Starting today, workers will take home more money in their paychecks because of the 5 percent federal tax reduction.

For example, a $520-a-week married worker with two children will net $5 a week more due to the decrease in the withholding tax.

The tax benefits get a bit bigger as income rises.

A single worker who earns $625 a week and claims only one withholding exemption would clear an extra $8 to $9 because of lower taxes.

As wages shrink, so does the amount of the net increase. The $312-a-week married worker, for instance, who claims two withholding exemptions, would receive an increase in net pay of about $3.50 a week thanks to lower taxes.

If those figures look small, compared to the size of the debate over the reductions, relax. More and bigger cuts are scheduled.

Even now, Internal Revenue Service statisticians are busy churning out new tax tables to take effect next summer when the second phase of the Reagan tax reduction program is set to be implemented. The 1982 reduction in federal withholding taxes will average about 10 percent. And the one scheduled for 1983 will average 19 percent. Finally, there will be a 23-percent reduction in 1984 and subsequent years.

But don't spend the money yet.

The government could modify, delay or drop the plan to lower federal taxes further.

Moreover, Social Security withholding will increase Jan. 1, wiping out some of the gains that workers get with today's tax reduction. For instance, a worker now pays 6.65 percent of his wages, up to $29,700, into Social Security. But he will pay 6.7 percent on wages up to $31,800 beginning in 1982.

Workers should find the changes in the federal tax withholding in the next paycheck they draw, according to Internal Revenue Service officials.

However, it is possible that some employers won't be able to implement the new tax changes as quickly as others. But any workers who pay more tax now than they should because of such delays will be able to recover the money when they file their 1981 tax returns, officials said.

"The employer should make a good faith effort to do this lower the withholding ," said Rod Young, an IRS representative. "But if he can't, the employe won't lose. The day of reckoning is in the spring when the employe does his tax return and finds out if he has overpaid or underpaid his tax for the year."