Magnetic field strength units — A/m, ampere-turn and oersted
H is what the current produces; B is what the material responds. Get the input unit wrong and B saturates unexpectedly.
Magnetic field strength (H) is the field the current source produces. Magnetic flux density (B) is the field that includes the material's response. Confusing units for the two lands you in the wrong part of a B–H curve.
The exact factors
1 A/m = 0.01257 Oe (Oe = oersted) > 1 Oe = 79.577 A/m ≈ 79.58 A/m > Amp-turn (At) is current × turns; 1 At on 1 m mean path = 1 A/m
Where each is used
A/m: SI-strict transformer and coil design, magnetic hysteresis loop axes. Oersted: US permanent-magnet datasheets, magnetic-particle NDT, legacy motor design. Amp-turn: solenoid and relay design.
Worked example — solenoid
A solenoid with 1000 turns and 0.5 A over a 0.1 m core produces H = NI/ℓ = 500 / 0.1 = 5000 A/m = 62.8 Oe. On a soft-iron B–H curve this is well below saturation (~15000 A/m). Read the datasheet as gauss/oersted, work in A/m internally, cross-check.
Recap
H drives B through μ. A/m in SI, Oe in datasheets, amp-turn in windings.
Try the converters mentioned in this article
FAQ
Is oersted still current?+
In US and some legacy datasheets, yes. Modern SI-consistent datasheets use A/m.
How does H relate to coercivity?+
Coercivity is the H field needed to demagnetise a saturated material, reported in Oe or A/m — usually thousands of oersteds for rare-earth magnets.
This article was written by The Turbo Unit Converter engineering desk (Applied metrology & engineering units) and last reviewed on 2026-07-07 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.