In reply to the Comment by Biel et al (2016 Nucl. Fusion57 038001) on our recent papers Costley et al (2015 Nucl Fusion55 033001) and Costley (2016 Nucl. Fusion56 066003), we point out that the fusion triple product, nTτE, and fusion power gain, Qfus, cannot be expressed solely in terms of independent engineering design variables such as major radius, R, and toroidal field, B; output performance variables such as normalised beta, βN, safety factor, q, and fusion power Pfus, have to be invoked. Further, we show that the density limit has the effect of largely cancelling the size dependence in nTτE and Qfus, which would otherwise be present, when these parameters are expressed in terms of Pfus. Considerations of engineering aspects are also briefly discussed.