Roman Numeral Chord Analyzer
Map a chord progression into Roman numerals in a given key.
Overview
The Roman numeral chord analyzer takes a chord progression in a chosen key and translates it into functional Roman numerals. Paste "C G Am F" with the key set to C major and the tool returns "I V vi IV." Try the same chords in G major and you get "IV I ii bVII." It's the reverse of the chord progression builder — useful for analysing songs you've transcribed by ear and understanding their underlying harmonic logic.
It's a study aid for theory students, a transcription helper for arrangers and orchestrators, and a sketch tool for songwriters who want to see whether a progression they like is a textbook pattern or something unusual. Once a song is in Roman numerals, it's instantly transposable to any key and comparable to other songs across genres.
How it works
Each chord is identified by its root note and quality. The tool finds the scale degree of the root within the chosen key — root C in C major is degree 1, root G is degree 5, root A is degree 6 — and matches the quality to the expected diatonic chord on that degree. Major qualities get uppercase numerals, minor get lowercase, diminished get the degree sign, augmented get the plus sign.
Non-diatonic chords (chords that don't fit the key) are flagged with flat or sharp accidentals: a Bb major chord in C major is bVII (a borrowed chord from the parallel minor); an E major chord in C major is V/vi (a secondary dominant resolving to A minor). The analyser also recognises modal interchange and tries to give the most musically sensible label, prioritising common interpretations like bVI bVII I in major-key rock contexts.
Examples
Key: C major, Chords: C G Am F → I V vi IV
Key: A minor, Chords: Am G F E → i bVII bVI V
Key: G major, Chords: G E7 Am D → I V/vi ii V
Key: D major, Chords: D Bm A G → I vi V IV
FAQ
What if a chord doesn't fit the key at all?
The tool labels it with the closest functional interpretation — usually a borrowed chord (bVII, bVI), a secondary dominant (V/V, V/vi), or a chromatic mediant. Some chords genuinely sit outside functional harmony and are best read literally.
How does the analyser distinguish I from i?
By chord quality. A major chord on the tonic is I (uppercase); a minor chord on the tonic is i (lowercase). Same root, different harmonic colour.
Is V7 the same as V in this notation?
Functionally yes — both are dominant. V7 specifies the dominant seventh chord (with a flat seventh on top); plain V usually means the major triad. The analyser preserves the seventh if it's present in the input.
Why might the same chord get different numerals in different keys?
A chord's function is relative to the key. F major is IV in C, I in F, V in Bb, and bVI in A minor. The numeral describes the chord's role, not its absolute identity.
Can it handle modulations within a song?
Best practice is to analyse each key area separately. Some tools let you mark a pivot chord that belongs to both keys.