The variables used in formulas that estimate energy requirements include weight, physical activity, height, and age. Weight is crucial because it influences the basal metabolic rate (BMR), which is the amount of energy expended while at rest. Physical activity level is also significant; individuals who engage in more physical activity will require more energy to support their increased activity levels. Height is considered because taller individuals typically have a larger body surface area and thus may have higher energy needs. Lastly, age is an important factor as energy requirements can decrease with age due to changes in metabolism, muscle mass, and overall activity levels. The variable '24-hour calorie intake' is not typically used in these formulas as it reflects consumption rather than requirements.