US paper sizes

Loose Sizes
Paper sizes in North America do not have a consistent aspect ratio like the ISO standard does. They have their own system that they follow. This means that scaling the paper sizes is more difficult. The US Letter paper size is the most popular size used throughout the US. It is also commonly used in Chile and the Philippines.

ANSI
The American National Standards Institute (ANSI) adopted ANSI/ASME Y14.1, which defined a series of paper sizes in 1996, based on the standard 8.5 inches × 11 inches (216mm × 279mm) Letter size, which was named ‘ANSI A’. This series is fairly similar to the ISO standard, in that if you cut a sheet in half, you will product two sheets of the next smaller size. Ledger/Tabloid is known as ‘ANSI B’. The most common and widely used size is ANSI A, also known as ‘Letter’.

Architectural Sizes
The Architectural series (ARCH) is used by architects in North America, and they prefer to use this series instead of ANSI, because the aspect ratios are ratios of smaller whole numbers (4:3 and 3:2). The ARCH series of paper sizes is defined in the ANSI/ASME Y14.1 standard. The ARCH sizes are commonly used by architects for their large format drawings.