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Wildfire behavioural parameters are assessed through metrics that can be expensive to measure with sufficient resolution in real time, such as rate of spread, intensity, and severity. Wildfire researchers and practitioners are thus in need of accurate, cost-effective, and user-friendly methods to estimate these metrics. Flame length is one such established proxy metric widely used to estimate fireline intensity, however direct measurements can be challenging. Char height on tree trunk s has been proposed as a cost-effective proxy for flame length, and thus fireline intensity, but its accuracy has not been widely tested. Based on research by Williams et al. (1998) in Australian eucalypt savannas, this study explores the relationship between char height on fence posts and flame length in a South African grassland fire context. Data were collected at 143 monitoring plots within 7 landscape-scale prescribed fires in Eastern Cape mesic montane grassland. Flame length was recorded in real time using installed wooden fence posts of known height as visual aids, and grouped by fire type (head, back, flank). Char height measurements were later recorded from the soil surface to the maximum height of charring on the fence post s. Across all fire types, the flame length (y) could be accurately estimated from char height (y = 1.42x + 0.971; R2 = 0.609), but there were some differences between fire types. For head fires, char height yielded a strong rank correlation (rs(37) = 0.807; p < 0.001) with flame length, while char height in flank fires had a moderate rank correlation (rs(25) = 0.532; p < 0.005). Back fires did not show a significant rank correlation between char height and flame length (rs(15) < 0.15; p > 0.567). Pragmatically, the simple doubling of post-hoc char height serves as a direct estimate of flame length. This research confirms that in mesic montane grasslands of southern Africa, char height is a reliable post-hoc indicator of flame length, particularly for head fires, and could have wide practical application as a rule-of-thumb in these grassland ecosystems.