Persons with obesity may be poor estimators of caloric content of food. Health care professionals encourage patients to consult nutritional labels as one strategy to assess and restrict caloric intake. Among subjects enrolled in a weight loss clinical trial, the objective is to determine the accuracy of subjects' estimates of caloric deficit needed to achieve the desired weight loss. A 6-month controlled trial demonstrated efficacy of a portion control tool to induce weight loss in 130 obese people with type 2 diabetes. All subjects had previously received dietary teaching from a dietician and a nurse. At baseline, patients were asked how much weight they would like to lose and to quantitatively estimate the caloric deficit required to achieve this weight loss. The stated amount of weight loss desired ranged from 4.5 to 73 kg, with an average of 26.6 kg (n = 127 respondents). Only 30% of participants were willing to estimate the required caloric deficit to lose their target weight. Subjects' per kilograms estimate of caloric deficit required ranged from 0.7 to 2 000 000 calories/kg with a median of 86 calories/kg. Nearly half of subjects (47.4%) underestimated the total required caloric deficit to achieve their target weight loss by greater than 100 000 calories. Despite attendance at a diabetes education centre, this population of obese individuals had a poor understanding of the quantitative relationship between caloric deficit and weight loss. Educational initiatives focused upon quantitative caloric intake and its impact on weight change may be needed to assist obese patients in setting appropriate weight loss goals and achieving the appropriate daily caloric restriction required for success.