As many as 18 million procrastinators are in for a shock when they look at their paychecks next month.

These are the people who have not yet filed a W-4 tax withholding form with their employer so that the federal income taxes taken from their checks would conform to the changes in tax law enacted last year. As an incentive to get taxpayers to make that adjustment, Congress directed that those who do not file by Oct. 1 -- Thursday -- will have their taxes adjusted anyway -- upward, in most cases.

The law requires that after Thursday, single taxpayers who have not filed a W-4 will be limited to only one withholding allowance, and married taxpayers can take no more than two. Most taxpayers with itemized deductions take more allowances than that, reducing the tax bite each pay period.

Another incentive to file the W-4 as soon as possible is the reduced likelihood of drawing a penalty for underwithholding.

Each allowance -- the amount of salary on which taxes are not withheld -- is worth $1,900 this year. A married taxpayer now taking 10 allowances, for instance, has no tax withheld on $19,000 of salary.

If that taxpayer is cut to two allowances, $15,200 more in annual income will be subject to withholding. For someone in the 25 percent bracket who gets paid once every two weeks, that comes to $146 more in taxes taken from each paycheck.

"If the employer is really on the ball, it should happen in the next paycheck" after Oct. 1, said Internal Revenue Service spokesman Steve Pyrek. He added, however, that the speed at which the paycheck changes will vary from company to company. Taxpayers still can straighten out their withholding after Thursday simply by filing the W-4.

Taxpayers reconcile the difference between the amount of taxes withheld and what they actually owe each spring, when they file their tax returns. But the new tax law, which reduces tax rates but repeals or limits many widely used deductions, may lead to greater disparities between the two figures than taxpayers are accustomed to because some people may owe far less or more than they did the previous year.

The law increased the penalties for withholding too little. If a taxpayer withholds less than 90 percent of the amount he owes or 100 percent of what he owed last year, he could be assessed with a penalty as well as an interest charge. Currently, the rate for interest and penalties is running at 10 percent for each.

Taxpayers who filed their W-4 forms by June 1 are exempt from an underwithholding penalty for 1987. Those who fail to make the Oct. 1 deadline face no additional penalty other than losing their extra withholding allowances, and they can restore those by filing the form with their employer.

Nationwide, the IRS estimates that about 20 percent of the 94 million taxpayers who are expected to file the W-4 have not yet done so. In the Washington area, several large employers show about the same pattern. At Giant Food Inc., 83 percent of the company's 23,000 workers have turned their forms in. About 80 percent of employes at The Washington Post Co. have completed their forms.

Taxpayers can file either the longer, more accurate W-4 form, or its shorter counterpart, the W-4A. Both include instructions on how to calculate the correct number of allowances, although the law's numerous changes are not all explained. For instance, taxpayers are directed to put the amount of nonwage income on one line of the form, but only interest or dividends are mentioned as possible examples. The form does not point out that all unemployment compensation is fully taxable this year and also should be included in calculating withholding