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A4 paper is 8.27 inches x 11.69 inches whereas US letter is 8½ inches by 11 inches.
How the paper system works is:
• 2 sheets of A4 equals A3,
• 2 sheets of A3 equals A2,
• 2 sheets of A2 equals A1.
• Half of A4 equals A5.
• Half of A5 equals A6.
• Half of A6 equals A7.
• Half of A7 equals A8.
I think it is an easy way to size paper.
All the Best!
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American to Britain