Purpose: Dose coverage of small targets in SBRT treatments depends on the density of the target and the surrounding lung. We studied the relationship between field size and dose heterogeneity for different size targets as a function of target and lung density. Methods: A 50 cm3 phantom was created in the Pinnacle (V9.2) planning system. A 23 cm diameter lung ROI was inserted in the phantom. 2 or 3 cm diameter targets were placed in the lung ROI and all centered on a 15 cm deep isocenter. Dose points were set at the lateral edges of each target, normal to the beam direction (i.e., at +/− cm for the 2 cm target and +/−1.5 cm for the 3 cm target). Field size was varied from the diameter of each target up to about 5×5 cm2. 100 cGy was delivered to the isocenter via a single 6 MV field and the ratio of the average dose to the lateral dose points to the isocenter dose was recorded for each combination of lung and target density. Lung densities, relative to water, were 0.25, 0.15, 0.1, 0.05 and 0.0. Target densities were 1.0, 0.9, 0.6, 0.4, 0.3, 0.2 and 0.1. Results: We found that uniformity of dose coverage is mainly influenced by lung density. For a typical average target density of 0.4–0.6 and lung density of 0.25 a 1 cm margin is needed to achieve 90% dose uniformity. If the lung density decreases to 0.1 the margin would have to be increased to 2 cm for the same uniformity. Conclusion: It is possible to create a nomographic‐type relationship between target size and field size as a function of lung and target density which can be used as a planning aid in the selection of adequate margins for SBRT cases, particularly for patients with COPD.