What formula is used by lenders in calculating P&I?

[Tom Seigold]

Seems like it should be a simple formula, but my googling is failing me... I'm trying to figure out what a lender uses to calculate the following:

Inputs: mortgage balance (after downpayment), interest rate, and loan term

Output: monthly payment (principle and interest)

[Malcomb Stapel]

Quote from @Tom Seigold:

Seems like it should be a simple formula, but my googling is failing me... I'm trying to figure out what a lender uses to calculate the following:

Inputs: mortgage balance (after downpayment), interest rate, and loan term

Output: monthly payment (principle and interest)

 Is it not as simple as plugging it into a mortgage calculator? I'm guessing you could even Google an amortization calculator if you wanted to break it down by payment over the length of the loan.