There is no fixed percentage. Most servicers set a practical minimum principal reduction, and the number varies by servicer, so get their recast policy in writing before you commit the funds. A quick refresher on the move itself: with a recast, you keep your existing loan and rate, make a large one-time principal payment, and the servicer re-amortizes the remaining balance over the remaining term so your monthly payment drops. It makes the most sense when your rate is better than the current market, because a refinance would cost you a rate worth keeping. Three things to know before you plan on one: - Conventional only. Fannie Mae and Freddie Mac loans permit re-amortization after a substantial principal paydown. FHA, VA, and USDA loans do not offer a recast. - The servicer's call. Even on a conventional loan, the servicer is not obligated to offer one. Confirming that yours does is step one. - A practical floor, not a set percentage. No servicer wants to process a recast for a few thousand dollars that barely moves the payment, so most set a minimum paydown and charge a processing fee. Call the company that services your loan and ask for the recast policy in writing: minimum amount, fee, and timeline. Then decide.