Magnetic Field Strength converter

The ampere per meter (A/m) is the base unit of magnetic field strength (often written H), the magnetomotive force spread over the length of the magnetic path it drives. The other units scale from it: a kiloampere per meter (kA/m) is 1,000 A/m, an ampere per centimeter (A/cm) is 100 A/m, and an ampere per millimeter (A/mm) is 1,000 A/m. The ampere-turn per meter (At/m) equals A/m exactly, since it's the same quantity viewed through a coil's turns rather than a bare current. The oersted (Oe) and gilbert per centimeter (Gb/cm), both from the CGS electromagnetic system, are identical to each other and equal about 79.57747154594767 A/m — a factor built from the same 4π/10 relationship that links gilberts to ampere-turns.

The featured pair converts oersted to ampere per meter with A/m = Oe × 79.57747155, and the reverse direction multiplies by about 0.01256637061.

A permanent magnet's coercivity rated at 10 oersted converts to 10 × 79.57747155 = 795.7747155 A/m. A field strength reading of 2 kA/m converts to oersted by first expressing it as 2,000 A/m, then multiplying by 0.01256637061, giving 2,000 × 0.01256637061 = 25.13274122 Oe.

Magnetic field strength (H) determines how strongly an external coil or magnet magnetizes a material, as distinct from the flux density (B) that actually results inside it; the two are related by the material's permeability. Permanent-magnet manufacturers specify coercivity — the field strength needed to demagnetize a magnet — often still in oersted even on modern datasheets, because the magnetics industry has been slow to fully retire the CGS unit. Electromagnetic coil and solenoid designers, by contrast, work almost entirely in A/m or kA/m, since that's the natural SI unit for current-driven fields.

A recurring point of confusion is that the oersted and gilbert per centimeter are not just similar — they're numerically identical, since both describe the same CGS quantity from slightly different historical naming conventions, which is why this page lists the same conversion factor for both. Another common mix-up is treating magnetic field strength (H, in A/m or Oe) as the same thing as magnetic flux density (B, in tesla or gauss, a different category) — they share the word "magnetic" and are proportional in free space, but inside a magnetic material they diverge according to that material's permeability, sometimes dramatically.

This H-versus-B distinction is exactly why permanent-magnet coercivity is sometimes quoted two different ways in the same datasheet: "intrinsic coercivity" (based on H, in A/m or Oe) and a separate "normal coercivity" (based on B) can differ substantially for the same magnet, and confusing the two when comparing magnet grades is a common and consequential mistake for anyone selecting magnetic materials rather than just converting between field-strength units.

Finally, it helps to remember that field strength from a coil falls off with distance and depends heavily on geometry, so a figure in A/m or Oe is only meaningful alongside where in the magnetic circuit it was measured. Doubling the number of turns in a coil while halving the current leaves the ampere-turn product, and therefore the resulting field strength at a given point in the circuit, essentially unchanged — a useful shortcut when comparing two differently wound coils meant to do the same job.