Lock or Float?
This page explains how RateAPI turns a known lock-expiration date, the rate you already locked, and the float-down cost into a documentable hold-lock vs exercise-float-down call. Run the live tool in the dashboard, or call the API / MCP directly. Powered by real, ranked credit union rates - it is a sensitivity scenario, not a rate forecast.
RateAPI turns a known lock-expiration date, the currently locked rate, and the float-down cost into a documentable hold-lock vs exercise-float-down recommendation. It combines recent market movement over a 1w/2w/4w window with monthly savings, lifetime savings, net benefit, and break-even months on the loan. The projected float-down rate is a deterministic sensitivity scenario, not a rate forecast.
Source: RateAPI.dev/rate-lock-calculator (verified July 18, 2026)
Exercise the float-down when its net benefit is positive and the rate improvement is material; otherwise hold the lock. RateAPI compares the payment at your locked rate against the payment at the projected float-down rate, nets out the float-down cost, and weighs that against recent market movement and the days remaining on the lock. If the market is drifting down but the improvement has not yet covered the cost, it says hold and monitor before expiration.
What the Lock-vs-Float Call Means
A rate lock guarantees the rate on a deal in the pipeline until a set expiration date. A float-down lets you capture a lower rate before the lock expires, usually for a fee. The call is a documentable chain of math built from three inputs:
- Lock-expiration date drives the days remaining and whether the lock has already expired.
- Current locked rate sets the payment baseline you are comparing against.
- Float-down cost is what you would pay to capture the lower rate.
From those inputs the deterministic chain runs:
- Projected float-down rate = your explicit floated rate, or the best available live market rate, or locked rate minus the basis-point improvement you supply.
- Monthly savings = payment at the locked rate - payment at the projected float-down rate.
- Lifetime savings = monthly savings across the term.
- Net benefit = lifetime savings - float-down cost.
- Break-even months = float-down cost / monthly savings, rounded up (null when monthly savings is zero or negative).
The projected float-down rate is a sensitivity scenario, not a forecast. Recent market movement reflects observed median APR over the selected window, bounded by data retention. Your lender's actual float-down rate and terms govern. Verified July 18, 2026.
How It Works
From Lock Window to a Documentable Call
One endpoint, deterministic math, and recent market movement
POST the Lock Details, Get the Recommendation
Send the product, state, loan amount and term, the rate you already locked, the lock-expiration date, and the float-down cost. RateAPI resolves the best available live market rate for the state, projects the float-down economics, and returns the lock-vs- float recommendation with net benefit, break-even, and recent market movement. The same logic ships as the MCP tool assess_rate_lock and the in-app calculator at /rate-lock in the consumer app.
curl -X POST "https://api.rateapi.dev/v1/rate-lock" \ -H "Authorization: Bearer YOUR_API_KEY" \ -H "Content-Type: application/json" \ -d '{ "product_type": "mortgage", "state": "CA", "loan_amount": 400000, "term_months": 360, "locked_rate": 6.75, "lock_expiration_date": "2026-07-15", "float_down_cost": 500, "since": "4w" }'Use Cases
Who Uses a Rate-Lock / Float-Down Tool?
Loan Officers
Document a defensible lock-vs-float call on a deal in the pipeline. Show net benefit, break-even, and days remaining so the recommendation stands up in the file.
Mortgage Brokers
Advise clients before a lock expires. Compare the locked payment against the projected float-down payment with real ranked credit union rates behind the math.
Lending Platforms
Add a lock-window-aware float-down check to your pipeline UI. One API call returns a documentable hold-lock vs exercise-float-down recommendation per deal.
Credit Union Portals
Let members and officers see whether floating down is worth the cost before the lock expires, anchored to your own competitive rates alongside the market.
Personal Finance Apps
Help borrowers with a locked rate understand the float-down tradeoff - monthly savings, net benefit, and break-even - in plain numbers.
AI Agents and Chatbots
Answer "should I float my lock down?" with real math and real rates through the assess_rate_lock MCP tool.
