# KeyRate - Complete Mortgage Calculator Reference

Free mortgage calculators — monthly payment, affordability, refinance, rent vs buy, extra payments, amortization, closing costs, down payment, and ARM vs fixed. Real formulas, instant results, visual breakdowns.

Last updated: 2026-08-07T22:15:22.870Z

## How KeyRate Works
KeyRate provides a comprehensive suite of mortgage and home financing calculators. All calculations run instantly in the browser with visual charts and amortization tables. No sign-up required, no data stored or transmitted.

## Calculators

### Monthly Payment Calculator
- URL: https://mortgage.thicket.sh/monthly-payment
- Purpose: Calculate your monthly mortgage payment including principal, interest, taxes, and insurance
- Inputs: Home price, down payment ($ or %), loan term (years), interest rate (%), property tax rate, homeowner's insurance (annual), PMI rate (if applicable), HOA fees
- Output: Monthly payment breakdown (principal + interest, taxes, insurance, PMI, HOA), total payment, total interest over loan life
- Formula: M = P * [r(1+r)^n] / [(1+r)^n - 1], where P = principal, r = monthly rate, n = total payments
- Use cases: Home shopping budgeting, comparing loan terms, understanding total cost of homeownership

### Affordability Calculator
- URL: https://mortgage.thicket.sh/affordability
- Purpose: Determine how much house you can afford based on income and debts
- Inputs: Annual gross income, monthly debts (car, student loans, credit cards), down payment available, interest rate, loan term, property tax rate, insurance estimate
- Output: Maximum home price, maximum monthly payment, recommended price range (conservative/moderate/aggressive)
- Method: Uses the 28/36 rule — housing costs should not exceed 28% of gross monthly income, total debt should not exceed 36%
- Use cases: First-time homebuyers, setting a budget before house hunting

### Refinance Calculator
- URL: https://mortgage.thicket.sh/refinance
- Purpose: Determine if refinancing your mortgage makes financial sense
- Inputs: Current loan balance, current monthly payment, current interest rate, remaining term, new interest rate, new loan term, closing costs
- Output: New monthly payment, monthly savings, total interest savings, break-even point (months), total cost comparison (current vs. refinanced)
- Method: Compares total cost of remaining current loan vs. new loan including closing costs
- Use cases: Evaluating refinance offers, deciding between rate-and-term vs. cash-out refinance

### Rent vs. Buy Calculator
- URL: https://mortgage.thicket.sh/rent-vs-buy
- Purpose: Compare the long-term financial impact of renting vs. buying a home
- Inputs: Home price, down payment, interest rate, loan term, monthly rent, annual rent increase, property appreciation rate, property tax rate, insurance, maintenance (% of home value), investment return rate (for rent savings)
- Output: Year-by-year comparison of total costs, net worth impact, break-even year, cumulative cost charts
- Method: Models equity building (buying) vs. investment growth (renting the difference), accounting for appreciation, maintenance, tax benefits, and opportunity cost
- Use cases: Major life decision analysis, financial planning, comparing markets

### Extra Payments Calculator
- URL: https://mortgage.thicket.sh/extra-payments
- Purpose: See how additional payments shorten your loan and reduce total interest
- Inputs: Loan amount, interest rate, loan term, extra monthly payment (or extra annual payment, or one-time payment)
- Output: New payoff date, years saved, total interest saved, side-by-side amortization comparison (with and without extra payments)
- Method: Recalculates amortization schedule with additional principal payments applied each period
- Use cases: Deciding how much extra to pay, visualizing the impact of biweekly payments or annual lump sums

### Amortization Calculator
- URL: https://mortgage.thicket.sh/amortization
- Purpose: Generate a full amortization schedule showing every payment over the loan life
- Inputs: Loan amount, interest rate, loan term
- Output: Month-by-month table showing: payment number, payment amount, principal portion, interest portion, remaining balance. Summary: total payments, total interest, payoff date
- Method: Standard amortization formula applied iteratively
- Use cases: Understanding how much goes to interest vs. principal over time, tax planning (interest deduction), loan comparison

### Closing Costs Calculator
- URL: https://mortgage.thicket.sh/closing-costs
- Purpose: Estimate total closing costs for a home purchase
- Inputs: Home price, loan amount, location (state), loan type
- Output: Itemized closing cost estimate including: origination fees, appraisal, title insurance, recording fees, prepaid taxes, prepaid insurance, escrow deposits. Total closing costs as dollar amount and percentage of home price
- Typical range: 2-5% of home price
- Use cases: Budgeting for home purchase, comparing lender fee sheets

### Down Payment Calculator
- URL: https://mortgage.thicket.sh/down-payment
- Purpose: Calculate how much to save for a down payment and how long it will take
- Inputs: Home price, target down payment percentage, current savings, monthly savings amount
- Output: Down payment amount needed, months to reach goal, PMI threshold analysis (shows impact of 5%, 10%, 15%, 20% down), monthly payment comparison at each down payment level
- PMI note: Private mortgage insurance is typically required below 20% down, adding 0.5-1.5% of loan amount annually
- Use cases: Savings planning, understanding the trade-off between down payment size and monthly cost

### ARM vs. Fixed Rate Calculator
- URL: https://mortgage.thicket.sh/arm-vs-fixed
- Purpose: Compare adjustable-rate mortgages (ARM) to fixed-rate mortgages
- Inputs: Loan amount, fixed rate, ARM initial rate, ARM adjustment period (e.g., 5/1, 7/1), ARM rate cap (periodic and lifetime), expected rate adjustment, loan term
- Output: Monthly payment comparison over time, total cost comparison, worst-case ARM scenario, best-case ARM scenario, break-even analysis
- ARM types explained:
  - 5/1 ARM: Fixed for 5 years, adjusts annually after
  - 7/1 ARM: Fixed for 7 years, adjusts annually after
  - 10/1 ARM: Fixed for 10 years, adjusts annually after
- Method: Models payment trajectories under multiple rate adjustment scenarios
- Use cases: Choosing between ARM and fixed, understanding rate risk, planning for refinance timing

## Key Formulas

### Standard Monthly Payment
M = P * [r(1+r)^n] / [(1+r)^n - 1]
- M = monthly payment
- P = loan principal
- r = monthly interest rate (annual rate / 12)
- n = total number of payments (years * 12)

### Total Interest
Total Interest = (M * n) - P

### 28/36 Rule (Affordability)
- Front-end ratio: Monthly housing cost / Gross monthly income <= 28%
- Back-end ratio: (Monthly housing cost + monthly debts) / Gross monthly income <= 36%

## Technical Details
- All calculations: Client-side JavaScript with instant results
- Charts: Interactive visual breakdowns (pie charts for payment composition, line charts for amortization)
- Precision: Calculations use standard banking rounding (to the cent)
- Privacy: No financial data is stored or transmitted

## Pages
- Homepage: https://mortgage.thicket.sh
- All calculators accessible from homepage navigation

## API Endpoints
- LLM Summary: https://mortgage.thicket.sh/llms.txt
- LLM Full Reference: https://mortgage.thicket.sh/llms-full.txt