Home › Reference › Vacuum magnetic permeabilityVacuum magnetic permeability = 1.256 637 061 27 × 10−6 N·A⁻²
Symbol μ₀
Value 1.256 637 061 27 × 10−6
Unit N·A⁻²
Uncertainty 0.000 000 000 20 × 10⁻⁶
Copy-ready 1.25663706127e-6Copy Before 2019 this was exactly 4π × 10⁻⁷ N·A⁻²; the redefinition of the ampere made it a measured quantity that still agrees with the old value to ten digits.
NIST, Fundamental Physical Constants — Complete Listing (2022 CODATA adjustment) , retrieved 2026-07-26.
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