Mortgage calculation with extra principal

Factual Questions
1

I have tried several online mortgage calculators and am getting different answers. Here are the details of the loan:

$94,000 - 20 year fixed mortgage at 3.45% Original payments were $543/month.

After 10 months I added an extra principal payment of $57/month for a new monthly payment of an even $600. Balance is now $84,622.

So far I have made 29 payments (2 years and 7 months). What is the remaining term to payoff the loan?

2

so the final loan from today(??) is:
$84,622 with payments of $600/mo. You should be able to find a calculator that does that.
Or, build a spreadsheet that calculates a line for principal minus $600 plus interest (principal*0.0345/12) each month.

My quickie spreadsheet tells me that you should be done paying off the last $77 on May 1, 2037

This assumes things like compound monthly, etc. and a month is 365.25/12 days.
2030-01-01 you would owe about $46,186.

3

This. Building your own spreadsheet allows all kinds of flexibility for trying whatever options you want.

4

Exactly. 4 columns - date, principal, payment, interest.
Next line - increment of above -
Date (first of month) is (previous date + 365.25/12)
Principal - equals principal above, minus payment from above line, but plus interest from above line
Payment - copy from above line
Interest - simple calculation based on (principal on this line * 0.0345/12)

Copy these lines all the way down.
Where does principal turn negative?
So for example if you plan to increase the payments in 5 years, find that date, change the payment amount.

Since things like exact date and interest accrued by days in month might be a little off, this won’t be exact to the penny, but within a few dollars anyway.

5

Excel also includes standard finance formulas. For the OP, the PMT function will be useful to calculate the minimum required payment based on the terms of the loan ($94K loan, 240 monthly payments, 3.45% annual interest rate).

https://exceljet.net/excel-functions/excel-pmt-function

The actual payment can just be that value for the first ten months ($543), and then $600 per month forever after.

I’ve used these kinds of homemade spreadsheets in the past to see how fast loans for cars and houses will get paid off when increasing my payments by various amounts.