Background Previously established correlations of flame length L with fireline intensity IB are based on theory and data which showed that flame zone depth D of a line fire could be neglected if L was much greater than D. Aims We evaluated this correlation for wildland fires where D is typically a non-negligible proportion of L (i.e. roughly L/D < ~2). Methods Experiments were conducted to measure flame length L from line-source fires using a gas burner where IB and D were controlled independently (0.014 ≤ L/D ≤ 13.6). Key results The resulting correlation showed D significantly reduced L for a given IB over the entire range of observations and was in accord with independent data from spreading fires. Flame length is reduced because the horizontal extent of deep flame zones entrains more air for combustion than assumed by theory involving only the vertical flame profile. Conclusions Analysis suggested that the noted variability among published correlations of L with IB may be partly explained by varying L/D ratios typical of wildland fires. Implications Fire behaviour modelling that relies on correlations of L with IB for scaling of heat transfer processes would likely benefit by including the effects of D.