All told, the potential savings to homeowners is about $1.67 billion a month, or more than $20 billion a year. If everyone took advantage of low rates, think of the boost it would give to the economy and housing market.

Black Knight, which tallied the universe of homeowners paying 4.5 percent or more on their loan who had good credit and at least 20 percent equity, last ran its refi analysis in December, after the Federal Reserve raised short-term borrowing costs and everyone was worried that mortgages would get more expensive.

Then came China and cheap oil and a global flight to safe financial assets, which instead made mortgages cheaper. Since the Fed rate hike, some 1.5 million people have joined the ranks of refi-ready.

Pay special heed to this message if you live in Hawaii, Washington, D.C., New York, California or Massachusetts. In those markets, homeowners have the biggest opportunities to save. In Hawaii, where more than 28,000 households are eligible for refinancing, the average savings would be $437 a month.

Of course there are good reasons not to refinance. If you’ll be selling soon, for example, a refi might not be worth it.

But I’m Underwater

Eight years after the financial collapse, the government is still helping homeowners get out from under bad, costly or inappropriate loans so they can benefit from today’s historic low interest rates.

Even if you have no equity, owe more on your home than it’s worth or have a bad credit score, the Home Affordable Refinance Program, or HARP, might be able to help.

At last check, almost 368,000 households were eligible for the program. That number surely has risen this year as rates have fallen.

Now’s the time to explore your HARP options. The program expires at the end of this year and isn’t likely to be renewed.