A commonly accepted method to measure the in situ gas diffusivity of topsoil is based on the diffusion of a test gas from an open-bottomed chamber that is inserted a few centimeters into the soil. The aim of this study is to develop an easy-to-use approach to determine the gas diffusivity of topsoil at sites where the chamber cannot be inserted into the soil. In the existing method, the diffusion of gas into the soil is a quasi-one-dimensional problem with an analytical solution for gas concentration in the chamber as a function of time t. This one-dimensionality is no longer given in the case of a chamber placed on top of the soil. In the newly suggested method, simulated time series of gas concentrations in a chamber inserted into the soil (Case 1) were compared to those for a chamber placed on top of the soil (Case 2), with equal gas diffusion coefficients for the soil, D-s. We determined a relationship that can be used to convert the chamber concentration for a given D-s at each t in Case 2 to Case 1. The values of D-s can be found iteratively. The method was validated using D-s values different from those used to calculate the function parameters. Practical field measurements of gas diffusivity were also conducted, which show that, on average, both methods yield the same results for an individual site.