If you are like most homeowners, your mortgage payments are due on a certain date every month for however long you hold the loan. For a typical 30-year mortgage, you make 12 monthly payments every year, or as many as 360 payments total over the life of the loan. But with one simple adjustment, you can lop thousands of dollars off your interest costs and slice six years or 72 monthly payments from your 30-year loan. How? Dump the monthly schedule and switch to biweekly payments.

With biweekly payments, you make a half-payment every two weeks instead of a full payment every calendar month. If you pay half the regular monthly amount every 14 days, that means at the end of 365 days, you've made 26 payments - the equivalent of 13 full payments, one every 28 days - rather than just 12 payments. Why? It's all in the units we use to measure time. Very simply: Thanks to the Gregorian calendar we use, a "month" varies in length from 31 to 28 days. But two weeks always equals 14 days, and four weeks is always 28 days. By making a full payment every 28 days instead of one every "month," you are making slightly more-frequent payments that convert that a 30-year mortgage into a 24-year loan.

Let's see how that affects a 30-year loan. Look at a $150,000 mortgage at 5.625 percent. Your monthly payment would be $863.48, which includes interest and principal but not taxes, insurance or any fees, dues, or assessments. If you make the standard 12 payments a year for 30 years you will pay off the initial loan of $150,000 plus $160,854.46 in interest. But make 13 payments a year and your total interest payment will be $129,819.02. That's a savings of $31,035.44. And your mortgage will be paid in 24 years. If you make biweekly payments, you would make half a payment every two weeks -- in this case, $431.74. Those 26 half-payments add up to 13 monthly payments every year.

The trick to understand here is that mortgage interest is computed every day on the unpaid principal. To illustrate this, look at the very first payment on the loan. You send in a check for $863.48 for 'payment one,' and $703.13 of it is used to pay interest. That means it reduces your principal by $160.35, leaving you with a new balance of $149,839.65. Since your principal has been reduced by a fraction, you will pay a fraction less in interest in month two-76 cents less, to be exact. That means in 'payment two,' $702.37 will be used to pay interest and $161.11 will go toward reducing the principal.

At the end of the first year your 12 payments will have totaled $10,361.76. Of that, $8,387.05 will pay interest, and only $1,974.71 will be used to reduce your principal, leaving an outstanding loan balance of $148,025.29. Now look at 13 payments, which will total $11,225.24 over a year. Of that $8,387.05 will have paid for interest, and $2,838.19 will have been credited toward principal, leaving an outstanding loan balance of $147,161.81.

If you can afford half a mortgage payment every two weeks, there are a few ways you can pay it. First, check with your lender. Although most mortgage companies do not accept partial payments, some lenders do offer biweekly payment programs you can join. There also are separate companies that will "warehouse" your biweekly payments and make them with whatever mortgage company you choose, usually for a fee. You can find these companies online and, in some large cities, in the telephone book.

If you decide to use a separate service to handle your biweekly payment, check their terms carefully because they can vary widely. Some will charge you a start-up fee for the payment service; others charge you a per-payment or monthly fee. Some charge both. Fees and costs can range from just a few dollars a month to a start-up fee of hundreds of dollars. Other companies actually do it for free. Why? Because they have use of your money for two weeks out of four.

Even though you are making half a mortgage payment every two weeks, the company sends a check for the full payment to your lender every four weeks, because the mortgage company probably doesn't accept partial payments. That means your biweekly company gets to keep that first half-payment in its account for two weeks earning interest. Two week's interest on $431.74 might not add up to much, but when that money is part of a pool of thousands of other checks, a pool that gets replenished on a regular basis, the interest does add up.

If you want to avoid possible fees and don't mind the extra effort, you can make 13 payments a year on your own - for free. Just set aside the additional money every two weeks, and then every time you have saved a full mortgage payment, send it to the mortgage company. Or, you can make your regular payment each month and save money all year in an interest-bearing account toward the 13th payment. Come December, send in the extra payment. If you make a 13th payment at the end of the mortgage's first year, the entire amount would be applied toward principal. That means you would start year two of the mortgage owing $147,161.96. By contrast, if you didn't make that 13th payment, it would take roughly six more months and six more monthly payments of $863.48 each to reduce your principal to that level.

Finally, if you don't like or can't afford the biweekly payment regimen, you always could just add an additional amount; say $50 or $100, whenever you can to your regular monthly principal payment. It still will help reduce the term of your loan and your total interest payments, even if you only do it occasionally. Check with your mortgage company first to ensure they will accept periodic additional principal payments and make sure you designate the added amount on the loan payment coupon as payment toward principal. Otherwise, it might be credited toward interest.

Biweekly mortgages make sense if you can afford half a payment every two weeks. If you can stick with a biweekly program, it can cut years off the life of a mortgage and save you thousands of dollars in interest payments.