I'm learning to play the minor versions of each scale. What BPM should I aim for, before I move onto the next scale?
You should play whatever speed you can with completely (completely, though!) relaxed hands. Except for the base of the finger being played, there should be no more muscular tension than if the hand was just sitting on a desk.
When you start to speed up, there's a tendency to "try-hard," which interferes with the passing of the thumb and makes scales sound bumpy. As the hand tires through extended practice, this can potentially even be dangerous-- leading to "tendinitis claw"
Rather than artificially focusing on bumping up the speed X bpm each week or whatever, I'd focus on how the hand FEELS. As you bump up each level, you'll have a little tension at first, and you'll know you're ready for the next level when that tension is totally gone.
- neck is relaxed
- shoulders are relaxed (specifically, not drawn up toward the neck)
- elbows are completely loose (check this by moving your body toward / away from the keyboard while playing)
- wrist is completely loose (check this by raising or lowering the wrist while playing)
- muscles on backs of fingers are not activated in any way
- the palm muscles are not being used at all.
- the webbing between the fingers has little muscles-- they should not be tired or stressed at all
- Also: once a note has been pressed, there should be very minimal pressure to sustain it. There's no reason to keep pushing down hard.
You should have total finger independence-- pushing with one finger shouldn't involve effort from any part of the hand or arm other than the finger itself.
The ABRSM 2021–2022 Piano Syllabus applies the following scale speeds for each exam grade, with all scales played in eighth-notes.1
Image source (PDF page 16)
Initial: quarter-note = 54 Grade 1: quarter-note = 60 Grade 2: quarter-note = 66 Grade 3: quarter-note = 80 Grade 4: quarter-note = 100 Grade 5: half-note = 60 Grade 6: half-note = 72 Grade 7: half-note = 80 Grade 8: half-note = 88
For minor scales, the progression is
Minor scales required by ABRSM. Each grade requires the scales from the previous grade. Initial: D; 1 octave; hands separately Grade 1: A, D; 2 octaves; hands separately Grade 2: A, D; 2 octaves; hands together Grade 3: E, G; 2 octaves; hands together B, C; 2 octaves; hands separately Grade 4: B, C; 2 octaves; hands together F, F#; 2 octaves; hands together Grade 5: F#, C#, G#, Eb, Bb; 2 octaves; hands together Grades 6-8: All; 4 octaves; hands together
When practicing scales, the most important element is relaxed, accurate movement. When both of these are achieved, speed generally takes care of itself. Practice slowly for efficiency rather than practicing for speed. The speed will come on its own.
1 ABRSM = Associated Board of the Royal Schools of Music is an internationally standard series of graded music exams. All information in this post comes from the Piano Syllabus for 2021-2022.