Bach's Sinfonias are monumental pedagogical works that explore contrapuntal textures and harmonic development in three-part writing. They are cornerstones of keyboard education, demanding clarity and precision.
Mastering the independent lines and intricate counterpoint is the primary challenge.
15 Sinfonias is rated Henle level 7 of 9 — late advanced repertoire, roughly ABRSM grade 8 and above. In the RCM system it corresponds to about level 10.
You should be comfortable at Henle level 7 — meaning you can already play level 6–7 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
15 Sinfonias is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.