Mortgage rates have been extremely low, and now millions of borrowers qualify for mortgage refinance.

CNBC's Diana Olick writes that 2.4 million borrowers could possibly save $200 or more on their mortgage payments, while 1.9 million can save $100 to $200 a month. This is the perfect time for borrowers to refinance, with mortgage rates expecting to rise in 2016.

"If rates go up 50 basis points from where they are now, 2.1 million borrowers will fall out of the running; a 100-basis-point increase would eliminate another million, leaving only 2 million potential refinance candidates, the lowest population of refinance candidates in recent history," said Ben Graboske, senior vice president at Black Knight Data & Analytics.

"While it's not a hard and fast rule that borrowers won't refinance into a higher rate in order to tap available equity — 23 percent of cash-out refi borrowers over the past six months did just that — for the most part, as rates rise, HELOCs will continue to become more popular to homeowners looking to tap available equity," said Graboske.

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