SOFR vs Prime Rate: How Index Rates Affect Your Mortgage

SOFR vs Prime Rate Mortgage: What I Wish Someone Had Told Me Before I Signed

So What Exactly Is SOFR?

SOFR stands for the Secured Overnight Financing Rate, and it replaced the old LIBOR benchmark that was basically caught up in a massive manipulation scandal. It’s based on actual overnight transactions in the U.S. Treasury repurchase market, which makes it way more transparent. Think of it as the interest rate banks charge each other for super short-term loans backed by government bonds.

When your mortgage is tied to SOFR, your adjustable rate gets calculated by adding a margin — usually around 2% to 3% — on top of whatever SOFR is sitting at. I remember my lender throwing around terms like “30-day SOFR average” and my eyes just glazed over. But honestly, once you get the basics down, it’s not that complicated.

And the Prime Rate? That Old Familiar Friend

The prime rate is something you’ve probably heard about a million times on the news. It’s the interest rate that commercial banks offer to their most creditworthy customers, and it moves pretty much in lockstep with the federal funds rate. Right now in 2025, it tends to hover around 7.5%, though that changes whenever the Fed makes a move.

Prime rate mortgages — and HELOCs especially — use this benchmark to set your variable rate. The bank adds or subtracts a margin from prime, and boom, that’s your rate. I actually had a HELOC tied to prime for years and didn’t even realize it until my payment jumped like crazy after a Fed rate hike.

The Real Differences That Actually Matter

Okay, so here’s where it gets interesting. Both SOFR and the prime rate are benchmark rates used to price adjustable-rate loans, but they behave pretty differently in practice.

Volatility: SOFR can fluctuate daily since it’s based on overnight transactions. The prime rate only changes when the Fed adjusts the federal funds rate, so it’s more predictable.
Transparency: SOFR is considered more transparent because it’s based on actual market transactions. Prime is essentially set by banks — they just follow the Fed’s lead.
Usage: Most new ARMs are tied to SOFR since LIBOR went away. HELOCs and some older variable-rate products still commonly reference the prime rate.
Margins: SOFR-based loans typically carry a higher margin to compensate for the lower base rate. Prime-based loans usually have a smaller margin since the base rate is already higher.

Which One Should You Actually Pick?

This is the question I kept asking my mortgage broker, and honestly, she was kinda vague about it. From what I’ve learned since then — sometimes the hard way — it depends on what type of loan product your going for. If you’re looking at a standard adjustable-rate mortgage, chances are it’ll be SOFR-based and you won’t really have a choice.

However, if you’re weighing a HELOC against a SOFR ARM for tapping home equity, the rate structure matters a lot. Prime-based HELOCs tend to be simpler to understand and more predictable month to month. SOFR-based ARMs might offer lower initial rates, but the adjustment periods can catch you off guard if you’re not paying attention.

My honest advice? Run the numbers for both scenarios using a good ARM calculator. Look at worst-case rate cap scenarios. Don’t just fall in love with the teaser rate — I made that mistake once and I’m still a little salty about it.

The Bottom Line on Your Rate Benchmark

Whether your mortgage references SOFR or the prime rate, understanding your benchmark is one of the smartest things you can do as a borrower. It affects every single payment adjustment you’ll ever face. Take the time to read your loan docs carefully — I know, boring — but future you will be grateful.

Want to keep learning about mortgage rates, loan structures, and how to make smarter borrowing decisions? Head over to the Mortgage Margin blog for more articles written in plain English. We’re all figuring this out together.