Vacuum 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-6

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.

Universal constants

Reduced Planck constantNewtonian constant of gravitationStandard acceleration of gravityVacuum electric permittivityVacuum magnetic permeabilityCharacteristic impedance of vacuum

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All physical constants

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