How the Calculator Works:
The calculator uses financial formulas to determine the car loan amount you can afford based on your monthly payments and interest rate. It also factors in the down payment, tradein value, and sales tax to give you an overall estimate of how much you can spend on a car.
Stepbystep explanation:

Loan Amount Calculation (Principal) The main calculation revolves around how much money you can afford to borrow based on your monthly car payment. This depends on:
 The monthly payment you wish to make.
 The loan term (in months or years).
 The interest rate (annual rate).
The formula to calculate the loan amount (principal) is:
$\text{Loan Amount} = \frac{\text{Monthly Payment} \times (1 – (1 + \text{Rate Monthly})^{\text{Term}})}{\text{Rate Monthly}}$Where:
 Monthly Payment = the payment amount you plan to make each month.
 Rate Monthly = the annual interest rate divided by 12 (months).
 Term = the total number of months over which you plan to pay off the loan.

Calculating Car Price Before Tax After determining the loan amount you can afford, the next step is to add the down payment and tradein value to find the car price before tax:
$\text{Car Price Before Tax} = \text{Loan Amount} + \text{Down Payment} + \text{TradeIn Value}$ 
Calculating Sales Tax Sales tax is calculated based on the car price before tax and the given tax rate:
$\text{Sales Tax} = \text{Car Price Before Tax} \times \frac{\text{Sales Tax Rate}}{100}$ 
Calculating Total Car Price (With Tax) The total car price is then calculated by adding the sales tax to the car price before tax:
$\text{Car Price With Tax} = \text{Car Price Before Tax} + \text{Sales Tax}$ 
Car Loan Needed The car loan needed is simply the loan amount (principal) required to finance the car purchase, excluding the down payment, tradein value, and sales tax.
Example Calculation:
Let’s walk through an example using the default values provided in the calculator.
 Monthly Payment: $350
 Sales Tax Rate: 7%
 Down Payment: $4,000
 TradeIn Value: $1,200
 Loan Term: 52 months
 Interest Rate: 3.5% annually
Step 1: Calculate the Loan Amount

Monthly Interest Rate:
$\text{Rate Monthly} = \frac{3.5}{12} = 0.0029167$ 
Loan Amount Calculation:
$\text{Loan Amount} = \frac{350 \times (1 – (1 + 0.0029167)^{52})}{0.0029167} \approx 17,735.51$
Step 2: Calculate Car Price Before Tax:
$\text{Car Price Before Tax} = 17,735.51 + 4,000 + 1,200 = 22,935.51$
Step 3: Calculate Sales Tax
$\text{Sales Tax} = 22,935.51 \times \frac{7}{100} = 1,605.49$
Step 4: Calculate Total Car Price With Tax:
$\text{Car Price With Tax} = 22,935.51 + 1,605.49 = 24,541.00$
Step 5: Car Loan Needed:
$$\text{CarLoanNeeded}=\text{LoanAmount}=17,735.51$$
Output Results in the Calculator:
The calculator would display the results in a table format as follows:
Calculation  Amount ($) 

Car Price You Can Afford  24,541.00 
Down Payment  4,000.00 
TradeIn Value  1,200.00 
Sales Tax  +1,605.49 
Car Loan Needed  17,735.51 