美国国旗（英語：Flag of the United States）是美利坚合众国的国旗。该旗的旗面由13道红白相间的宽条构成，左上角还有一个包含了50颗白色小五角星的藍色長方形。50顆小星代表了美國的50個州，而13条間紋则象征着美国最早建国时的13個殖民地。紅色象征勇气，白色象征真理，蓝色則象征正义。
这面旗帜俗称“星条旗”（Stars and Stripes）、中文俗稱「花旗」，正式名稱「合众国旗」（The Flag of the United States）或美國旗（American Flag）。它在正式成为美国国旗后曾经过26次修改。最早期的美国国旗只有13颗星，之后每一个州加入合众国就在国旗上加上一颗星，但是宽条的数目不变。根据1818年4月4日通过的《国旗法案》（Flag Act of 1818），只能在7月4日美国独立日这一天对国旗作出更改。1959年8月21日夏威夷正式成为美国的一个州后，次年國慶日，国旗上的49顆星被改為50顆——這也是美國國旗距今最近的一次修改。
^Note that the flag ratio (B/A in the diagram) is not absolutely fixed. Although the diagram in Executive Order 10834 gives a ratio of 1.9, earlier in the order is a list of flag sizes authorized for executive agencies. This list permits eleven specific flag sizes (specified by height and width) for such agencies: 20.00 × 38.00; 10.00 × 19.00; 8.95 × 17.00; 7.00 × 11.00; 5.00 × 9.50; 4.33 × 5.50; 3.50 × 6.65; 3.00 × 4.00; 3.00 × 5.70; 2.37 × 4.50; and 1.32 × 2.50. Eight of these sizes conform to the 1.9 ratio, within a small rounding error (less than 0.01). However, three of the authorized sizes vary significantly: 1.57 (for 7.00 × 11.00), 1.27 (for 4.33 × 5.50) and 1.33 (for 3.00 × 4.00).
^Ex. ord. No. 10834, August 21, 1959, 24 F.R. 6865 (governing flags "manufactured or purchased for the use of executive agencies", Section 22). Section 32 explains that, "As used in this order, the term "executive agencies" means the executive departments and independent establishments in the executive branch of the Government, including wholly owned Government corporations."
^In the 9th edition of the Standard Color Card of America, "White", "Old Glory Red", and "Old Glory Blue" were, respectively, Cable No. 70001, Cable No. 70180, and Cable No. 70075. The Munsell renotation coordinates for these were taken directly from the Reimann et al. paper, the CIELAB D65 coordinates were found by converting the xyY values in that paper to be relative to CIE Illuminant D65 from Illuminant C using the CAT02 chromatic adaptation transform, and relative to a perfect diffuse reflector as white. The "relative" values in the table were found by taking Cable No. 70001’s luminosity to be that of the white point, and were converted to D65 or D50 also using the CAT02 transformation. The values for CMYK were found by converting from the CIELAB D50 values using the Adobe CMM and the GRACoL 2006 ICC profile in Adobe Photoshop.
^Helen Taylor, Lucille Knoche, and Walter Granville, eds. (1950), Descriptive Color Names Dictionary. Container Corporation of America.
^The values in this table were taken by converting the CIELAB D50 coordinates for PMS 193 C, 200 C, and 281 C given in the Pantone color palettes of Adobe Photoshop to sRGB and GRACoL using the Adobe CMM and the GRACoL 2006 and sRGB profiles in Photoshop. Since PMS 281 C (blue) is outside the gamut of sRGB, and both PMS 281 C and PMS 183 C (red) are outside the gamut of GRACoL, the RGB and CMYK values for those in the table below are necessarily less-colorful approximations to the colors of the Pantone swatches.
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