With apologies to Barbie, math isn’t that hard after all. Looking at today’s crazy-low interest rates, homeowners who want to refinance are realizing that it’s worth it to bring some cash to the table to get that lower rate.

See, if you have 20% equity in your home you often qualify for the lowest of the low rates — and you avoid paying any PMI. But with home values having dropped, some folks who once had that 20% equity find they don’t anymore. When they go to lower their rates, they need to do a “cash-in” refinance in which they bring enough money to hit that magic 20% equity mark.

But it’s worth it. As Dan Green over at the Mortgage Reports points out, “Each dollar you knock from your mortgage balance is a dollar on which you won’t pay interest for the next 30 years.”

The result of this is that, for the first time ever, half of homeowners who refinance — half! — are bringing money to the table, and reaping the benefits.

Green goes into much more detail, and it’s a good read. So go. Read it.