There isn’t an app that I know of, but it’s very easy to calculate in the mobile version of excel or in Google sheets.
If there is not, and if I knew how to create an app, I would do that and sell it. I had a very simple financial program on my old MAC 512 that would do about a dozen different calculations that were very useful, including this one. It had a cool interface. Seems like if a MAC 512 could do it, an iPhone should have no trouble with it.
I also had an excel template that would do the same thing but it wasn’t as elegant.
That doesn’t calculate your monthly payment does it? I remember when I was getting a mortgage, the banker used a simple formula similar to that to give an approximate monthly payment. I freaked him out when I pulled out my Radio Shack calculator (1985) and calculated it to the penny. I was taking a financial class for my degree at the time and had just learned the formulas for stuff like that. Haven’t used them since.
Naw just the portion of the payment that would be applied to interest. So if the payment was $100 a month, the first month maybe $80 would be interest and the last month maybe $20 interest. I had one of those calculators at work. Can’t remember what it was good for (this was before PCs) but you could do amortization schedules on it and used it for net present value calculations from time to time. Killed a few trees anyway. Computers are much easier.
There are some apps for loan amortization. There are also many websites with loan calculators. Just do a search for “loan amortization calculator” in your favorite web search engine.
Here’s one for auto loans. This site will do home mortgage calculations too, among others.
Σk= (n×(n+1)/2) for k=1 to n . 12×13/2 = 78, 24×25/2=300
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Nerd. Keep up the good work. 
There are DOZENS of loan calculators. Some are extremely good with a lot of detail.
iPhones do have an app for it. Converter Plus (Converter +).
Search templates or conversion formulas created by other people from within the App or write your own formula by editing a template.
The formulas can get as complex as you like using standard notations. I managed to set up a Property investment template for quickly assessing comparative rates of return on specific properties with fully amortized compounding returns over specified periods.
Easier to use on an iPhone screen than Excel.
I don’t know why anyone would want a computer application for a a loan type like that, today’s loans are simple interest, not rules of 78.
They get the higher selling price.
They get a kickback from the affiliated finance.
They get a kickback from the finance insurance.
They try to get you to use their affiliated auto insurance for another kickback.
They add in the unpaid balance from your previous loan to increase kickback.
To sweeten the pot they add in an incentive package to get you to bring it in for maintenance only valid with them.
I would rather buy a one year old low mileage vehicle from Carmax than go through the hassle with a dealer. Its had it’s early failures and recalls and been thoroughly checked out.
Funny you mention carmax. Are you sure they thoroughly check out their cars? Please. The last car I purchased from carmax was a POS. It was filthy. Although it ran decent but some problems came up later. I’d not trust carmax more than any other car dealer. I’m wanting to go new this time.
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Substitute “commission” for “kickback” and you are correct.
And you don’t wonder why a one year old vehicle is on a used vehicle lot so soon .
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I have bought several from Carmax and they were perfect. Their warranty, on top of any mfr’s warranty left, was good. The no haggle price was fair, the trade-in was good. A couple of times, the vehicle was brought in from another location for my inspection before committing. The only problem I ever had, if you could call it a problem, was a bouncy ride on a 2011 Ranger I bought in 2012. Turned out the tires were inflated to 38 PSI. S/B 30 PSI. That fixed that. You walk in with a title and drive out an hour later with the vehicle. I don’t know why Carmax had the truck but a 2004 Cheve Malibu I bought from them with 9000 miles on it in 2005 had never been registered as sold with GM. My daughter bought a couple of Jeeps from them for wheels for my teen age granddaughters and that worked out well for them. Jeeps got popular with millennials and they sold the Jeeps years later for more than they paid for them.
On the kickback thing it was different for used cars with independent dealers. They sold cars to people that they knew were unlikely to afford them, got them back on repossession. One retired used car dealer laughingly told me he sold the same car seven times.
We used to call that “delivering a car on a rubber band”.
a lot or most of Carmax vehicles are off lease or rental car sales