According to IAEA TRS 398 recommendations the determination of absorbed dose with plane-parallel ionisation chambers calibrated in terms of N(K,Q0) can be done using N(D,W,Q0) = (M(Q0)(free air)/M(Q0)(surface)) N(K,Q0)B[(mu(en)/rho)(W,air)](free air) P(Q0). This equation takes into account only the scattering from the stem of the soft ionisation chamber, not the total scattering published in the scientific literature. That makes it difficult to perform dosimetry with field sizes different from those used in the standardisation laboratory to calibrate the chamber. This paper describes a method to calculate D(W,Q0) by using either N(K,Q0) or N(D,W,Q0) for different radiation field sizes.