A 1:1 to 2:1 civilian to combatant ratio is reported by John Spencer, urban war researcher and chair at the Modern War Institute.

Spencer has said it will take years of research to confirm this ratio and how it was achieved. He considers it important to understand because the typical ratio in urban wars, including wars the U.S. has prosecuted, exceeds 6 innocent civilians killed per enemy combatant killed. Some urban wars see 12 innocent civilians killed per combatant killed.

I think all of these ratios are horrifying. A low ratio can't be considered either good or exculpatory, as to whether violations of international humanitarian law have occurred. Civilians always bear a disproportionate impact in all urban wars. The case studies that have been completed are worth reading.

https://mwi.westpoint.edu/urban-warfare-project/urban-warfar...