The fiance visa is an unusual one. Although technically it's a nonimmigrant visa (which lasts only 90 days), its purpose is not only to allow the K-1 visa holder to enter the U.S. and marry the U.S. citizen sponsor, but to allow the immigrant to apply for a green card if desired.
So, while the financial standards for most nonimmigrant (temporary) visas require simply showing that the applicant has enough money to cover his or her time in the United States, fiance visa applicants must show a bit more. It depends in part on the consular officer conducting the visa interview. Each consulate has a lot of discretion in evaluating the income needed for a fiancé visa to be approved.
Fiance visa applicants are often asked to show that their sponsor is able to support them at 100% of the federal Poverty Guidelines.These guidelines are published annually and can be found on Form I-864P, published by U.S. Citizenship and Immigration Services (USCIS). The sponsor may be asked to fill out an Affidavit of Support on USCIS Form I-134 (which is much shorter than the I-864 Affidavit that sponsors of immigrants for green cards must fill out).
However, the information provided on Form I-134 or otherwise will stay in the applicant's file. And, within a mere few months of arriving in the U.S., the applicant should have married and may have chosen to apply for a U.S. green card. So, it is to the fiancés’ advantage if the sponsor can show an income level at or above the 125% guidelines. (Showing anything less at this point may raise questions during the eventual green card interview.) In addition, some consular officers are particularly strict in dealing with fiancé visas, and consider the 125% level that will be required of the applicant.
Other officers may simply apply the “eyeball” test for fiancés -- if the applicant looks young and healthy, that's enough to justify granting the K-1 fiance visa (which is, after all, good for only 90 days and comes only with the requirement that the couple marry, not that the immigrant apply for a green card -- he or she can always choose to return home after the wedding).
If you are having trouble meeting the consulate’s standards for getting a fiancé visa, one option is to have another family member or friend in the United States agree to serve as a joint sponsor. A joint sponsor is someone who agrees to share responsibility for the K-1 visa holder's financial support with the primary sponsor, up to the full amount of the support. In other words, if the U.S. sponsor can’t support the K-1 visa holder, the joint sponsor can be held 100% responsible for the support (and is not allowed to merely choose a percentage or limit on how much to pitch in). Joint sponsors can indicate their willingness to help by signing a separate Affidavit of Support on Form I-134.
Both the advantage and the disadvantage of the Form I-134 Affidavit of Support is that it is not considered to have much legal weight. In other words, although a U.S. government agency could take a sponsor to court to enforce it, they never seem to do so. Perhaps they realize they wouldn’t win. That’s an advantage because makes it easier to persuade someone to sign the form, without endangering that person's financial future. It’s also a disadvantage: Because the consular officials know that the affidavit is hardly worth the paper it’s written on, they may disregard the joint sponsor. For this reason, if you go the joint sponsor route, you might want to supplement the person’s affidavit with a letter or a sworn statement expressing his or her deep commitment to supporting the visa applicant.