Staytill For British visitors the US limit is short and firm: ninety days on an ESTA, no extension, no change of status. The trap is the side trip north or south, which does not give you a fresh start.
Under the Visa Waiver Program you are admitted for a stay of up to 90 days without a visa.12
Leaving for a week in Toronto or Cancún feels like leaving the US. For the count, it is a pause. If you leave for a contiguous territory or adjacent island, you may be readmitted for the balance of your original admission period.8
New York, a week in Toronto, and back
A Visa Waiver Program visitor is in a class that cannot change to another nonimmigrant status, except to U status.1011
The balance of your first admission is the number to watch after any trip across a land border. It is only visible if you kept the days.
Staytill, the iPhone app that counts your days in the US and how many you have left Visa Waiver Program: how long you can stay in the USSubstantial presence test: the IRS 183-day formula
How long can I stay in Canada? 
How long can I stay in Mexico? 
How long can I stay in the UK? stays of up to 90 days without a visa for a period not exceeding 90 days The Department requires all prospective VWP travelers to obtain pre-travel authorization via U.S. Customs and Border Protection’s (CBP) Electronic System for Travel Authorization (ESTA) prior to boarding a plane or ship bound for the United States. U.S. Customs and Border Protection officers determine admissibility upon travelers' arrival. to contest, other than on the basis of an application for asylum, any action for removal of the alien The United Kingdom refers only to British citizens who have the unrestricted right of permanent abode in the United Kingdom (England, Scotland, Wales, Northern Ireland, the Channel Islands, and the Isle of Man) Canadian visitors can usually stay in the United States for 6 months without a visa. may be readmitted to the United States after a departure to foreign contiguous territory or adjacent island for the balance of his or her original Visa Waiver Program admission period A nonimmigrant in any of the following classes is ineligible for an extension of stay: (i) B-1 or B-2 where admission was pursuant to the Visa Waiver Pilot Program Any alien admitted as a Visa Waiver Pilot Program visitor under the provisions of section 217 of the Act and part 217 of this chapter. The prohibition against a change of nonimmigrant status for the categories of aliens described in paragraphs (a)(1) through (6) of this section is inapplicable to aliens applying for a change of nonimmigrant status to that of a nonimmigrant under section 101(a)(15)(U) If an emergency prevents an alien admitted under this part from departing from the United States within his or her period of authorized stay grant a period of satisfactory departure not to exceed 30 days 31 days during the current year, and 183 days during the 3-year period that includes the current year and the 2 years immediately before that such individual was present in the United States on at least 31 days during the calendar year equals or exceeds 183 days You will be considered a United States resident for tax purposes if you meet the substantial presence test for the calendar year. Know a rule that changed, or a line we got wrong? Send us the source. We fix the page and log it.
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