From the Earth's magnetic field to a 1.3 T neodymium magnet: how to convert magnetic quantities without confusing B and H.
Magnetism is the set of physical phenomena associated with magnetic fields, which exert force on magnetic materials and moving electric charges. In the International System of Units (SI), the fundamental magnetic quantities have their own units: magnetic flux is measured in weber (Wb), magnetic flux density (also called magnetic induction) in tesla (T), and magnetic field intensity in ampere per metre (A/m). The name weber honors the German physicist Wilhelm Weber, and the name tesla honors the Serbian-American inventor Nikola Tesla, a pioneer of alternating current.
Magnetic flux density, measured in tesla, is the most widely used magnetic quantity in practice. One tesla equals one weber per square metre (1 T = 1 Wb/m²). To grasp the scale, the Earth's magnetic field at the surface is approximately 25 to 65 microtesla (μT), depending on latitude; a common refrigerator magnet produces about 5 millitesla (mT) near its surface; and a clinical magnetic resonance imaging (MRI) scanner typically operates at 1.5 T to 3 T, with research equipment reaching 7 T or more.
A historical unit still common in physics and older literature is the gauss (G), belonging to the CGS system: 1 tesla = 10,000 gauss, or 1 G = 0.0001 T = 100 μT. Thus, an Earth's magnetic field of 50 μT equals 0.5 G. The gauss and the tesla measure the same quantity (magnetic flux density), but in different systems — the gauss in CGS and the tesla in SI. In modern scientific work, the tesla is the standard, but the gauss still appears in magnet specifications, in geophysics, and in older instruments.
Magnetic field intensity, measured in ampere per metre (A/m) in the SI, describes the magnetic field generated by electric currents, independent of the material. In the CGS system, the corresponding unit is the oersted (Oe), honoring Hans Christian Ørsted, who in 1820 discovered the relationship between electricity and magnetism. The approximate conversion is 1 Oe ≈ 79.577 A/m. It is important to distinguish magnetic field intensity (A/m) from magnetic flux density (T): they are related by the permeability of the medium (B = μ × H), and confusing the two is a common conceptual error.
The main conversion formulas are: 1 T = 10,000 G; 1 mT = 10 G; 1 μT = 0.01 G = 10 mG; 1 Wb = 1 V·s = 10⁸ maxwell (CGS unit of flux); 1 Oe ≈ 79.577 A/m. The maxwell (Mx) is the CGS unit of magnetic flux, equivalent to 10⁻⁸ Wb. Since magnetism involves several distinct quantities (flux, flux density, field intensity, magnetomotive force), it is essential to know which quantity is being converted before applying any factor — converting tesla to A/m, for example, makes no sense without knowing the material's permeability.
A practical example: a grade N42 neodymium magnet can produce a magnetic flux density of about 1.3 T near its surface. Converting to gauss: 1.3 × 10,000 = 13,000 G. In the reverse direction, a magnetic field sensor measuring 450 μT (typical of the Earth's field at mid-latitude) equals 450 × 0.01 = 4.5 G, or 0.45 mT. These values are routine in geophysics, in compass navigation, and in the design of magnetic (Hall effect) sensors used in automobiles and electronics.
Care with precision is important because magnetic fields can vary over enormous scales — from nanotesla in geophysical measurements to tens of tesla in laboratory magnets. In magnetic resonance imaging, field homogeneity is measured in parts per million (ppm), and variations of just a few μT within the imaging volume can cause artifacts. Use exact factors (1 T = 10,000 G, not 1 T ≈ 10,000 G) and round only the final result; in geophysics, keep nanotesla resolution; in industrial magnet applications, millitesla is usually sufficient.
Real applications of magnetism are vast: medical magnetic resonance imaging uses fields of 1.5 to 7 T to image the body, the hard drive and magnetic tape industry stores data in microscopic fields, geophysics maps magnetic anomalies for ore prospecting, and electric power depends on magnetic fields in transformers and generators. The TudoConversor magnetism converter calculates between tesla, gauss, weber, maxwell, ampere per metre, oersted, and other units with official factors and double precision, ensuring reliable results in any direction.