Metallic nanostructures with nanogap features can confine electromagnetic fields into extremely small volumes. In particular, as the gap size is scaled down to sub-nanometer regime, the quantum effects for localized field enhancement reveal the ultimate capability for light–matter interaction. Although the enhancement factor approaching the quantum upper limit has been reported, the grand challenge for surface-enhanced vibrational spectroscopic sensing remains in the inherent randomness, preventing uniformly distributed localized fields over large areas. Herein, a strategy to fabricate high-density random metallic nanopatterns with accurately controlled nanogaps, defined by atomic-layer-deposition and self-assembled-monolayer processes, is reported. As the gap size approaches the quantum regime of ≈0.78 nm, its potential for quantitative sensing, based on a record-high uniformity with the relative standard deviation of 4.3% over a large area of 22 mm × 60 mm, is demonstrated. This superior feature paves the way towards more affordable and quantitative sensing using quantum-limit-approaching nanogap structures.