Mortgage Calculators

This JS Library contains functions commonly used with mortgage calculations.

Background

While trying to build a set of mortgage calculators for a client, we were hoping to find a Javascript library that contained the functions.
After hours of research, we were unable to find any Javascript library. However, we found a bunch of spreadsheets and after thorough investigation, developed our own functions.

Getting Started

To get started you can download the minified code src/mortgage-calculators.js or clone the entire repo and import it into your next project.

Modules

This library contains 7 different modules used for different

1. Monthly Mortgage Payment
2. Monthly Mortgage Payment With Extra Monthly Payments
3. 15 Year VS 30 Year Mortgage
4. How Much Can I Borrow ?
5. Fixed Rate VS ARM
6. Refinance Mortgage
7. Should I Buy or Rent a home?

Monthly Mortgage Payment

Description:

This module calculates the monthly mortgage payment based on the loan amount, annual interest rate, and the term in years.

Usage:

mortgageCalculators.monthlyMortgagePayments({
    loanAmount : 100000,
    interestRate : 3.75, (Annual Rate, will be converted to a monthly percentage in calculations)
    termInYears : 30  
});

Response:

Returns 463.12;

Monthly Mortgage Payment With Extra Monthly Payments

Description:

This module calculates the monthly monthly payment from above with an extra monthly payment amount. It returns an object with annual and monthly breakdowns.

Usage:

mortgageCalculators.monthlyMortgagePaymentsWithExtraPayments({
    loanAmount : 200000,
    interestRate : 6.75,
    termInYears : 5,
    extraPaymentAmount : 300
});

Response:

{
    withExtraPayment : {
        totalMonthlyPayment : 4236.69,
        interestRate : 6.75,
        term : 5,
        totalCost : 233018.07,
        payments : [{ // Array of Annual Break Down
            annualInterestPayment: 12322.85,
            annualPrincipalPayment: 38517.47,
            balance: 161482.53,
            monthlyBreakdown: [{ // Array of monthly breakdowns
                monthlyPayment : 4236.69,
                principalPayment :  3111.69,
                interestPayment :  1125,
                balance :  196888.31
            }...]
        }...]
    },
    withoutExtraPayment : {
        totalMonthlyPayment : 3936.69,
        interestRate : 6.75,
        term : 5,
        totalCost : 236201.53
    }
}

15 Year VS 30 Year Mortgage

Description:

This module calculates the comparison of a 15-year mortgage with a 30-year mortgage based on the loan amount, and the interest rate of the respective year.

Usage:

mortgageCalculators.compareFifteenVsThirtyYearMortgages({
    loanAmount : 100000,
    interestRate1 : 3.25,
    interestRate2 : 3.75
});

Response:

{
    fifteenYearMortgage : {
        monthlyMortgagePayment : 702.67,
        totalPayments : 126480.38,
        totalInterest : 26480.38
    },
    thirtyYearMortgage : {
        monthlyMortgagePayment : 463.12,
        totalPayments : 166721.61,
        totalInterest : 66721.61
    }
}

How Much Can I Borrow?

Description:

This module calculates how much you could be expected to borrow from a lender based on various parameters. It returns an aggressive and conservative break down using commonly used debt-to-payment ratios.

Usage:

mortgageCalculators.howMuchCanIBorrow({
    interestRate : 3.75,
    grossMonthlyIncome : 4000,
    monthlyDebtPayment :  400,
    termInYears : 30,
    downPayment : 5,
    yearlyPropertyTax : 1200,
    yearlyPropertyInsurance : 1200        
});

Response:

{
  aggressive: {
    downPayment: 12092.01,
    loanAmount: 241840.27,
    priceOfHome: 253932.28
  },
  conservative: {
    downPayment: 9069.01,
    loanAmount: 181380.2,
    priceOfHome: 190449.21
  },
  futureMonthlyPayment: {
    aggressive: {
      principalAndInterest: 1320,
      taxesAndInsurance: 200,
      totalMonthlyPayment: 1520
    },
    conservative: {
      principalAndInterest: 1040,
      taxesAndInsurance: 200,
      totalMonthlyPayment: 1240
    }
  }
}

Fixed Rate VS ARM

Description:

This module calculates the comparison between a fixed rate mortgage and an adjustable rate mortgage based on various parameters.

Usage:

mortgageCalculators.comparefixedRateVsARM({
    loanAmount : 100000,
    termInYears : 30,
    interestRate : 6.5,
    monthsBeforeFirstAdjustment : 36,
    monthsBetweenAdjustments : 12,
    expectedAdjustmentRate : 0.25,
    initialInterestRate : 6.5,
    maximumInterestRate : 12
});

Response:

 {
    fixedRate : {
        monthlyMortgagePayment : 632.07
    },
    ARM : {
        initialMonthlyMortgagePayment : 632.07,
        maxMonthlyMortgagePayment : 889.32
    }
}

Refinance Mortgage

Description:

This module calculates new values for a new monthly mortgage after an n number of payments are already made. Returns an object with old and new mortgage values and the interest potentially saved by refinancing.

Usage:

mortgageCalculators.refinanceMortgage({
    interestRate : 3.75,
    loanAmount : 100000,
    termInYears : 30,
    paymentsMade : 12,
    newInterestRate : 2.75,
    newTermInYears : 30    
});

Response:

{
    interestSaved : 16899.79,
    oldMonthlyMortgage : {
        monthlyMortgagePayment : 463.12,
        remainingInterest : 63003
    },
    newMonthlyMortgage : {
        newMortgageTotal : 98161.22,
        monthlyMortgagePayment : 400.73,
        remainingInterest : 46103.21
    }
}

Should I Buy or Rent a Home?

Description:

This module calculates the potential benefits of buying a home as opposed to renting a home based on various given parameters. Returns a break down of the various costs of both buying vs renting, and a final value of money saved by buying. This final value may be negative to indicate a benefit of renting rather than buying.

Usage:

mortgageCalculators.compareBuyVsRent({
    monthlyRent : 800,
    expectedAnnualRentIncrease : 5,
    purchasePrice : 200000,
    downPayment : 5,
    interestRate : 6,
    termInYears : 30,
    closingCosts : 1.5,
    annualAppreciation : 3,
    howLongBeforeSelling : 10,
    currentAnnualInterestOnDownPayment : 3,
    incomeTaxRate : 28
});

Response:

{
    currentValueOfHome: 268783.28,
    totalOwedToBank: 167371.45,
    equityOnHome: 101411.83,
    netCostOfBuying: 11326.5,
    netCostOfRenting: 120747.77,
    benefitOfBuying: 109421.27
}