The present study was conducted as part of a comprehensive work to establish National Diagnostic Reference Levels (NDRLs) in Sri Lanka for the first time. DRLs can be used as an effective optimization tool for identifying unusually high or low patient doses during X‐ray examinations. This study aims to propose institutional DRLs (IDRLs) by measuring the kerma‐area product (KAP) of adult patients undergoing routine projection X‐ray examinations. The median and the 75th percentile KAP values obtained were compared with that of the single institution KAP values reported from India and Greece. This descriptive cross‐sectional study was conducted in a public hospital in Uva province, Sri Lanka, with 400 adult patients aged 18–87 years and weighing 58 ± 20 kg. The patient‐specific information (age, sex, weight, and height) and corresponding exposure parameters (tube voltage and current‐exposure time product) were obtained. The KAP values were measured, and descriptive statistics were utilized for data analysis. The median KAP values obtained were proposed as IDRLs. The IDRLs in Gy.cm2 were 0.23 for cervical spine anterior–posterior (AP), 0.19 for cervical spine lateral (LAT), 0.10 for chest posterior–anterior (PA), 0.06 for knee joint AP, 0.05 for knee joint LAT, 1.47 for KUB AP, 0.85 for lumbar spine AP, 1.97 for lumbar spine LAT, 0.29 for shoulder joint AP, 0.61 for skull PA, and 0.60 for skull LAT examinations. The maximum to minimum ratio of KAP values ranged from 2.4 for KUB AP to 6.3 for the cervical spine AP examinations. The median and the 75th percentile of most of the examinations were comparable to corresponding KAP values reported by the countries mentioned above, except for the skull PA and LAT examinations. Accordingly, interquartile ranges of exposure parameters are recommended for skull examinations to improve the optimization of patient doses.