The SWAC (Standards Western Automatic Computer) was an early electronic digital computer built in 1950 by the U.S. National Bureau of Standards (NBS) in Los Angeles, California.

[1][self-published source] Like the SEAC which was built about the same time, the SWAC was a small-scale interim computer designed to be built quickly and put into operation while the NBS waited for more powerful computers to be completed (in particular, the RAYDAC by Raytheon).

It had only seven basic operations: add, subtract, and fixed-point multiply;[2] comparison, data extraction, input and output.

In January 1952, Raphael M. Robinson used the SWAC to discover five Mersenne primes—the largest prime numbers known at the time, with 157, 183,[5] 386, 664 and 687 digits.

Additionally, the SWAC was vital in doing the intense calculation required for the X-ray analysis of the structure of vitamin B12 done by Dorothy Hodgkin.