The maximum amount eligible for EMI conversion is generally capped by your available credit limit at the time of conversion, since the outstanding EMI balance continues to be counted against that limit for the duration of the plan. Some banks also apply an additional cap specific to EMI conversions, separate from your overall credit limit, particularly for larger transactions, so the effective maximum you can convert is whichever is lower: your available credit headroom or any bank-specific EMI conversion cap that applies to your card.
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