Rate Lock Extension vs Floating Cost Estimator

Introduction

A mortgage rate lock can protect you from rising rates while your loan is processed, but locks expire. If your closing date slips past the lock period, lenders may offer a rate lock extension for a fee. The alternative is to let the lock expire and float—accepting whatever rate is available at closing.

This estimator compares those two paths using your loan details, your lender’s extension pricing, and your assumptions about how rates might move before closing. It is designed to be practical: you’ll see a plain-language recommendation, plus a scenario table you can download as CSV.

How to use this calculator

1. Enter your loan basics: loan amount, term, and your currently locked rate.
2. Enter timing: days until the lock expires and your estimated days until closing. The calculator uses the gap to determine whether an extension is needed.
3. Enter extension pricing: the extension fee as a percent of the loan, any flat fee, and the extension length in days.
4. Enter floating assumptions: the possible rate increase amount, the probability of an increase, the probability of a decrease, and the possible decrease amount. Any remaining probability is treated as “no meaningful change.”
5. Add hedging costs (optional): if you expect to pay for a hedge or option while floating, include it here.
6. Select Analyze to populate the results and the cash-flow summary table. Use Download CSV to export the table.

What the results mean (in plain English)

The results compare two estimated costs over the first five years of the loan:

• Extend: five-year interest at your locked rate plus the extension fee (if an extension is needed based on your timeline).
• Float: a probability-weighted five-year interest estimate across “rates up / rates down / no change,” plus any hedging cost you entered.

If the extension path is lower, the calculator will say extending is financially favorable based on your inputs. If floating is lower, it will say floating may offer better expected value. This is not a guarantee—just a structured way to compare the trade-offs.

Formula and assumptions

Monthly payment is estimated using the standard fixed-rate amortizing mortgage formula. The calculator then simulates the first 60 payments to estimate five-year interest for each rate scenario.

• Fixed-rate only: adjustable-rate mortgages and special products are not modeled.
• Three rate outcomes: up, down, or unchanged, using the probabilities you enter.
• Timing logic: if your estimated closing date is within the lock period, extension cost is treated as $0.
• Extension fee proration: the percent-based fee is prorated by extensionDays / 30 (matching the existing script behavior).
• Costs not included: taxes, insurance, mortgage insurance, discount points, and lender-specific rules unless you approximate them via the fee fields.

Worked example

Example inputs (similar to the defaults): a $400,000 loan for 30 years at a locked rate of 6.25%. The lock expires in 5 days, but closing is expected in 25 days, so you may need extra coverage. Your lender quotes an extension fee of 0.25% of the loan for a 30-day extension and no flat fee.

For floating, you assume a 60% chance rates rise by 0.50%, a 10% chance rates fall by 0.25%, and the remaining 30% chance rates stay about the same. After you click Analyze, the calculator will:

• Estimate the extension fee (if needed) and show the locked payment.
• Compute payments and five-year interest for the “up,” “down,” and “no change” floating scenarios.
• Combine those floating outcomes into an expected (probability-weighted) result.

Use the table to sanity-check whether your probabilities and rate-move sizes reflect your real decision. If you want a conservative view, increase the “rate increases” probability or the increase amount and re-run.

Understanding extension fees and timing (what lenders commonly do)

Extension pricing is not standardized. Some lenders quote a flat dollar amount, others quote a percentage of the loan amount, and many use a combination. Some price extensions in 7-day or 15-day increments; others only offer 30-day blocks. This calculator keeps the inputs flexible so you can mirror the quote you received.

The timing fields are intentionally simple: you enter days until lock expires and estimated days until closing. If closing occurs after the lock expires, the calculator treats the difference as a gap and assumes you need an extension to cover that gap. If closing is expected before expiration, the extension fee is treated as zero.

If your lender requires multiple extensions (for example, a 15-day extension followed by another 15-day extension), you can approximate that by entering the total extension days and the total fee you expect to pay. If your lender charges a higher fee for the second extension, you can reflect that by increasing the percentage or flat fee.

How to think about “floating” probabilities

Floating is a risk decision. The calculator asks for three outcomes: rates increase, rates decrease, or rates stay roughly the same. You provide the size of the increase and decrease, plus the probability of each. Any remaining probability is treated as “no change.”

If you are unsure how to set probabilities, start with a simple approach: choose a base case where you think rates are most likely to be unchanged, then add a smaller probability for a meaningful move up or down. If you want to be cautious, tilt the probabilities toward an increase. If you have a strong view that rates may fall, increase the decrease probability or the decrease amount.

The output is an expected value comparison. Expected value is not a promise; it is a weighted average. A decision can be “better on average” but still have a meaningful chance of being worse. If you are highly risk-averse, you may prefer the certainty of an extension even when the expected value is close.

Interpreting the five-year interest window

The table uses five-year interest as a consistent yardstick. Why five years? Many borrowers refinance, move, or otherwise change their mortgage within a few years, and five years is long enough to show meaningful differences without requiring a full 30-year projection.

If you expect to keep the loan much longer than five years, the long-run impact of a higher rate can be larger than what you see here. If you expect to sell or refinance within one to three years, the difference may be smaller. You can still use the calculator: treat the five-year number as a standardized comparison, and focus on the direction and magnitude of the difference.

Common scenarios where extending can make sense

Extending is often considered when (1) rates have risen since you locked, (2) your closing is delayed for reasons outside your control, or (3) you value certainty and want to avoid last-minute surprises. Even if floating has a slightly better expected value, the worst-case outcome of floating may be unacceptable if your budget is tight.

On the other hand, floating can make sense when (1) you believe rates are likely to fall, (2) the extension fee is unusually high, or (3) you have a float-down option or other protection that reduces the downside. This calculator does not model float-down clauses directly, but you can approximate them by reducing the expected increase amount or the probability of an increase.

Quick checklist before you decide

• Confirm the lock expiration date and whether weekends/holidays affect the lender’s definition of “days.”
• Ask how the extension is priced: per-day, per-week, or per-30-days, and whether the fee is refundable if you close early.
• Verify what is covered: some locks cover only the rate, while others also cover points/credits.
• Re-check your closing timeline: if the gap is small, a shorter extension (if available) may be cheaper than a full 30-day block.
• Stress test the float case: run a higher increase amount or probability to see your downside exposure.