Why a rate lock matters
Mortgage rates can move daily, sometimes more than once in a day. Locking protects you from market swings while the file is being processed, which can be especially important on larger loans where even a small rate change materially affects the payment. Once locked, your rate is held regardless of what the market does before closing, as long as you close within the lock window.
A quick example
On a $500,000 loan, a rate that drifts from 6.50% to 6.75% while you shop adds roughly $80 a month, or close to $29,000 over a 30-year term. Locking early removes that risk. The trade-off is that if rates fall after you lock, you are generally held to the higher number unless your lock includes a float-down option.
How it affects your closing timeline
Lock periods are finite — often 30, 45, or 60 days — so the lock and your closing date have to line up. If closing slips past the lock’s expiration, you may owe an extension fee or have to re-lock at current pricing. Choosing a realistic lock length up front, and keeping your loan documents moving, is the simplest way to avoid paying twice.
Common mistake
Many borrowers focus only on the rate number and miss the fee structure. Two locks can show the same interest rate while carrying very different upfront costs. Compare the rate and its points or lender credits together on your Loan Estimate, and factor those into your closing costs before you commit to a lock.