The best free circuit simulator, by what you need it for

Start with the job. Building intuition about how current and voltage behave calls for animation and quick changes. Designing a circuit that has to work calls for SPICE analysis and manufacturers’ models. A lab course calls for instruments: a scope, a generator and a meter you wire in the way you would on a bench. And a project with a microcontroller needs a simulator that runs its code beside the circuit.

Then the machine. Chromebooks and locked-down lab PCs rule out desktop installs, and a phone rules out most desktop tools entirely. Finally, check what free includes: some tools limit circuit size or private work on their free tier, and the limit matters more for a class than for one curious person.

LTspice, from Analog Devices, is free for Windows and macOS, with no limits on circuit size, analyses such as Monte Carlo and distortion, and a very large library of manufacturers’ models. Qucs-S is free and open source on Linux, Windows, macOS and FreeBSD, over ngspice, Xyce, SpiceOpus and Qucsator, with RF analyses such as S-parameters and harmonic balance. For serious analog design, use one of these. Neither runs in a browser or on a Chromebook.

Free in the browser with no size limit, on computers, Chromebooks, tablets and phones. Circuits run on a real 0.1 inch breadboard with a scope, a function generator, a multimeter and a bench supply wired in as parts, and analog parts are solved with real device models. SPICE decks run in the editor on an engine checked against ngspice on 42 benchmark circuits, without BSIM models, Monte Carlo or distortion. Arduino, ESP32 and Pico boards run real compiled code on the same canvas.

A high school physics or electronics student on a Chromebook: Falstad to see why a circuit does what it does, and Mokxi to wire the same circuit on a breadboard and measure it with a meter and a scope. A college student in a circuits course: LTspice for homework that asks for transient and AC analysis, on a laptop that can install it, and a browser tool for the lab machines that cannot.

A hobbyist with an Arduino or an ESP32 on order: Mokxi or Tinkercad Circuits, so the code and the circuit can be tried together before the parts arrive. Someone who mostly works from a phone: EveryCircuit’s apps, or a browser tool in the phone’s browser. An engineer checking a design against real part models: LTspice or Qucs-S.