Mortgage Rate Lock Extension Calculator
Enter your loan amount, locked rate, possible replacement rate, and lender extension quote. The calculator estimates the extension fee, payment difference, break-even point, and net savings for the time you expect to keep the mortgage.
Rate lock extension fee$500.00
- Monthly payment at locked rate
- $2,528.27
- Monthly payment if repriced
- $2,594.39
- Monthly payment savings
- $66.12
- Interest-based break-even time
- 6 months
- Break-even replacement rate
- 6.52%
- Net savings over holding period
- $6,539.02
- Lower-cost scenario
- Extending the lock is lower cost over the planned holding period.
Only the fee field matching the selected quote method is included in the extension cost.
Assumes a fixed-rate, fully amortizing mortgage with monthly principal-and-interest payments.
How to use this calculator
- Enter the mortgage amount and the full amortization term.
- Enter the protected locked rate and the rate you would receive if the lock expired.
- Choose how the lender quoted the extension fee, then fill in the matching fee field.
- Enter how long you expect to keep the loan before selling or refinancing.
- Compare the fee, break-even time, and net savings before deciding what to ask the lender.
How the rate lock extension estimate works
A mortgage rate lock extension is usually priced as a percent of the loan amount, a number of basis points per day, or one flat dollar charge. This calculator converts the selected quote to dollars, then compares the locked rate with the replacement rate you would use if the lock expired.
The extension fee is calculated from the quote method you select:
E = L x f / 100when the quote is a percent of the mortgage amount.E = L x b / 10000 x dwhen the quote is basis points per day.E = Fwhen the quote is one flat fee.
In those formulas, L is the mortgage amount, f is the quoted percentage, b is basis points per day, d is the number of extension days, and F is the flat fee. Only the field that matches the selected quote method is included in the cost.
How the mortgage comparison is calculated
For each rate, the tool uses the standard fixed-rate mortgage payment formula. Let n be the number of monthly payments, and let i be the annual rate divided by 1200. The monthly payment is M = L x i / (1 - (1 + i)^-n). If the rate is 0%, the payment is simply L / n.
After k payments, the remaining balance is B = L(1 + i)^k - M((1 + i)^k - 1) / i. Cumulative interest through that month is C = kM - (L - B). The calculator measures the interest avoided by keeping the locked rate as C(replacement rate) - C(locked rate).
The interest-based break-even time is the first month when that avoided interest equals or exceeds the extension fee. Net savings over your holding period equals interest avoided through that holding period, minus the extension fee. The break-even replacement rate is solved numerically: it is the replacement rate that would make holding-period interest savings exactly equal the extension fee.
What moves the result most
The rate gap is usually the biggest driver. A small extension fee can make sense when the expiring quote is materially higher than the locked rate and you plan to keep the mortgage for several years. The result gets weaker when the replacement rate is close to the locked rate, the extension is expensive, or you expect to refinance or sell soon.
What this calculator leaves out
This is a principal-and-interest comparison only. It excludes taxes, homeowners insurance, mortgage insurance, points, lender credits, other closing costs, float-down rights, approval risk, and responsibility for the delay. Lender rules can also matter: some lenders use minimum extension periods, escalating extension schedules, worst-case repricing, or program-specific rules that this model does not attempt to predict.
Worked example
Suppose the mortgage amount is $400,000 for 30 years. The current lock is 6.5%, the replacement rate is 6.75%, the extension is 15 days, and the lender quotes 0.125% of the loan amount.
The extension fee is $400,000 x 0.125%, or $500. The locked-rate payment is about $2,528.27, while the repriced payment is about $2,594.39, so keeping the lock lowers the principal-and-interest payment by about $66.12 per month.
If the borrower expects to keep the mortgage for seven years, the locked rate avoids about $7,039.02 of interest over that period. After subtracting the $500 fee, the net savings are about $6,539.02. The interest avoided reaches the fee in month 6, and the break-even replacement rate is about 6.518%.
Common questions
How much does a mortgage rate lock extension cost?
The cost depends on the lender quote. Some quotes are a percent of the loan amount, some are basis points per day, and some are a flat fee. This calculator converts the quote you enter into a dollar fee.
Is a rate lock extension fee charged per day or as a percentage?
It can be either. A lender may quote one total percentage for the extension, a daily basis-point charge multiplied by the extension days, or a fixed dollar fee. Use the wording in the written quote rather than assuming a standard schedule.
When is paying to extend a mortgage rate lock worth it?
It is more likely to be worth it when the locked rate is below the replacement rate by enough to recover the fee during the time you expect to keep the mortgage. A short holding period or a small rate difference can make repricing cheaper even when the monthly payment is higher.
Who pays if the delay was caused by the lender, builder, seller, or borrower?
Payment responsibility depends on the lock agreement, the purchase contract, and the lender or builder program. This calculator only measures the cost tradeoff. Ask for the fee, any credit, and the reason for the delay in writing.
What happens if a mortgage rate lock expires before closing?
The loan may need to be repriced at current market terms, extended for a fee, or handled under a lender-specific policy. Some programs use worst-case pricing or have limited extension options. Confirm the available choices with the lender before the expiration date.
Why is break-even not just the fee divided by monthly payment savings?
Monthly payment savings include both principal timing and interest. The break-even shown here uses cumulative interest avoided, which is a cleaner way to compare the cost of keeping one fixed rate instead of another over the same mortgage term.