Mortgage Rate Lock Calculator
Decide whether to lock your mortgage rate or float, compare lock periods (30/45/60/90 days), and see exactly how each rate-change scenario affects your monthly payment and total interest cost — free, private, no signup required.
| Lock Period | Typical Fee (%) | Fee in Dollars | Extension Cost Est. | Notes |
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| Rate Change Scenario | Resulting Rate | Monthly (Float) | Monthly (Locked) | Monthly Savings | Lifetime Interest Diff | Best Choice |
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What Is a Mortgage Rate Lock and How Does It Work?
A mortgage rate lock is a lender's commitment to hold a specific interest rate for a defined period — typically 30, 45, 60, or 90 days — while your loan closes. According to the Consumer Financial Protection Bureau (cfpb.gov), a rate lock protects the borrower from market rate increases between the time of application and loan closing. If rates rise 0.50% during your lock period, you still close at the originally quoted rate, potentially saving hundreds per month.
Rate locks are not free: lenders typically charge a fee of 0.125% to 0.5% of the loan amount depending on the lock period length. A 30-day lock on a $400,000 loan may cost $500–$1,000, while a 90-day lock could cost $1,500–$2,000 or more. The tradeoff is certainty: you know exactly what your monthly payment will be. Freddie Mac (freddiemac.com) data shows that mortgage rates can move 0.25%–0.75% in a single month during volatile markets, making rate lock decisions consequential.
Lock vs Float — When Each Strategy Wins
The decision to lock or float depends on where rates are heading and how risk-tolerant you are. Here are the key considerations for each strategy:
- Lock your rate when: Rates are rising or near recent lows. The Fed has signaled tightening. Your close date is more than 30 days away. You have a tight budget and cannot absorb a higher payment. The lock fee is less than one month's potential payment increase.
- Float your rate when: Rates are falling or trending lower. Economic data suggests a rate-cut cycle. Your close date is within 2 weeks and the short window limits upside risk. You have the financial buffer to absorb a rate increase of 0.25%–0.50%.
- Float-down option: Some lenders offer a "float-down" provision that lets you lock a rate but still capture a lower rate if rates fall significantly (typically 0.25%+ drop). These usually cost an additional 0.25%–0.50% but provide downside protection on both sides.
The break-even analysis is the mathematical core of this decision: if the rate lock fee costs $1,000 and locking saves you $50/month versus floating at a higher rate, you break even in 20 months. If you plan to keep the home and loan longer than that, locking made financial sense.
Rate Lock Extension Costs and Risks
Rate lock expirations are a common closing-day surprise. If your loan closing is delayed — due to appraisal issues, title problems, or document gathering — and your rate lock expires, you face two options: pay for an extension or let the lock expire and re-lock at current market rates.
- Extension cost: Lenders typically charge 0.15%–0.375% of the loan amount per 15-day extension period. On a $400,000 loan, a 15-day extension may cost $600–$1,500.
- Who pays when the lender causes the delay: If the delay is your lender's fault, some lenders will absorb the extension cost. Get this in writing at the time of lock. The CFPB advises borrowers to ask about extension policies upfront.
- Re-lock risk: If you let a lock expire and rates have risen, you will close at the new, higher market rate. A 0.50% increase on a $400,000 30-year loan adds approximately $130/month — $46,800 over the loan's life.
- Best practice: Choose a lock period at least 15 days longer than your expected closing timeline to build in a buffer. If closing is expected in 35 days, get a 45-day lock.
How This Calculator Computes Rate Lock Scenarios
This calculator uses the standard mortgage amortization formula to compute monthly principal and interest payments at both the locked rate and the rate resulting from each scenario. The monthly payment formula is: M = P × [r(1+r)^n] / [(1+r)^n − 1], where P is the loan principal, r is the monthly interest rate, and n is the total number of payments.
The break-even analysis divides the rate lock fee in dollars by the monthly savings achieved by locking (compared to floating at the scenario rate). When the floating rate is lower than the locked rate (rates dropped), the calculator correctly identifies floating as the better strategy. The lifetime interest difference compounds the monthly difference over the full loan term, illustrating the true long-run cost of rate decisions.
Lock fee estimates are based on industry-standard fee schedules: 30-day locks at ~0.125%–0.25%, 45-day at ~0.25%, 60-day at ~0.375%, and 90-day at ~0.50%. Extension costs are estimated at 0.25% per 15-day period. Sources: cfpb.gov, freddiemac.com. Last updated: May 2026.
Frequently Asked Questions
What is a mortgage rate lock?
A mortgage rate lock is a lender's guarantee to hold a specific interest rate for a set period — typically 30, 45, 60, or 90 days — while your loan is being processed and closed. If market rates rise during your lock period, you still receive the locked rate. If rates fall, you are locked in at the higher rate (unless your lock includes a float-down provision). Rate locks typically cost 0.125%–0.5% of the loan amount depending on the lock period length.
How much does a mortgage rate lock cost?
Rate lock fees vary by lender and lock period. A 30-day lock typically costs 0.125%–0.25% of the loan amount — about $500–$1,000 on a $400,000 loan. A 45-day lock may cost 0.25%, a 60-day lock about 0.375%, and a 90-day lock 0.5% or more. Some lenders offer free 30-day locks but charge for longer periods. Always ask your loan officer for the exact fee and get it in writing.
Should I lock my mortgage rate now or wait?
Lock if rates are rising, near recent lows, or you cannot afford a higher payment. Float if rates are trending downward and you have financial flexibility. The CFPB advises that most borrowers benefit from locking because the risk of rates rising (and increasing your payment for the life of the loan) typically outweighs the chance of saving money by floating. Use this calculator's break-even analysis to see how many months it takes to recoup the lock fee.
What happens if my rate lock expires before closing?
If your rate lock expires before you close on your home, you have two options: pay a rate lock extension fee (typically 0.15%–0.375% per 15-day extension) or re-lock at current market rates. If market rates have risen since your original lock, you will close at the new, higher rate — which can significantly increase your monthly payment. The CFPB recommends choosing a lock period at least 15 days longer than your expected closing date as a buffer.
What is a float-down rate lock?
A float-down provision allows you to lock a rate but still capture a lower rate if interest rates drop significantly (typically 0.25% or more) before closing. Float-down locks cost an additional 0.25%–0.50% of the loan amount but protect you on both sides: your rate cannot go up past the lock, and it can go down if rates drop enough. Not all lenders offer this option, and the terms (minimum drop threshold, timing) vary widely.
How is the break-even point calculated for a rate lock?
The break-even point is calculated by dividing the rate lock fee (in dollars) by the monthly savings from locking versus floating at the scenario rate. For example, if a lock costs $1,200 and saves you $60/month compared to floating at a higher rate, you break even in 20 months (1,200 ÷ 60 = 20). If you keep the loan for longer than 20 months, the lock was financially beneficial. If you sell or refinance before then, floating might have been better.