As of today, there are 39 hydrogen fueling stations in the U.S., all but four of them in the state of California.

They serve roughly 39 million Californians, whereas 284 million Americans outside the state have no access to hydrogen fuel-cell cars nor stations at which to refill them.

Germany, on the other hand, has 83 million residents—and 45 hydrogen fueling stations that are accessible to the public (though some require prior notice).

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Japan leads the way on hydrogen stations, however, which is hardly surprising given that two of the three makers now offering hydrogen vehicles to the public are from that country.

That country now has 91 hydrogen stations, according to an assessment by H2Stations reported by FuelCellWorks last Wednesday, to serve a population of 127 million people.

The article notes that Germany had the highest rate of increase last year, adding 24 operating stations to the 2016 total of 21.

2015 Hyundai Tucson Fuel Cell, 2016 Toyota Mirai at hydrogen fueling station, Fountain Valley, CA

2015 Hyundai Tucson Fuel Cell, 2016 Toyota Mirai at hydrogen fueling station, Fountain Valley, CA

Another way to put the numbers into perspective is by land mass: California has 164,000 square miles, Japan has 146,000 square miles, and Germany has 138,000.

Summarized, Germany has 45 stations in the most compact area, serving twice as many people as the 35 now operating in California.

Japan's 91 hydrogen stations serve three times as many people as California's 35.

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It struck us a useful metric might be to calculate the number of hydrogen stations spread over the product of the number of residents and area served.

Those results provide a somewhat different story on the relative density of hydrogen stations versus the overall number.

Even after accounting for its larger area, California has the highest number of hydrogen stations per trillion residents per square mile: 5.5 as of today.

Mercedes-Benz GLC F-Cell

Mercedes-Benz GLC F-Cell

The comparable figure for Japan is 4.9, and Germany lags somewhat at 3.9. (Though some may argue the stations should be considered only by population, not by both people and area, which is a justifiable position.)

Other factors make this analysis loose at best: California may have more drivers among its residents than Germany or Japan, which have the functioning mass-transit networks that the Golden State lacks.

Both California and Japan also include substantial regions of uninhabited mountainous terrain, however, with their populations more concentrated on the coasts.

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Still, as Honda, Hyundai, and Toyota soon to produce thousands of hydrogen fuel-cell vehicles a year for public sale or lease, the question of fuel availability will remain key to their potential for adoption.

EDITOR'S NOTE: An early version of this story contained a math error and insufficient explanation of how we calculated the density of hydrogen stations for each of the three areas. We thank dedicated reader John C. Briggs for pointing them out.

Hat tip: Miguel Angel