ISO 14346 permits fillet welds in rectangular hollow section (RHS) joints to be designed either (i) to develop the capacity of the connected brace or (ii) to resist the actual load(s) in the brace (i.e. as “fit-for-purpose”). However, no methods for design according to approach (ii) are given in the ISO or any European standard. In this paper, data from 41 weld-critical tests on RHS gapped K, T, Y and X joints under brace axial load(s) are analysed to determine a reliable method for the design of fillet welds as “fit-for-purpose”. “Weld effective length” formulae are evaluated in conjunction with EN 1993-1-8 (directional and simplified methods) according to the standard procedure of EN 1990. A reliable method, based on ISO 14346, is proposed for calculating the design resistance of all-around fillet welds in RHS gapped K, T, Y and X joints. Updates to the minimum fillet weld throat thicknesses required to develop the capacity of a connected RHS brace are also discussed.