Friday, April 7, 2017

Retro Review: Hewlett Packard HP 20S and 21S

Retro Review:  Hewlett Packard HP 20S and 21S

Company:  Hewlett Packard
Type:  Scientific Programming
Memory:  9 Registers, 99 Programing Steps, 6 Pre-loaded programs
Years:  1988? – 1993? (21S); 1988 – 2002(?) (20S), original price was around $50
Operating System:  Algebraic
Batteries:  3 x LR44

HP 21S:  The Rarer Cousin of the HP 20S

The HP 21S is an algebraic, keystroke programming calculator.  I paid $35, which is not bad considering the keyboard and the display are in supreme quality, the keys are a pleasure to touch, and the learning curve is easy to operate the calculator.   

The HP 21S was originally released by Hewlett Packard along with the more familiar cousin the HP 20S.   Both had a great dark brown keyboard with white font for primary labels, orange and blue for shift key fonts (my favorite!).  This design has been used several Hewlett Packard calculators from the late 1980s/early 1990s, including the HP 32SII and the HP 48 SX.  Sometime during the 1990s, the HP 20S garnered a purple/green shift font scheme, to match the HP 48 G series. 

What the 20S and 21S Have in Common

Both are algebraic programming calculators.  Each calculator has 10 memory registers (R0 through R9) with storage arithmetic available (STO+, STO-, STO*, and STO÷).  Both models have the standard array of scientific functions including trigonometric, logarithmic, exponential, combination, permutation, integer and fraction parts, and decimal/hours-minutes-seconds conversions.  However, there is no fraction mode and all numbers are real numbers. 

One thing is to consider is that the factorial function (n!) only accepts positive integers. 



 It does take a little getting used to working the [INPUT] key, especially for those who work with HP calculators, since we’re used to RPN.  The INPUT key stores the number in a temporary “x” slot.  The INPUT key places a role in several functions:
Rectangular to Polar Conversion:  x, [INPUT], y, [left shift], [STO] (>P) 
(θ is displayed, r is stored)
Polar to Rectangular Conversion:  r, [INPUT], θ, [right shift], [STO] (>R) 
(y is displayed, x is stored)
Combinations/Permutations:  n, [INPUT], r, [right shift], [ 0 ] (combination) or [ . ] (permutation)
Percent Change: 
HP 20S:  old, [INPUT], new, [left shift], [ 1/x ] (%CHG)
HP 21S:  old, [INPUT], new, [right shift], [ 5 ] (%CHG)



Both the HP 20S and HP 21S have one variable, one weighted variable, two variable statistics, and linear regression.  There is no separate mode to be entered, just enter data with the [ Σ+ ] and go.  Upon entering and clearing data though, the following registers become holders of statistical sums:

R4 = n

R5 = Σx

R6 = Σy

R7 = Σx^2

R8 = Σy^2

R9 = Σxy

What is really nice is that both models have the sums indicated in gray at the bottom right hand corner of the keys as reminders.

Both the HP 20S and the HP 21S are keystroke programmable.  Both allow for labels A – F, 0 – 9.  In addition there are two tests:  x=0? and x≤y?.

The x≤y? Test:  x, [INPUT]*, y, [right shift], [ 7 ] (x≤y?).   A way to remember this command is:  hidden value ≤ displayed valued?

* According to the manuals, any other arithmetic/pending operation can be used such as [ + ] and [ ÷ ] are also allowed.

Each digit takes a step.  For example, 250 takes three steps, one for the 2, the 5, and the 0.

Differences between the HP 20S and HP 21S

Scientific Operations

HP 20S
HP 21S
Hyperbolic functions, base conversions (binary, octal, hexadecimal, binary), metric/US conversions (kg/lb, °C/°F, cm/in, l/gal)
Upper tail areas and inverse (z, t, F, Chi-squared), random numbers, seeding

Preloaded Programs


The HP 20S and HP 21S have six-preloaded programs.  The can be loaded by the key sequence [left shift], [ ← ] (LOAD).  While the HP 21S programs can be loaded at any time, the HP 20S must be in programming mode to load programs.


Unfortunately, user-created programs cannot be stored into permanent memory which would have been nice.


Preloaded Programs HP 20S


Root Finder
62 steps
Numerical Integration
58 steps
Complex Arithmetic
75 steps
3 x 3 Matrices
98 steps
Quadratic Equation
65 steps
Curve Fitting (logarithmic, exponential, power)
77 steps


Preloaded Programs HP 21S


One Sample Stat Tests
82 steps
Two Sample Stat Tests
82 steps
Linear Regression Stat Tests (used with One Sample Tests)
86 steps
Chi Square Tests
53 steps
Binomial Distribution
39 steps
Time Value of Money Solver (Finance)
99 steps


Final Verdict

Do I recommend buying the HP 21S and/or the HP 20S?  Assuming the price is reasonable, Yes, Yes, and Yes!  It is shame that the HP 20S and HP 21S are no longer produced because these are nice calculators.

The original price was around $45 - $55.    

Let’s get to the features.  I think it would be appropriate comparing the features of both the HP 21S and HP 20S at the same time. 


This blog is property of Edward Shore, 2017


  1. Hi, Eddie, thanks for your blog entries. I have a 20S and I really like it, but unfortunately it has a serious data input bug. Check it on HPMuseum, if interested:



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