Forth Online Compiler / Interpreter

Write and run Forth code online directly from your browser with our free Online Forth Compiler / Interpreter. Practice stack-based programming, Reverse Polish Notation (RPN), custom words, arithmetic, and stack manipulation without installing Gforth or configuring a local Forth environment.

How to Use This Forth Compiler

  1. Write or paste your Forth code
    Enter your program in the Code Editor. You can test arithmetic operations, stack commands, output statements, custom words, loops, and other Forth features supported by the environment.
  2. Click the Run button
    Select Run to execute your Forth code. If the program contains an invalid word, stack error, or syntax issue, review the returned feedback, correct the code, and run it again.
  3. Check the Output screen
    Your program result appears in the Output panel. Use the available controls to copy, download, or delete the output before testing another Forth program.

Key Features

  • Free Online Forth Compiler / Interpreter: Write and execute Forth directly from a modern web browser.
  • Zero Installation: No need to install Gforth or configure a local interpreter for supported programs.
  • Instant Execution: Click Run and quickly review program output or execution feedback.
  • Stack-Based Programming: Practice working with Forth’s data stack and postfix expression model.
  • Reverse Polish Notation (RPN): Test arithmetic and logical expressions without traditional infix syntax.
  • Stack Manipulation: Experiment with words such as DUP, DROP, SWAP, and OVER.
  • Custom Words: Define reusable operations using Forth’s : and ; syntax.
  • Dark and Light Mode: Switch the Code Editor theme for comfortable coding in different environments.
  • Output Controls: Copy, download, or clear generated output directly from the Output panel.
1. How do I print output in Forth using ." "?

Forth commonly uses ." to print a string.

CR moves the output to a new line.

You can also print numbers using .:

This pushes 10 and 20 onto the stack, adds them, and prints the result.

Forth performs most operations using a data stack. Values are pushed onto the stack first, and operations then consume those values.

For example:

The execution works conceptually like this:

You can also inspect and manipulate values:

DUP duplicates the top stack value, so the expression effectively calculates 10 + 10.

No. You can use this Online Forth Compiler / Interpreter directly from your browser without installing Gforth locally for supported programs.

A local Forth environment may still be useful for hardware interaction, embedded development, native libraries, larger projects, or implementation-specific features.

Forth lets you create new reusable commands, called words, using : and ;.

For example:

The square word duplicates the top stack value and multiplies the two copies.

You can also define words that combine several operations:

This makes Forth highly extensible because new words can be built from existing ones and then reused throughout your program.