Want to calculate a carbon footprint?

"Carbon neutrality" is definitely in, with everyone from Al Gore to U2 to the Mariners calculating their carbon footprints and offsetting it with donations to wind-energy plants or tree-planting programs.

And now the average environmentally conscious citizen can get in on the act. All you have to do is pick from a large offering of online carbon calculators, plug in numbers from utility bills and transportation habits and press "submit."

The calculators compute the amount of carbon produced each year -- your so-called "carbon footprint" -- and tell you how much money to give to compensate for it. With a few clicks of the mouse, anyone can achieve the ultimate state of environmental perfection: carbon neutrality.

That is, if you can believe the calculator you're using.

A recent University of Washington study found that when the same values were used with 10 different online calculators, the results varied greatly. In one category, the bottom line for a typical American homeowner varied by more than 32,800 pounds of carbon produced per year.

The variation suggests tallies of carbon emissions have been oversimplified to produce a "one-click" solution to an extremely complicated problem -- global warming. Some experts fear calculators suggesting a person plant a few trees to offset driving a gas guzzler may actually discourage needed lifestyle changes that can benefit the planet.

"Everyone assumes that every calculator they use will produce an accurate result, but in reality, there are vast inconsistencies between the calculations being done," said Anne Steinemann, a UW civil and environmental engineering professor who headed the research. "I was really surprised by the magnitude of inconsistency."

That inconsistency isn't news to King County Executive Ron Sims, who is working toward creating a standard calculator for local governments to use.

"I wouldn't grab a calculator and say, 'Here's the ton I just reduced,' because you don't know that for sure," he said. "I don't think a calculator can give you that kind of guide."

Since there's no oversight of the increasingly popular calculators, Steinemann agreed that there's nothing preventing anyone from running a Web site that claims to calculate carbon footprints.

And if the results can't be trusted, that means some people may be changing their lifestyle or donating money for carbon "offsets," such as those offered by public utilities, under false assumptions.

The Seattle P-I did its own test of the calculators, based on the utility bills, miles driven, miles flown and other data submitted by environmental reporter Lisa Stiffler.

Stiffler and her husband, Brent Roraback, a software company manager, are like many environmentally conscious Seattleites. They recycle, compost, own a hybrid car and keep their electricity bills much lower than the national average of $100 for two people.

The biggest carbon producer in their lives is probably the 23,000 miles Roraback flies each year for his job.

Using the calculators cited in the UW study, the couple's carbon footprint varied significantly.

The Bonneville Environmental Foundation calculator computed that the couple produces 75,795 pounds of carbon dioxide per year, while the U.S. Environmental Protection Agency tallied them at a more modest 27,029 pounds.

That's a greater variation than in Steinemann's study, which broke the calculators down into types of household and transportation emissions. One reason for the wide range is that emissions from air travel, the couple's largest carbon producer, are often calculated very differently, said Clark Williams-Derry, research director at Sightline Institute, a nonprofit research center that studies carbon calculators.

Another part of the variation stems from the fact that different calculators include different behaviors in their calculations. For example, some ask for the amount of garbage a household produces each year, while others use the national average (1,606 pounds), and others don't include garbage at all.

Another reason for the differences is that most calculators use different internal numbers as conversion factors and standards to come up with their results, Steinemann said.

"There are so many different ways to calculate, using different variables, different standards and different assumptions," she said. "There's no one absolute right best number, so each calculator seems to use something different."

Web sites offering the calculators also often don't let the user see what those numbers are.

"The other problem is that individual calculators don't tell you the assumptions behind their calculations," Steinemann said. "Even if there isn't one standard calculator, they should at least be able to be transparent, so people know what's being included, what isn't being included, and what's not being calculated."

Of the 10 Web sites with calculators, only two show the differences that specific lifestyle changes, such as driving less and recycling more, would make on a person's "footprint."

Half offered payment plans a person can use to pay back the environment for the carbon produced. For Stiffler, those price tags ranged from $50 to plant trees (American Forests) to $1,012 (Bonneville) to invest in wind energy.

Since the calculators' results vary so much and probably do not include every source of carbon emissions in a person's lifestyle, these offsets may not really balance out carbon production, Steinemann said.

"The calculators probably underestimate emissions, and so offsets may not cover everything," she said.

The end result is that the Mariners, U2 and even Gore may not be as "neutral" as they think.

Williams-Derry agreed that calculators could be misleading for people using them to make specific choices, such as whether to heat a home with electricity or gas. But, overall, he still sees the calculators as useful.

"If you're just trying to identify big areas in your life that are producing carbon, there's no question that there's a value there," he said. "Imperfect information is better than none at all. Otherwise we'd be operating completely in the dark."

But Sims thinks the calculators may do more harm than good. Instead of encouraging people to examine their lifestyles and look for things they can change, the calculators offer an "easy out" where consumers can simply donate money to make their impact disappear.

"I want to move away from calculators and tell people more what they can do," he said. "There are individual things we know we need to do in our lifestyles, but I think that if you use a calculator to make a change, you may find yourself being disappointed over time."

Small lifestyle changes will make a far bigger impact than buying offsets, Sims said.

"We need to create a culture of people doing those small things that will result in big change."

CARBON CALCULATORS

The Web sites offer calculators that the UW study examined:

American Forests, nonprofit forest conservation organization -- americanforests.org/resources/ccc

BeGreen, from Green Mountain Energy Co., which sells environmentally friendly energy products and carbon offsets -- begreennow.com/calculator

Bonneville Environmental Foundation, nonprofit that markets renewable energy products -- greentagsusa.org/greentags/calculator

CarbonCounter, from The Climate Trust, an environmental nonprofit -- carboncounter.org

Chuck Wright Consulting, sustainable energy consulting firm -- chuck-wright.com/calculators/

Clear Water, environmental advocacy group for the Hudson River -- clearwater.org/carbon.html

The Conservation Fund, environmental nonprofit -- gozero.conservationfund.org/calc/calculate

Environmental Protection Agency -- epa.gov/climatechange/emissions/ind_calculator.html

SafeClimate, from World Resources Institute, an environmental think tank -- safeclimate.net

TerraPass, social enterprise that sells carbon offsets -- terrapass.com/carbon-footprint-calculator