The difficulties in measuring Byram's fireline intensity have led many researchers to derive an empirical relation between the fireline intensity and flame length, which is easier to measure at the practical (firefighting) level. In this article, we address both the estimation of Byram's fireline intensity by comparison with oxygen consumption calorimetry (OC) measurement and the test of formulations for fireline intensity versus flame length. We directly measured the fireline intensity for spreading fires on a laboratory scale under conditions of no wind and no slope by OC. The fires were set across beds of Pinus pinaster needles, Avena fatua straw, and Genista salzmannii spines. Fireline intensity obtained by OC ranged from 28 to 160 kW/m, depending on species and load. Byram's index of fire intensity ranged from 38 to 185 kW/m. It was observed that Byram's intensity overestimated the fireline intensity measured by OC. Combustion efficiency was introduced in Byram's formulation, which led to a better estimate of fireline intensity. The indirect estimation of fireline intensity through the observation of the flame length was carried out using a stereovision system. Mean flame length ranged from 0.35 to 0.84m according to the species and the fuel load. A new relationship was established between the fireline intensity and the flame length.