The calculators here indicate how different extra payment plans affect the payoff date, the total amount of interest paid. and the borrower’s equity in the property.

Many mortgage borrowers want to pay down their debt before term, and are interested in how different extra payment plans will affect the payoff date, the amount of interest they have to pay, how long it will take to get out from underwater, and more. As background, these articles are recommended: Can I Do My Own Biweekly Mortgage?  Mortgage Prepayment as Investment: Another Look; Using a Calculator to Prepay an ARM; and A Great Investment in Loan Repayment. 

Mortgage Payoff Calculator: Extra Payments. (2a)
Allows borrowers to specify almost any combination of payments, payment intervals and payment periods, and see the impact on the amortization schedule, payoff date and total interest paid.

Mortgage Payoff Calculator: Extra Monthly Payments to Pay Off in Specified Period. (2c)
For borrowers who have a target payoff date and want to know how much extra they must pay, above their required monthly payment, to pay off their loan within that period.

Mortgage Payoff Calculator: Biweekly Payments Applied Monthly. (2b)
For borrowers who want to know when their loan will pay off, and how much interest they will save, if they shift to a biweekly payment plan, and if they make extra voluntary payments in addition to their required monthly payment.

Mortgage Payoff Calculator: Biweekly Payments Applied Biweekly. (2bi)
For borrowers who want to know when their loan will pay off, and how much interest they will save, if they shift to a biweekly payment plan on which the biweekly payments are credited biweekly rather than monthly.

Mortgage Payoff Calculator: Property Appreciation and Extra Monthly Payments on Underwater Mortgages. (2d)
For borrowers with negative equity (they are "underwater") who want to know how long it will take before some combination of normal amortization, extra payments and, property appreciation eliminate the negative equity.

Mortgage Payoff Calculator: Extra Monthly Payments and Property Value Appreciation to Reach a Desired Equity in a Specified Period (2e)

For borrowers with negative equity (they are "underwater") who want to know how much extra they must pay to reach a target equity within a specified period.