You do not combine your income with theirs
This is the central misunderstanding. A joint sponsor does not top up what you are missing: they must meet the entire requirement on their own. If you are $2,000 short, it is not enough for them to earn $2,000 more. They must reach 125% of the Federal Poverty Guidelines for their own household, counting the immigrant being sponsored.
You still sign your own I-864 even though you do not qualify. The joint sponsor signs a separate one.
Who can be one
A U.S. citizen or lawful permanent resident, at least 18 years old, domiciled in the United States. They do not need to be related to you or the immigrant — a friend, a coworker or a neighbor can serve.
Up to two joint sponsors can be used in a case, but each covers different immigrants. They do not split responsibility for the same person.
Count the immigrant in the joint sponsor's household
The most common error when evaluating a joint sponsor is forgetting to add the immigrant to their household. If that person lives alone, their household for this calculation is not one person but two: themselves and the immigrant. Sponsoring a family of four raises their household by four.
The military exception does not apply here
The I-864P table lowers the threshold to 100% for active-duty service members, but only for the sponsor who is petitioning for their spouse or child. A joint sponsor is not the petitioner, so the 125% threshold applies even if they are on active duty. That is why this calculator does not offer that option.
What that person is agreeing to
Worth stating plainly before you ask someone for this favor. A joint sponsor takes on a legally enforceable contract with the U.S. government. If the immigrant receives certain means-tested public benefits, the government can seek reimbursement from them. The obligation lasts until the immigrant becomes a citizen, earns 40 qualifying work quarters, permanently leaves the country, or dies.
It is not a formality. It is a responsibility that can last years.