Direct answer
Your bank loyalty tax is the interest you forgo by leaving savings at a low APY instead of a competitive available APY. It is balance times APY gap, expressed in dollars per year.
Why this is not generic
Personal calculator
Shareable receipt
Monthly public index
The math is simple
Annual loyalty tax equals balance multiplied by the gap between your APY and a better available APY, divided by 100.
The receipt makes it shareable
The Loyalty Tax Receipt turns the calculation into a screenshot-ready artifact without including private account data by default.
The index makes it citable
The Bank Gap Index publishes the representative national version as a monthly data release.