Why? My friend is car shopping.

My friend is car shopping, and I wanna help her make sense of what changes to what numbers will affect the monthly payment and how much.

I believe one of the best ways to build a sense of understanding and confidence in a decision with many factors is to play with a model of the system.

Prior Art: Top Google Results

What calculators currently exist? What do they get right? What do they get wrong?

Let's google "monthly car calculator" and look at the top 5 non-ad results.

NerdWallet

Things I like:

• clean UI
• "total interest cost"
• default numbers
• hides advanced options - sales tax, title/registration fees, include taxes and fees in loan

Things I'd change:

• would be nice to have the loan terms expressed in years as well, i.e ~4.2 years
• sliders can be easier on mobile than input fields
• I dont need trade-in value and amount owed on trade-in

Calculator.net

Things I like:

• "upfront payment"
• "total loan interest"
• percent of interest donut chart

Things I'd change:

• UI wastes space and is busy on the eyes
• that amortization schedule line chart doesnt seem to add much
• annual schedule doesnt add much

Cars.com

Things I like:

• clean UI
• clear monthly payment UI
• includes credit rating

Things I'd change:

• assumes 0% APR?!?
• length of loan only has 4 options

Bankrate

Things I like:

• compact UI

Things I'd change:

• mobile UI needs a little spacing help
• set a default interest rate

Capital One

Things I like:

• compact UI

Things I'd change:

• no defaults

How Can We Do Better?

Visualizing the System

When you are buying a car the biggest things you have control over:

• price of the car (by choosing a different car)
• loan term length

It would be nice to see how these two things affect the monthly payment.

With a basic calculator, like all of the above, the user needs to repeatedly input different numbers and hold previous calculations in short-term memory to compare.

I'd like to build a calculator that shows the user how the monthly payment changes as they change the price of the car and the length of the loan.

This would require showing some graph/chart of the monthly payment as the user changes the car's price and the loan's length.

Other Small Improvements

There are other small improvements I'd like to make:

• default numbers - set a bit higher than average so the user feels good as they improve the numbers and shrink the monthly payment
• sliders - easier to use on mobile
• express loan length in years as well as months
• fitting the UI to mobile and as close to a single screen as possible