The Museum of HP Calculators


The first of the 20 series was essentially an HP-35 in a smaller, less expensive package. Since the HP-21 had 33% more ROM than the HP-35 it had room for a few extra features, namely:

The HP-21 also implemented other changes that appeared in post HP-35 calculators. For example, it provided yx rather than xy and the CLR key left the storage register intact.

Front view (~47K)
Three quarter view (~79K)

Manual Available

HP-21 features

Technology and Packaging

Dimensions and Weight
Price: $125

Introduction-Discontinuation: 1975-1978

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