TO MAKE UP your mind about the windfall oil profits tax, now under vigorous debate in the Senate, it helps to begin with a little arithmetic. President Carter has committed the country -- properly and necessarily -- to decontrolling oil prices over the next two years. If you assume, as most people do, that the world price will be over $30 a barrel by then, the gross revenues of the U.S. domestic oil producers will double over those two years. Their revenues will increase by some $60 billion.

The windfall profits tax has nothing to do, in fact, with profits. It is an excise tax -- that is, a tax on each barrel of oil produced. The questions now before the Senate are how high to set that tax, and whether to vary it on the different categories of oil. The bill being debated in the Senate, drafted by its Finance Committee, would raise about half as much money as the version passed by the House last June. Which is right?

The evidence weighs heavily in favor of the House bill. The oil industry objects that the House bill would destroy incentives for further exploration. That's nonsense. Under the House bill, a barrel of newly discovered oil sold at $30 would pay a windfall tax of $6.50. That's hardly confiscatory -- particularly when you remember that a similar barrel of newly discovered oil today is sold, under the controls, for less than $14, and exploration continues at a high rate.

Under the Seante bill, newly discovered oil would pay no tax at all. It would be exempt, to stimulate further discovery. But the Congressional Budget Office, like most other analysts, warns that domestic production is very unlikely to rise, regardless of prices offered. The only real question is how fast production falls. The higher the price, the slower that decline -- but large differences in price incentives seem to offer only modest differences in the amounts of oil that will be found and brought to the market.

There are several basic principles that this new tax ought to reflect. It ought to follow, in general, the House bill in cutting down excessive incentives. But it would well follow the Senate bill in tilting decisively in favor of new discoveries, rather than heavy production from old fields. Third, the idea of segregated trust funds is a fundeamentally bad one. Both bills would put this tax's revenues in trust funds; the normal appropriations process works a great deal better.

Without a stiff excise tax on oil, the rush of revenues from decontrol and a soaring world price promise severe damage to the American economy. If the companies put all of the money into drilling, they will rapidly push exploration far beyond the point of diminishing returns. If they use the money to diversity into other businesses, the implications for competition and diversity in the American business world are unwholesome. This tax is a device to maintain a crucial balance that is in danger of being lost.