Mortgage Lookback Period: 45-Day Window, Index Capture, and Caps

On an adjustable-rate mortgage, the lookback period is the fixed window before your rate change date when the servicer captures the index value that will set your new rate. For nearly every ARM written today, that window is 45 days. Whatever the index reads on that single date becomes the number plugged into your rate calculation, and it stays frozen even if the market moves sharply in the weeks between the capture and your first new payment.

What the 45-Day Window Actually Does

Your ARM note names a “change date” when the interest rate resets. The lookback period tells the servicer exactly which day’s index reading to use. On the standard Fannie Mae adjustable-rate note, the most recent index value available 45 days before the change date becomes the “Current Index” for that adjustment.¹ Once captured, it is locked. A rate spike three weeks later doesn’t touch you. A rate drop doesn’t help you. Your next rate has already been decided.

The 45-day figure is the industry standard. When federal regulators updated ARM servicing rules in 2013 and 2014, they observed that an overwhelming majority of conventional ARMs already used a 45-day lookback and aligned FHA and VA programs to match.² The buffer gives the servicer time to run the math, generate the required disclosure, and mail it before the new payment comes due.

If the 45th day happens to fall on a weekend or holiday when the index isn’t published, the servicer uses the most recent value available before that date. The Fannie Mae note builds this in by referencing the “most recent Index value available” rather than requiring a reading published on the exact day.¹

Check your own note before assuming 45 days. That is the standard, but the number in your contract governs your loan.

How That Captured Index Becomes Your New Rate

Every ARM adjustment uses the same formula: the index value on the lookback date plus a fixed margin equals your new interest rate. The margin is set at closing and doesn’t change for the life of the loan.³ You’ll find it stated in your promissory note as a percentage such as 2.75%. For conforming loans eligible for sale to Fannie Mae, the margin cannot exceed 300 basis points.⁴ Non-conforming and jumbo ARMs can carry higher margins.

The index itself is an external benchmark outside your lender’s control. Two dominate current ARM contracts. The Secured Overnight Financing Rate (SOFR) reflects the cost of overnight borrowing collateralized by Treasury securities and is published each business day by the Federal Reserve Bank of New York.⁵ Many ARMs reference a 30-day or 90-day compounded SOFR average rather than the daily spot rate, and the New York Fed publishes those averages alongside the daily figure.⁶ Constant Maturity Treasury (CMT) rates represent Treasury yields interpolated to fixed maturities such as one year, and appear on the Federal Reserve’s H.15 release.⁷ Both are freely available, so the same number your servicer uses is one you can look up yourself.

Here’s how the math runs. If the 30-day average SOFR on your lookback date is 4.25% and your margin is 2.75%, your fully indexed rate is 7.00%. Even if SOFR jumps to 4.50% the following week, your rate stays at 7.00% until the next change date, because 4.25% was locked in on the lookback date.

One note on older loans: if your ARM originated before mid-2023 and originally referenced LIBOR, it should have transitioned automatically to a spread-adjusted SOFR index under the Adjustable Interest Rate (LIBOR) Act after LIBOR publication ended on June 30, 2023.⁸ Your most recent rate-change notice should state which index now governs your loan.

When Caps or Floors Override the Calculation

The index-plus-margin result isn’t always your new rate. Caps limit how far your rate can move. Most ARMs use a three-number cap structure written as something like 2/2/5 or 5/2/5.

Initial adjustment cap: how far the rate can move at the first reset after the fixed introductory period. Commonly two or five percentage points.
Subsequent adjustment cap: how far the rate can move at each later change. Usually one or two percentage points.
Lifetime cap: the maximum total change over the life of the loan. Five percentage points is typical.⁹

If your starting rate was 5.00% and your caps are 2/2/5, the first adjustment cannot exceed 7.00% even if index-plus-margin produces 7.50%. The cap prevails. That unused half-point may not disappear, though. Some contracts allow the lender to carry forward the trimmed portion and apply it at a later adjustment, which Regulation Z calls “previously foregone interest rate increases.”¹⁰

Floors work in the opposite direction. The Fannie Mae note treats any negative index value as zero, which effectively creates a floor at your margin.¹ Some loans add a separate lifetime floor preventing the rate from falling more than a set number of points below the initial rate.⁹ The specific floor language in your note determines how much benefit you actually see when rates fall.

