The IRS does not simply count the days you were here. It adds all of this year's days, a third of last year's and a sixth of the year before — and it ignores days you were on a student or trainee visa. Get it wrong and you file the wrong return.
Count every day you were physically present in the United States, including partial days. Travel days count.
Days on certain visas are exempt — they do not count at all. This is the step most calculators skip.
General guidance only, not tax or legal advice. The substantial presence test has exceptions this tool does not cover — the closer connection exception, treaty tie-breaker rules, days you could not leave for a medical condition that arose in the US, and days as a regular commuter from Canada or Mexico. Nothing you type leaves this browser.