Calorie count on exercise machines

I have found lately that the “calories burned” shown on the elliptical machines and other stationary equipment has been way off. At level 6 (of 10) (138-145 steps per minute) for 45 minutes, the machines have shown 100 calories (hope not) and 1100 calories (highly doubtful). Without using a bodybugg or fitbit type unit (not allowed to wear at work and other reasons), is there a better way to estimate calories? I assume there is a formula that uses weight combined with number of steps or similar. Google search of “calorie burn formula” was not helpful. Thanks.