You're looking for Young's penetration equations, developed at Sandia National Lab in the 90s. Your favorite LLM can walk through the math step by step if you're interested, but a US WW2 era AN-M64 500 lb bomb gives results of 15-20 ft penetration in average soil. Reality might be a little less because free-falling bombs have forward velocity from the aircraft that dropped them, so they hit at angle instead of penetrating straight down.