Mortgage Rate Lock Calculator

Last reviewed June 1, 2026

Compare locking a mortgage rate now against floating until closing. Enter the loan amount, offered rate, expected rate move, lock fee, and holding period to see payment risk and break-even math.

Core formula

Lock cost / monthly savings

If the lock costs $1,000 and avoids a $50 higher payment, break-even is 20 months.

Main risk

Payment can move before closing

A 25 bps change can matter most on larger loan balances and tight DTI files.

Final source

Written Loan Estimate

The lender quote controls APR, points, cash to close, lock terms, and five-year cost.

Rate Lock vs Float Calculator

Model whether a mortgage rate lock fee is worth it if rates rise or fall before closing.

Try a lock scenario
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Lock Decision Signal
Lock Math Looks Favorable

If the rate rises 25 bps and you keep the loan about 60 months, the lower payment can outweigh the lock cost.

Locked Payment
$2,528.27
at 6.500%
Floating Payment
$2,594.39
if rate becomes 6.750%
Payment Difference
$66.12
lock saves per month
Lock Cost
$0
no payment break-even
Expected rate move+25 bps
Floating scenario rate6.750%
Payment difference over first 5 years or hold period$3,967
Interest during modeled hold period, locked$126,140
Interest during modeled hold period, floating$131,166
Net lock value over modeled holding period$3,967

How to Use the Rate Lock Calculator

A mortgage rate lock protects a quoted rate for a set period. The value of that lock depends on the loan amount, expected rate move, lock fee, closing timeline, and how long the borrower keeps the loan before refinancing or selling. This calculator turns those inputs into payment difference, lock-cost break-even, and holding-period math.

Locking can win when

Rates are likely to rise, the lock fee is small, the loan balance is large, or the borrower cannot afford a higher payment before closing.

Floating can win when

Rates fall before closing, the lock fee is high, the closing date is uncertain, or the borrower can tolerate a higher payment if the market moves against them.

Float-down terms matter

A float-down option can help if rates drop, but the minimum drop, fee, exercise window, and relock rules vary by lender.

Before You Lock a Mortgage Rate

  • Use the same loan amount, term, down payment, property type, and occupancy across lender quotes.
  • Compare APR, points, lender credits, Section A origination charges, cash to close, and five-year cost.
  • Ask whether the lock has a fee, a longer-lock premium, an extension cost, or a float-down option.
  • Match the lock period to the closing timeline plus a realistic buffer for appraisal, title, and underwriting delays.
  • Use the calculator for planning math, then let the written Loan Estimate and lock agreement decide the final comparison.

Source checkpoint

Amortio uses standard fixed-rate amortization math for payment estimates. The calculator is a planning tool, not a lender quote, approval, lock agreement, or financial advice.

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