What is the formula for counting days in the US for a foreign national?

In the U.S., a foreign national typically counts days of presence based on the Substantial Presence Test. This test includes all the days physically present in the U.S. during the current year, plus one-third of the days in the previous year, and one-sixth of the days in the year before that. If the total equals or exceeds 183 days, the individual is considered a resident for tax purposes. Certain exceptions apply, such as for specific visa types or short visits.