Short answer: The smallest black holes known in the universe are stellar-mass black holes with masses around a few solar masses, and there is theoretical discussion about even tinier, primordial or quantum-scale holes. The lightest widely cited observational example is about 3.8 solar masses and roughly 24–25 kilometers across, placing it among the smallest confirmed black holes. Theoretically, black holes could be much smaller (even down to the Planck scale) if primordial or exotic processes occurred, but such tiny objects would be incredibly short-lived or speculative in nature. For practical purposes, the smallest confirmed black holes are in the 3–4 solar mass range with diameters of tens of kilometers.