Verifying the Number Your Servicer Used

You don’t have to wait for a bill to check the math. The process takes about ten minutes.

Find your change date in the note. Count back the number of days your note specifies for the lookback (45 days on most contracts). That’s the capture date.
Look up the index value for that date. Use the New York Fed’s SOFR pages for SOFR-based loans and the Federal Reserve’s H.15 release for CMT loans. If your note references a 30-day or 90-day SOFR average, use the averages series, not the daily spot rate.
Add your margin, exactly as written in the note.
Apply the caps. If the sum exceeds what your cap structure allows for this adjustment, the cap sets the rate. Check whether your contract permits carryover of any trimmed portion.
Check the floor. If the sum sits below any minimum stated in your note, the floor takes effect instead.

Compare the result to your rate-change notice. Federal law requires that notice to list the index used, the source where you can verify it, the margin, the current and new rates, the current and new payments, the applicable caps, and any prepayment penalty that could apply if you refinance in response. Under Regulation Z, the notice must arrive between 60 and 120 days before the first adjusted payment is due for most ARMs, with a shorter 25-to-120-day window for ARMs adjusting every 60 days or more often and for certain pre-2015 loans with shorter lookback periods.¹⁰

If the Rate Adjustment Looks Wrong

When your independent calculation doesn’t match the notice, RESPA gives you a formal error-resolution process. Send a written Notice of Error to the address your servicer designates for disputes, which is often different from the address for payments.¹¹ Identify the specific mistake: the wrong index date, the wrong index value, the wrong margin, a miscalculated cap.

Once received, the servicer must acknowledge your notice in writing within five business days and then investigate and respond within 30 business days, with the option to extend by 15 business days if it notifies you in writing before the initial deadline. No fee may be charged for handling the dispute. If the servicer catches the error early, it can skip the acknowledgment step by correcting the mistake and notifying you within five business days.¹²

A late or missing rate-change notice is a separate issue. The Truth in Lending Act allows individual claims for statutory damages between $400 and $4,000 on mortgages secured by real property, plus actual damages and reasonable attorney’s fees.¹³ The strongest starting position for any dispute is the exact index value for your lookback date, pulled directly from the New York Fed or the Federal Reserve, next to the exact margin in your note. With both numbers in hand, the correct rate is a matter of arithmetic.

1
Fannie Mae. Adjustable Rate Note
2
Federal Register. Federal Housing Administration (FHA) Adjustable Rate Mortgage Notification Requirements and Look-Back Period for FHA-Insured Single Family Mortgages
3
Consumer Financial Protection Bureau. For an Adjustable-Rate Mortgage (ARM), What Are the Index and Margin, and How Do They Work?
4
Fannie Mae. Adjustable-Rate Mortgages (ARMs)
5
Federal Reserve Bank of New York. Secured Overnight Financing Rate Data
6
Federal Reserve Bank of New York. SOFR Averages and Index Data
7
Federal Reserve Board. H.15 – Selected Interest Rates (Daily)
8
Office of the Law Revision Counsel. 12 USC Ch. 55 Adjustable Interest Rate (LIBOR)
9
Consumer Financial Protection Bureau. What Are Rate Caps With an Adjustable-Rate Mortgage (ARM) and How Do They Work?
10
eCFR. 12 CFR 1026.20 – Disclosure Requirements Regarding Post-Consummation Events
11
Consumer Financial Protection Bureau. What Is a Qualified Written Request (QWR)?
12
eCFR. 12 CFR 1024.35 – Error Resolution Procedures
13
Consumer Financial Protection Bureau. CFPB Laws and Regulations TILA