Actual describes what really happened or exists, measured in the world. Theoretical describes what should happen according to a model or calculation. A theoretical maximum is what the physics allows; the actual figure is what you got. The gap between them is usually where the interesting question is.
The core difference
- actual — observed, measured, real: actual output, the actual cost
- theoretical — derived from a model, ideal conditions assumed: theoretical maximum, theoretical yield
They are most often used together precisely to name a gap: theoretical yield versus actual yield, theoretical bandwidth versus actual throughput.
The test
Ask where the number came from. Measured from the world → actual. Calculated from assumptions → theoretical. If nobody has run the experiment, the figure cannot be actual however confident the calculation.
Theoretical does not mean doubtful
In everyday speech "theoretical" often implies unproven or impractical — "that's only theoretical". In scientific use it means the opposite of empirical, not the opposite of true: theoretical physics is rigorous work, and a theoretical maximum is a hard limit rather than a guess.
The same trap sits under theory, which in science means a well-tested explanatory framework, not a hunch. That everyday sense is hypothesis.
Actual and its false friends
Actual is a classic trap for speakers of several European languages, where the cognate means current or topical: French actuel, German aktuell, Spanish actual all mean present-day. English actual never means that — "the actual situation" means the real one, not the current one.
The related pairs
- nominal vs actual — the stated or rated figure against the measured one
- empirical vs theoretical — derived from observation against derived from principle
- practice vs theory — the same distinction as a pair of nouns
