This on-line calculator operates on constructive real numbers. Numbers are represented exactly internally to the online calculator, and then evaluated on demand to guarantee an error in the displayed result that is strictly less than one in the least significant displayed digit. It is possible to scroll the display to the right to generate essentially arbitrary precision in the result.
The online calculator uses Reverse Polish Notation (RPN). To add 1 and 2, either click  [Enter]  [+] buttons on the calculator keypad.
How to use online calculator
The RPN online calculator emulator window is composed of several regions. From top to bottom they are.
The online calculator display. This displays numbers entered into the on-line RPN calculator and results. Newly entered or computed numbers appear toward the bottom of the display. If more numbers are entered than fit into the display, a scrollbar on the right can be used to move between them.
Calculator commands can be entered either by using the keypad (see below) or by typing with the display as the input focus.
Right-to-left scrolling control for the calculator display. Immediately below the display is a scrollbar and a small text window. Either can be used to control right-to-left scrolling of the display. At any point, the small text window displays the weight of the rightmost displayed digit. If the window reads -125, then the rightmost displayed digit is 125 digits to the right of the decimal point, and thus has weight 10125.
This value can be adjusted either by editing the text window, or by using the slide control. Note that very large negative values are likely to require substantial time and memory to update the display. (Values below -1000 are likely to produce annoying pauses. Values below -10,000,000 are likely to result in error messages, possibly after a very long pause.)
Large positive values are likely to result in all significant digits moving to beyond the right side of the display. This is indicated by a display entry of -->.
Note that the scrollbar has a comparatively limited range, and its resolution is far better near 0 than away from 0.
The pnline calculator keypad area. This includes a control panel area on the left, and the main calculator keypad on the right. The control panel area includes several buttons whose labels change in response to the state of the calculator. These are often useful simply for determining calculator state. The calculator history window. This includes recently entered commands, in the form in which they could have been typed on the computer keyboard. The text area is edit-able and, with the aid of the replay button, can be used to correct and replay previously entered commands. (On many platforms it also provides a somewhat clumsy mechanism for pasting in numbers from other sources which bypasses the restrictions on clipboard access from Java applets.)
Calculator keypad and keyboard commands
Most commands can be entered either by clicking a keypad button on the emulatorís screen, or by typing a single character on the computer keyboard. Here we list first the keypad label for the command, and then the equivalent keyboard characters for each command. They are listed approximately in the order in which they appear on the keypad.
1/x, % Replace the last entry in the display by its reciprocal. The entry must be nonzero. A zero entry will cause the calculator to run until it is explicitly stopped, exhausts memory, or exceeds an internal limit on requested precision. (The calculator cannot determine whether an entry is exactly zero. You will be asked if you want to proceed if the entry appears to be zero. The recommended answer is "no" unless you believe the entry to be very small but nonzero.)
sqrt, r, R Replace the last displayed entry by its square root. The entry must be nonnegative.
exp, x, X Replace the last displayed entry y by ey.
ln, l, L Replace the last displayed entry by its natural logarithm, i.e. logarithm base e. The entry must be positive. The base 10 logarithm can be computed by dividing by ln(10).
x^y, ^ If the second to the last entry is x and the last entry is y, replace them both by xy. Since this deals with arbitrary exponents, x is required to be positive.
C/CE, # Clear either an incompletely entered number, or the bottom entry on the stack. The key on the keyboard deletes just the last entered digit. If there is no incomplete entry, behaves like #.
sin, s Replace the last entry by its sine. By default, the argument is in radians. In degree mode, this and the next two functions interpret their argument as a number of degrees. cos, k Replace the last entry by its cosine.
tan, t Replace the last entry by its tangent.
pi, p Add the mathematical constant pi (the ratio of a circleís circumference to its diameter) to the end of the stack.
C All, @ Delete all numbers and any incomplete entries from the stack.
asin, S Replace the last entry by its arcsine. The argument must be between -1 and 1, inclusive. By default, the result is in radians and between -pi/2 and pi/2. In degree mode, the answer is between -90 and 90.
acos, K Replace the last entry by its arccosine. The argument must be between -1 and 1, inclusive. The result is between 0 and pi (0 and 180).
atan, T Replace the last entry by its arctangent. The result is between -pi/2 and pi/2, exclusive, or between -90 and 90. /, / Divide the second to the last entry by the last one. Replace both by the result. The last one must be nonzero.
xchg, i, I Exchange the last two entries on the stack. *, * Replace the last two stack entries by their product.
Store M, = Copy the top of stack to a separate memory location. -, - Subtract the last stack entry from the previous one. Replace both by the result.
get M, g Append the contents of the memory to the stack as the last entry. +, + Replace the last two entries on the stack by their sum.
copy, q, Q Append a copy of the previously last stack entry as the last stack entry. +/-, ~ Change the sign of the last stack entry, i.e. multiply it by -1.
enter, $, , Convert the previously entered digits (and possibly decimal point) into a real number and append it to the end of the stack. All other operations that affect or use the last stack entry do this implicitly before they perform their operation. Thus this is needed only if multiple numbers are appended to the stack without intervening arithmetic operations.
0-9, 0-9 Decimal digits used to enter numbers. ., . Decimal point for numeric entry. (no keypad equivalent), a-f Hexadecimal digits. Can be entered only from keyboard. Valid only in hexadecimal mode.
RPN online calculator control panel
The RPN calculator control panel provides several additional keys and checkboxes which are used for purposes other than entering numbers or performing operation on the stack. Several of them also serve to indicate the RPN calculatorís state.
stop or STOP!, (no keyboard equivalent) Aborts the operation of the calculator, e.g. after a nonterminating computation was inadvertently started. A label of "stop" indicates that the calculator is currently idle, and the button has no effect. If the button is labeled "STOP!", then a calculation is in progress. (Note that some Java implementations force a delayed response to this button.)
HELP!, (no keyboard equivalent) Pop up a window with short instructions for the calculator.
replay, (no keyboard equivalent) Interpret the selected text in the history window as command input to the calculator. This can be used to replay a previous computation with different inputs, or to add a number that was previously pasted into the history window to the calculator stack. Select the right commands in the history buffer before pushing this button.
write, w, W Write the last entry on the stack to the RPN calculator emulatorís Java console. The output includes all digits to the left of the rightmost one currently displayed, including those to the left of the display window. (Most browsers do not make the Java console visible by default, but provide an option to make it visible. Most also make it possible to copy text from the Java console to elsewhere on the desktop. It is generally not possible for Java Applets to write directly to the clipboard. Hence this option was chosen.)
base 16, ! A checkbox that allows selection of base 16 (hexadecimal) mode. This affects only number entry and display. To convert between bases, first enter the number and then switch bases.
degrees, " A checkbox that allows selection of degree mode. Affects trignonometric and inverse trigonometric functions. Default is radian mode.