Expressions provided by various authors for the weighted roentgens‐to‐rads conversion factorf¯are examined and compared. It is shown thatf‐values should be weighted by the relative exposure at each energy interval;f¯arrived at in this way is equivalent to thef¯obtained from the ratio of mass energy absorption coefficients weighted by energy fluence as described in ICRU Report 17. It is also equivalent tof¯as calculated directly as the ratio of relative total absorbed dose and relative total exposure.