The email shows up 41 days before closing. Your lender has repriced the loan twice this week and wants an answer by Thursday: lock the rate now, or let it float and hope the market moves your way. You pull up a mortgage rate lock calculator, stare at the fields, and realise you’re not sure what half of them mean.
That’s the usual experience. Standard advice says lock when rates are rising and float when they’re falling, which is technically true and practically useless at 11pm the night before a deadline. The narrower question is the one that actually decides things: what does this specific lock cost me, and how far do rates have to move before that cost was worth it?
The Premium You’re Paying for a Promise
A rate lock is insurance, and insurance has a price. Sometimes the price is built into your rate. Sometimes it’s a line item at closing. Often it’s both.
A 30-day lock on a 30-year fixed is typically free. A 60-day lock usually runs somewhere between 0.125% and 0.5% of the loan amount, and longer windows cost more because the lender is holding interest-rate risk on your behalf for longer. On a $400,000 loan, a 0.25% lock fee is $1,000 cash at closing.
So what does that $1,000 buy? Protection against the market moving between now and your funding date. If rates climb a quarter point before you close, your principal-and-interest payment on $400,000 rises by roughly $66 a month. Divide the $1,000 fee by that $66 and the lock breaks even in about 15 months of homeownership — assuming rates actually do move against you. If they don’t, you paid $1,000 for nothing, which is the part nobody puts in the brochure.
The Fields That Change the Answer
Most mortgage rate lock calculators ask for a handful of inputs. A few of them swing the result hard, and a few barely matter.
- Loan amount. Every fee in this calculation is a percentage of it, so a $600,000 loan doubles the dollar cost of the same lock structure compared with a $300,000 one.
- Lock length versus your actual closing date. Count business days, not calendar days, and build in a buffer. Underwriters miss dates constantly.
- Upfront fee versus rate bump. A lender might offer a free 45-day lock at 6.75% or the same lock at 6.625% for 0.25 points. These are not equivalent, and the calculator should show you both paths side by side.
- Float-down provision. This is the single most valuable field and the one most people skip.
- Extension cost. Usually quoted per 15 days, usually 0.125% of the loan.
If you’re still working out what a given rate does to your monthly budget in the first place, it’s worth getting that number nailed down before you start optimising lock terms. The interest rate field is the one that decides what you can actually afford, and a quick run through a mortgage calculator by interest rate will tell you whether an eighth of a point is a nuisance or a dealbreaker.
Comparing Two Lock Offers Without Getting Fooled
Lenders rarely quote lock costs in the same format, which makes side-by-side comparison harder than it should be. One charges 0.375 points for 60 days. Another charges nothing but prices the rate 0.25% higher. A third throws in a free 45-day lock and charges $500 per 15-day extension.
Convert everything to one number: total cost over the first five years, including the lock fee and the extra interest from a higher rate. A half-point difference in rate on a $400,000 loan is roughly $130 a month, which is $7,800 over five years — far more than any lock fee you’ll encounter. Running that comparison properly, the way a mortgage rate comparison calculator does, usually reveals that the rate itself matters more than the lock structure. People agonise over a $600 lock fee while accepting a rate that costs them six times that.
Float-Down Options: Where the Real Value Sits
A float-down lets you lock today and still capture a better rate if the market improves before closing. Terms vary wildly, so read the fine print before you assume you’re covered.
How they usually work
Most float-downs trigger only if rates drop by a set threshold — commonly 0.25% or 0.5% — and only within a defined window, often the final 30 days before closing. Some allow one re-lock, not unlimited. Others charge a fee of a few hundred dollars for the privilege.
The catch
A float-down is not a free option on the market. It’s a partial hedge, and lenders price it accordingly. If rates fall 0.125%, a float-down with a 0.25% threshold does nothing for you. If rates fall 0.5%, you capture the improvement but the lender keeps the difference between your original rate and the new market rate until the trigger hits. Ask specifically: what’s the threshold, what’s the window, how many times can I use it, and what does it cost?
Extension Fees and the Closing-Date Trap
This is where rate locks quietly turn expensive. A 15-day extension at 0.125% on a $400,000 loan costs $500. Need two of them because the appraisal came back late and the title company found a lien? That’s $1,000 gone, and you’re now paying more for the lock than you would have paid for a longer lock upfront.
Worse, many locks don’t cover delays caused by the borrower at all. Missing a document request, a late bank statement, a gift letter that never got signed — those can void the lock entirely and force a reprice at whatever the market is that day. Budget for a 60-day lock if your purchase involves a short sale, a self-employed borrower, or anything with a condo questionnaire attached.
Buying Points Inside a Lock
Discount points and rate locks interact in a way that catches people out. Points are locked along with the rate, so if you pay 1.5 points to buy the rate down and then a float-down triggers, you may end up having paid for a buydown you no longer needed. If you’re considering it, model the payback first — a mortgage discount points calculator will show you how many months of payments each point has to save before it’s worth the cash, which is usually longer than the sales pitch implies.
When Floating Is the Smarter Bet
Floating makes sense in a few specific situations, not as a general philosophy. You might float if your closing is more than 90 days out and lock fees for that window are steep, if you have genuine slack in your budget and could absorb a quarter-point rise without changing your plans, or if you’re early in house-hunting and haven’t even had an offer accepted.
Locking generally wins when your closing is inside 45 days, when your debt-to-income ratio is tight enough that a higher payment would break approval, or when you’ve already stretched to the top of your budget. There’s also a case for caring less about the lock entirely: if you plan to pay extra every month, the loan term shrinks fast enough that rate shopping matters less than you’d think. A mortgage interest savings calculator running an extra $200 a month will usually show more total interest saved than a successful quarter-point float.
Worth stress-testing the worst case too. Before you decide to float, look at what your payment becomes if rates jump half a point between now and closing — the payment on a 30-year loan at 7 percent interest versus your current quote is a useful gut check. If that number makes you wince, you already have your answer.
Questions to Ask Before You Sign the Lock
Get these answered in writing, not over the phone. The lock agreement is a contract, and the details that hurt people are always in the paragraph nobody read.
- What exact date and time does the lock expire, and is it business days or calendar days?
- What happens if we close late for a reason outside my control — appraisal delay, title issue, lender backlog?
- Is there a float-down, and what’s the threshold, window, and fee?
- What does an extension cost, per how many days, and is it capped?
- Does the lock cover the rate, the points, the lender credits, or all three?
- If I’m denied or the deal falls through, is any lock fee refunded?
Then put the closing date on your calendar with a two-week buffer in front of the lock expiration, and tell your loan officer you’re watching it. Locks expire quietly, and the reprice that follows is never in your favour.