Rule 1 — the FBAR (the low, $10,000 trigger)
If you're a US person and the combined peak value of all your foreign financial accounts was more than $10,000 at any single moment during the year, you must file an FBAR (FinCEN Form 114). Two things trip people up: it's the combined total across all accounts (three $4,000 accounts = $12,000, over the line), and it's the highest value at any point, not the year-end balance.
Rule 2 — FATCA Form 8938 (higher thresholds, filed with your taxes)
Form 8938 is filed with your Form 1040 and only kicks in at higher amounts, which depend on where you live and your filing status:
Living in the US: single (or married filing separately) — over $50,000 on the last day of the year, or over $75,000 at any time. Married filing jointly — $100,000 / $150,000.
Living abroad (you're a bona fide resident of a foreign country all year, or you were physically present in a foreign country 330+ days in a 12-month period): single — over $200,000 / $300,000. Married filing jointly — $400,000 / $600,000.
All of these are "more than" figures — landing exactly on the number doesn't trigger the form.
The trap: they don't cancel each other out
Filing Form 8938 does not satisfy your FBAR obligation, and vice-versa. They go to different agencies. Plenty of people have to file both — for example, a US resident with $80,000 in a Spanish account crosses the FBAR ($10,000) and the single US 8938 last-day threshold ($50,000), so both apply.
Why it's worth getting right
The penalties are wildly out of proportion to the effort. A non-willful FBAR penalty runs up to roughly $16,500; Form 8938 failure-to-file is $10,000 (with more piling on after IRS notice). Both forms are free to file. If you've missed prior years, the IRS streamlined filing procedures exist for non-willful cases — worth asking a cross-border professional about before you file.
Check which forms you owe → Free checker: enter where you live, your status, and your account values — see FBAR and Form 8938 at once.