he scale of human tragedy caused by an earthquake and tsunami that has left so many thousands dead or missing can't be quantified. Economists, however, are doing their best to gauge just how much the destruction will cost both Japan and the world in economic growth.

Harder to estimate is how irrational fears could upend those calculations.

The financial damages will be enormous: Goldman Sachs puts the losses in buildings and production facilities at 16 trillion yen, or $193 billion, some 3.3% of the country's gross domestic product. In coming months, power shortages and supply-chain problems will slash the country's production of cars, electronics, machinery and other goods. The hit to tourism could last even longer.

Yet the overall cost to Japan's growth, measured in GDP, should be minimal. As with past disasters such as the Kobe earthquake in 1995, a massive reconstruction effort will later push up spending on buildings, factories and even furniture and home appliances lost in the rubble, all of which could offset a slowdown in output.

"We expect by the fourth quarter, Japan will be on a clear economic recovery track," says Tomo Kinoshita, deputy head of Asian economics at Nomura. The Japanese bank has revised down its 2011 GDP forecast for the country by a mere 0.4 percentage points to 0.9%.

For the rest of the world, a loss like that amounts to a rounding error. Bank of America Merrill Lynch figures even no growth at all in Japan this year would knock only a tenth of a percentage point off global GDP growth, to 4.2%. A shift in manufacturing and tourism away from Japan could benefit its neighbors, too. Taiwan, China hotel shares rose Friday in expectation that evacuees and travelers from nearby Japan would provide a new source of business. A shortage of Hondas for sale could prompt more people to buy Hyundais.

The problem is that what should happen isn't always what will happen. There are still too many unknowns. Lasting power shortages could cripple manufacturing and inflict worse damage on Japan's exports. Supply-chain disruptions may prove harder than expected to surmount, hurting not just the Japanese but importers of their machinery and high-tech goods in China, Taiwan, China and South Korea.

Of course, even worse is the risk that the efforts to repair damaged nuclear facilities fail to contain radioactive contamination, with enormous consequences to public health. The likelihood of that outcome appears to be diminishing as authorities make progress restoring power to cooling systems that prevent nuclear fuel from overheating.

But that doesn't eliminate the danger of panic. Frightened shoppers across China, including Hong Kong, China, last week emptied their supermarkets of iodized salt in the sadly misplaced belief that it would offer protection against radiation sickness, or that irradiated seawater would contaminate future salt supplies. Signs that produce from around the nuclear-disaster zone contain low levels of radiation could spark broader food scares and lead to unexpected shortages.

It's not hard to imagine how panic might spread in the financial arena as well. In a report last week, Wells Fargo Securities warned of "psychological damage" that could lead to a selloff in the stock markets. "A full-blown financial contagion can not be ruled out," it added.

When markets freeze up, so does economic activity, as consumers hoard their savings and companies rethink their investment plans. This is what creates recessions.

If we're not careful, fear, not logic, will create the reality that determines the outcome of events. Should that be the case, savvy investors might spot an opportunity to snap up cheap assets. For everyone else, the Japan's tsunami would end up leaving a wider swath of destruction than the spreadsheets are predicting.

Write to Peter Stein at peter.stein@wsj.com