Abstract The conventional method for computing the apparent resistivity of any location within a rectangular loop is to use the associated vertical magnetic field or its time derivative by a well-developed central-loop equation. For central part of the loop, this definition of apparent resistivity is accurate, and the survey configuration is designated as modified central-loop TEM. However, when applied to the non-central part of the rectangular loop that often demonstrates uneven distribution of magnetic field the use of this equation runs the risk of producing computational errors. Instead of the uneven distribution of vertical magnetic field, ratio response of the orthogonal horizontal field components has a uniform field distribution. Furthermore, the decay curves of ratio response reveal the properties of translation and scaling. By examining the decay curves for different resistivity, a new definition of ratio apparent resistivity is proposed, which is applicable for any point in a loop with the uniform distribution of ratio response. With the proposed explicit function between ratio response and apparent resistivity, our new algorithm avoids the time-consuming iteration by solving a non-linear equation. Both theoretical modeling and real data example indicate the added value of our method on computational efficiency and accuracy.