Throughout, the volunteers wore masks that precisely measured their energy expenditure, which means how many calories they were using.
Unsurprisingly, sitting was not very taxing. The volunteers generally burned about 20 calories during their 15 minutes of sitting, whether they were typing or staring at a television screen.
More unexpected, standing up was barely more demanding. While standing for 15 minutes, the volunteers burned about 2 additional calories compared to when they sat down. ...
Over all, in fact, the researchers concluded, someone who stood up while working instead of sitting would burn about 8 or 9 extra calories per hour. (Just for comparison, a single cup of coffee with cream and sugar contains around 50 calories.)
But walking was a different matter. When the volunteers walked for 15 minutes, even at a fairly easy pace, they burned about three times as many calories as when they sat or stood.
--Gretchen Reynolds, NYT, on why I have a treadmill desk