More than a million people have calculated their carbon footprint using Government's Act on CO2 calculator since it was launched last June.
The online tool allows people in the UK to work out how much carbon they use from home energy, appliances and transport, and develops a personalised action plan for users.
Climate Change Minister Joan Ruddock announced the milestone on World Environment Day, on Thursday.
"A significant part of the fight against climate change is about the action we can take as individuals," she said.
"On World Environment Day it is really encouraging that so many people are deciding to Act on CO2 to help the environment."
Ms Ruddock spent the day in Birmingham as the city held a Climate Change Festival organised by Birmingham City Council and CABE (Commissions for Architecture and the Built Environment).
CABE and the council installed a 29-metre nickel-plated pylon in the centre of the city during the nine-day festival to highlight the large proportion of Britain's carbon emissions that come from the built environment.
Deputy council leader Paul Tilsley said: "The pylon brings home the reality of climate change. It makes the issue visible and local and immediate."
The pylon was designed by Block Architecture. Explaining its significance, company director Graeme Williamson said: "Pylons allow our cities to use energy without seeing the chimneys and cooling towers of the power station.
"Like so many things in our lives these days, they illustrate how the consequences of our choices are hidden from us."
World Environment Day takes place every year on June 5 and is run by UNEP (the United Nations Environmental Programme). This year's theme was moving towards a low carbon economy.
The Act On CO2 carbon calculator can be found here.
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