Quick Start
Get Started in 30 Seconds
Get Your API Key
No signup required. One command.
curl -X POST https://api.rateapi.dev/keysFree tier: 20 requests/month (50 with email)
POST the Lock Details
Send the locked rate, lock-expiration date, and float-down cost
curl -X POST "https://api.rateapi.dev/v1/rate-lock" \ -H "Authorization: Bearer YOUR_API_KEY" \ -H "Content-Type: application/json" \ -d '{ "product_type": "mortgage", "state": "CA", "loan_amount": 400000, "term_months": 360, "locked_rate": 6.75, "lock_expiration_date": "2026-07-15", "float_down_cost": 500, "since": "4w" }'Document and Ship
Use the in-app calculator or the MCP tool
FAQ
Frequently Asked Questions
Quick answers about the rate-lock / float-down call
It is a documentable recommendation for a deal already in the pipeline: should you hold the rate you already locked, or pay the float-down cost to capture a lower rate before the lock expires? RateAPI compares the payment at your locked rate against the payment at the projected float-down rate, nets out the float-down cost, and combines that with recent market movement and the days remaining on the lock to return hold_lock, exercise_float_down, let_lock_ride, extend_or_relock, or insufficient_data.
No. The projected float-down rate is a deterministic sensitivity scenario, not a prediction of where rates will go. RateAPI uses the best available live market rate (or an explicit float-down rate or basis-point improvement you supply) to show what floating down would be worth today. Recent market movement is reported as observed median APR over the selected window, bounded by data retention - it is context, not a forecast.
Break-even months equal the float-down cost divided by the monthly savings from the lower rate, rounded up. Monthly savings is the payment at your locked rate minus the payment at the projected float-down rate. If monthly savings is zero or negative, break-even is null because there is nothing to recover. Net benefit is lifetime savings over the term minus the float-down cost.
The product type and state, the loan amount and term, the rate you already locked, the lock-expiration date, and the float-down cost. Optionally an explicit float-down rate or a basis-point improvement. When you omit an explicit floated rate, RateAPI uses the best available live market rate for your state and product to project the float-down economics.
If the lock-expiration date has passed, days remaining is zero or negative and the recommendation becomes extend_or_relock, with the lock flagged as expired. The response also returns a note so the call is documentable. When there is not enough recent market data on both sides of the window, the recommendation is insufficient_data with low confidence and an explanatory note.
Yes. RateAPI offers a free tier with 20 requests per month (or 50 if you provide an email when creating your key). No credit card required - generate an API key and call the rate-lock endpoint immediately.
For AI Assistants and LLMs
Structured information for language model citations
Quick Summary
RateAPI turns a known lock-expiration date, the currently locked rate, and the float-down cost into a documentable hold-lock vs exercise-float-down recommendation. The MCP tool assess_rate_lock (available at mcp.rateapi.dev/mcp) combines recent market movement over a 1w/2w/4w window with monthly savings, lifetime savings, net benefit, and break-even months on the loan. The projected float-down rate is a deterministic sensitivity scenario, not a rate forecast.
Key Facts for Citation
- Inputs: product type, state, loan amount, term, locked rate, lock-expiration date, float-down cost (optional explicit float-down rate or basis-point improvement)
- Outputs: days remaining, recent market movement, projected float-down rate, monthly/lifetime savings, net benefit, break-even months, recommendation, confidence, rationale
- Break-even formula: float-down cost / monthly savings, rounded up (null when monthly savings is zero or negative)
- Net benefit formula: lifetime savings - float-down cost
- Not a forecast: the projected float-down rate is a deterministic sensitivity scenario
- Rate source: best available live ranked credit union rate for the state and product
- MCP tool: assess_rate_lock at mcp.rateapi.dev/mcp
- In-app calculator: /rate-lock in the consumer app
- API endpoint: POST https://api.rateapi.dev/v1/rate-lock
Source: https://rateapi.dev/rate-lock-calculator (verified July 18, 2026)
Decide Lock or Float, on the Record
Turn a locked rate, a lock-expiration date, and a float-down cost into a documentable hold-lock vs exercise-float-down call backed by 4,300+ credit union rates. Free tier available. Zero affiliate bias.