Features in optional ROMs are listed as Option. 

Display and Keyboard  HP55  HP25 
Display type  LED  LED 
Display format  1 Line  1 Line 
FIXed number format  Yes  Yes 
SCIentific number format  Yes  Yes 
ENGineering number format    Yes 
STanDard / ALL number format     
Alpha messages and prompts     
Softkeys labeled by mag. card or overlay     
Softkeys labeled by display menus     
Softkeys with neither labeling system     
Status annunciators     
Redefinable keyboard     
Logic  HP55  HP25 
Logic system  RPN  RPN 
Stack levels  4  4 
Last x  Yes  Yes 
Undo     
Precedence     
Parentheses     
Storage Addition (STO +)    Yes 
Storage Arithmetic (STO +, , ×, ÷)    Yes 
Storage Exponentiation (STO ^)     
Recall Arithmetic     
Named variables     
Multiple data types     
Directories     
Math  HP55  HP25 
+, , ×, ÷  Yes  Yes 
1/x  Yes  Yes 
sq. root  Yes  Yes 
x^{2}  Yes  Yes 
LOG x  Yes  Yes 
10^{x}  Yes  Yes 
LN x  Yes  Yes 
e^{x}  Yes  Yes 
y^{x}  Yes  Yes 
pi  Yes  Yes 
%  Yes  Yes 
% change     
INTeger truncation and ABSolute value    Yes 
Trigonometric functions (SIN, COS etc.)  Yes  Yes 
Hyperbolic trig. (SINH, COSH etc.)     
# of trig. modes (degrees, radians, grads)  3  3 
Advanced Math  HP55  HP25 
Solver / root finder     
Numeric integration     
Complex numbers     
Matrix operations     
Matrix writer     
Equation writer     
Symbolic operations     
Fractions     
Plotting and graphics (display)     
Plotting and graphics (printer / plotter)     
Statistical  HP55  HP25 
Mean, standard deviation (# of variables)  2  1 
Summation of n, x, x^{2}  Yes  Yes 
Summation of n, x, x^{2}, y  Yes  Yes 
Summation of n, x, x^{2}, y, xy  Yes  Yes 
Summation of n, x, x^{2}, y, y^{2}, xy  Yes   
Summation of lists     
Random number generator     
Linear regression, estimate  Yes   
Curve fitting     
Correlation coefficient     
Normal distribution     
Factorial  Yes   
Gamma     
Combinations and permutations     
Scientific and Time  
Polar / rectangular conversions  Yes  Yes 
Base conversions (2, 8, 10, 16)     
Arithmetic in bases 2, 8 and 16     
Bit operations (AND, OR, XOR, ...)     
Selectable word size (164 bits)     
Selectable complement mode (1's, 2's, unsigned)     
Decimal hrs / hrs.min.sec. conversions  Yes  Yes 
Hrs.min.sec arithmetic  Yes   
Time of day clock     
Timer  Yes   
Dynamic calculations (timer × rate)     
Alarms     
Unit Conversions  
Centimeters / inches     
Millimeters / inches  Yes   
Meters / feet  Yes   
Kilograms / pounds (mass)  Yes   
Newtons / pounds (force)  Yes   
British thermal units / joules  Yes   
Liters / gallons  Yes   
° Celsius / ° Fahrenheit  Yes   
Other     
Userdefined units     
Time Value of Money  HP55  HP25 
Number of payments (n)     
Interest rate (i)     
Present Value (PV)     
Payment (PMT)     
Future Value (FV)     
Cash Flow Analysis  HP55  HP25 
Net present value (NPV)     
Internal rate of return (IRR)     
Amortization  HP55  HP25 
Accumulated Interest (ACC INT)     
Balance (BAL)     
Depreciation  HP55  HP25 
Straight line (SL)     
Sum of the years digits (SOYD)     
Declining balance (DB)     
Bonds and calendar functions  HP55  HP25 
Bond price     
Bond yield     
Date arithmetic / calendar functions     
Memory and Programming  HP55  HP25 
Programming language(s)  RPN  RPN 
RAM / Max # of variables or addressable registers  20 Registers  8 Registers 
Max # of program steps  50  49 
RAM / Max vars or regs with full optional RAM     
Max # of prog steps with full optional RAM     
Merged program steps   
Yes 
Manual register / program memory allocation     
Automatic register / program memory allocation   
 
Overlapped register and program memory     
Continuous memory   
 
Flags     
Conditional tests  2  8 
Subroutine levels     
Indexed looping (increment by +/ 1, end at 0)     
Indexed looping (choice of increment and end)     
Multiple index / loop registers     
Line number addressing    Yes 
Branching by relative line number     
Global program label addressing     
Local program label addressing     
Indirect / computed addressing and control     
Multiple indirection registers     
Pause    Yes 
Alpha messages and prompts in programs     
Alpha program listings     
Alpha string manipulation     
Program step insert / delete     
I/O and Expansion Features  HP55  HP25 
ROM modules     
RAM modules or expansion option     
General expansion slots / connector     
Magnetic card reader/writer     
Tape drive     
Disk drive     
Upgradeable firmware     
Optical wand     
Serial port     
Infrared input     
Infrared output     
Printer     
Networking     
Other     
General  HP55  HP25 
Sounds     
Self test     
Hard case (in addition to soft case)  Yes   
Additional Features     
Batteries  Rechargeable  Rechargeable 
Length (inches)  5.8"  5.1" 
Width  3.2"  2.7" 
Height  1.3"  1.2" 
Weight (oz or lbs)  8.7 oz  6 oz 
Year of introduction  1975  1975 
Features in optional ROMs are listed as Option. 
HP55 notes:

HP25 notes:

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