can choose either path, grand juries tend to be used for complex investigations, organized crime cases, or politically sensitive matters where the prosecutor wants a community body to share responsibility for the charging decision.
If You’ve Been Summoned
Federal law uses the same baseline qualifications for grand and trial jurors. You must be a U.S. citizen, at least 18, a resident of the judicial district for at least a year, and able to read, write, and speak English well enough to follow the proceedings. Anyone with a pending felony charge or an unrestored felony conviction is disqualified, as is anyone whose mental or physical condition would prevent satisfactory service.
Courts can excuse jurors for hardship. Permanent excuses are sometimes available for people over 70, anyone who served on a federal jury within the past two years, and volunteer firefighters or rescue squad members. Temporary deferrals are granted at the court’s discretion when serving at that particular time would cause undue hardship.
Federal jurors are paid $50 per day. Trial jurors who serve more than 10 days on a single case can receive up to $60 per day at the judge’s discretion. Grand jurors get the same bump after 45 days of service. Federal jurors also receive a mileage allowance. State court pay varies dramatically, from nothing in a couple of states to $50 per day in the highest-paying jurisdictions, with most states falling well below the federal rate. Nearly every state prohibits employers from firing or retaliating against employees for serving, though most do not require employers to pay wages during the absence.
Ignoring a federal jury summons is a federal offense. The court can order you to appear and explain yourself, and if you can’t show good cause, you face a fine of up to $1,000, up to three days in jail, mandatory community service, or any combination of the three. Courts usually start with a show-cause order served by the U.S. Marshals before escalating. State penalties follow a similar pattern: a missed summons triggers a failure-to-appear notice, then a show-cause hearing, and potentially a contempt finding. The simplest response is to answer the summons promptly, even if you need to request a deferral or hardship excuse.