This paper deals with duality based approaches of field computations. It presents two methods that allow to obtain upper bounds for the error on the forces or torques. The first one is based on a double evaluation of the global coenergy for two values of the position. The second one resembles the popular Maxwell's tensor method and requires the field computation only once, but respecting an additional rule. For both methods, the force or torque are considered in respect of their average value on an interval of displacement, taken as configuration parameter.