16. WHITEHALL, PALM BEACH, FLA: 60,000 square feet. Henry Flagler, a co-founder of Standard Oil who built this mansion 1902, died after falling down a flight of marble stairs in the home at age 83.
15. THE BREAKERS, NEWPORT, RI: 62,482 square feet. The mansion, completed in 1895, was built as the summer home of Cornelius Vanderbilt II.
14. MAR-A-LAGO, PALM BEACH, FL: 62,500 square feet. The estate was built by Marjorie Merriweather Post in 1924, It's currently owned by the Trump Organization and run as a private club.
13. GREY TOWERS CASTLE, GLENSIDE, PA: 66,341 square feet. Built in 1893 by William Welsh Harrison, a co-owner of the Franklin Sugar Refinery, the building is now part of the campus of Arcadia University.
12. HEARST CASTLE, SAN SIMEON, CA: 60,645 square feet. This castle, built for newspaper magnate William Randolph Hearst and completed in 1947, has 56 bedrooms and 61 bathrooms.
9. (tie) IDLE HOUR, OAKDALE, NY: 70,000 square feet. Originally built by William K. Vanderbilt, the mansion served for decades as the home of Dowling College, which closed in August 2016.
9. (tie) LYNNEWOOD HALL, ELKINS PARK, PA: 70,000 square feet. The grounds of this 110-room, Gilded Age estate were used to train military dogs during World War II.
9. (tie) WOODLEA, BRIARCLIFF MANOR, NY: 70,000 square feet. Originally built as a private residence, the mansion has been home to Sleepy Hollow Country Club since it was founded in 1911.
8. INDIAN NECK HALL, OAKDALE, NY: 75,000 square feet. Built as the country home of Singer Sewing Machine Co. president Frederick Gilbert Bourne, the property was acquired by St. John's University in 2001.
7. FLORHAM, FLORHAM PARK, NJ: 80,000 square feet. Built in 1997 for businessman Hamilton McKown Twombly, the mansion is now part of Fairleigh Dickinson University's Florham Campus.
6. MEADOW BROOK HALL, ROCHESTER HILLS, MI: 88,000 square feet. Scenes from "Transformers 3" were filmed at this estate, built by the wife of auto pioneer John Francis Dodge.
5. SHADOW LAWN, WEST LONG BRANCH, NJ: 90,000 square feet. Built for Woolworth Co. president Hubert T. Parsons in 1927, the building was purchased by Monmouth University and renamed Woodrow Wilson Hall.
4. WINTERTHUR, WINTERTHUR, DE: 96,582 square feet. This 175-room house, once the home of Henry Francis du Pont, is now a museum that houses around 85,000 objects.
3. ARDEN HOUSE, HARRIMAN, NY: 97,188 square feet. Once owned by railroad magnate Edward Henry Harriman, the home was purchased in 2011 by a Chinese-backed nonprofit, the Research Center on Natural Conservation.
2. OHEKA CASTLE, HUNTINGTON, NY: 109,000 square feet. Completed in 1915, this Long Island mansion was built as the country home of financier and philanthropist Otto Kahn. Now a hotel and wedding destination.
1. BILTMORE ESTATE, ASHEVILLE, NC: 175,000 square feet. This 8,000-acre estate is still owned by one of builder George Washington Vanderbilt II's descendants.
