Death is inevitable. So are taxes, at least for most of us.

But disability is something most people give little thought to. It happens, of course, but only to other people, right?

Wrong. "Depending on how you look at the statistics, there are more people who are disabled each year than there are who die," said John McIntyre of Alexandra Armstrong Advisors Inc., a financial planning firm here.

Considering all the illnesses, automobile accidents, industrial injuries and other unpleasant events that can befall us, this isn't so surprising. Though some disabilities are permanent, short-term ones are much more common. But for a family in need of every penny that every wage earner brings in, having one paycheck cut off for six or eight months because of an injury or illness can be a disaster.

But, said McIntyre, "far fewer people have a good disability {insurance} program than have good life insurance."

Disability insurance replaces income that is lost if you are ill or injured and unable to work. A prudent plan gives you enough replacement income to get you by. At the same time, the more coverage you have, the more it costs, so you don't want to buy too much.

The accompanying work sheet, supplied by Alexandra Armstrong Advisors, will help you figure out how much disability insurance would be required to keep your income at the same level as when you are working.

You, of course, must assess how much flexibility there is in your living standard. If many of your expenses are discretionary and you could actually live comfortably on less, this exercise may overstate your needs. The same may be true if your pay is small compared with your spouse's. But if you have large fixed expenses, such as a big mortgage or a child in college, a substantial income cut could be disastrous.

The first step is to add up all the income you would still have if you couldn't work -- your spouse's salary, any rental income you might have, disability benefits provided by your employer, plus assets that generate cash or could easily be converted to do so.

Social Security, which does provide disability benefits, isn't included here, said Kris Maley of Alexandra Armstrong Advisors, because its definitions of disability are so tough. It is meant for very long-term disability and you might not qualify if you could still work at some other job, though not at the one you had before, she said, so "we take a conservative approach and don't include it."

The example assumes that the disabled person's employer provided some coverage and paid for it. This is fairly common, Maley said, but if the employer has paid the premiums, the benefits are taxable to the employee. If you buy the insurance yourself, benefits are not taxable, she said.

Step 2 is to look at your expenses, a figure you can get from your family budget, if you keep one. Otherwise, you should sit down with your checkbook and look through several months of outlays to get a sense of how much you spend. Be careful to include items like taxes and tuition and car insurance that are paid only annually or a couple of times a year.

Then, before you simply plug the number in, go over it and see how much it could be reduced. You don't want to buy "gobs and gobs of insurance" that you don't need, Maley said. "Try to make it a bare-bones minimum, but be realistic."

Step 3 is to subtract expenses from income -- if the result is positive, you don't need any more insurance. If it is negative, this is your need on an annual basis. Divide the amount by 12 to reach the monthly need figure.

If you find you need more coverage, check with your employer first. Many offer optional additional coverage that you pay for, but at group rates.

If you have to buy an individual policy, read it carefully, Maley advised. There are significant differences among policies. While benefits usually run for life if you are permanently disabled, some policies stop paying at age 65. "You really need to read through all the benefits," she said.