Ohm
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The ohm, symbol Ω, is the SI coherent derived unit of electric resistance, equal to one volt per ampere. [1] In electrical testing it appears in prefixed forms, among them the megohm used for insulation resistance and the microhm used for the resistance of contacts and connections. [2] [3] [4] [5]
Definition
The NIST Guide to the SI lists the ohm among the coherent derived units that have special names and symbols, and expresses it in other SI units as the volt per ampere. [1] In SI base units the ohm is the square meter times the kilogram, divided by the cube of the second and the square of the ampere. [6]
Multiples and submultiples
SI prefixes form decimal multiples and submultiples of the ohm. Applying mega gives the megohm, equal to 1,000,000 Ω, and applying micro gives the microhm, equal to 0.000001 Ω. [2] [3]
Relationships
The volt, the unit of electric potential difference, is the watt per ampere, and the ampere is an SI base unit. [7] [8] The siemens, the unit of electric conductance, is the ampere per volt and is therefore the reciprocal of the ohm. [9]
Used in
IEEE 43-2013 expresses insulation resistance in megohms. [4] Measurements of the small resistances of contacts, joints and bolted connections made with a low-resistance ohmmeter fall at the other end of the scale, where submultiples such as the microhm apply. [5] [3]
Notes
- NIST SP 811, web page updated 2025-08-18 edition, clause Table 3. In the NIST Guide to the SI, the ohm, symbol Ω, is the SI coherent derived unit of electric resistance and is expressed in other SI units as the volt per ampere.Value: V/A. Datum qty.ohm.definition
- NIST SP 811, web page updated 2025-08-18 edition, clause Table 5. Applying the SI prefix mega, symbol M, which denotes the factor one million, to the ohm gives the megohm, equal to one million ohms.Value: 1,000,000 Ω. Datum qty.ohm.megohm
- NIST SP 811, web page updated 2025-08-18 edition, clause Table 5. Applying the SI prefix micro, symbol μ, which denotes the factor one millionth, to the ohm gives the microhm, equal to one millionth of an ohm.Value: 0.000001 Ω. Datum qty.ohm.microhm
- IEEE 43-2013, clause 9.2. IEEE 43-2013 expresses insulation resistance in megohms. Datum tests.insulation-resistance.megohm-unit
- Electrical Testing Reference editorial taxonomy, 2026-10 edition. A low-resistance ohmmeter is an instrument that measures small resistances, such as those of contacts, joints and bolted connections, by passing a known direct current through the object and measuring the voltage drop that the current produces across it. Datum instr.low-resistance-ohmmeter.definition
- NIST SP 811, web page updated 2025-08-18 edition, clause Table 3. Expressed in SI base units, the ohm equals the square meter times the kilogram divided by the cube of the second and the square of the ampere.Value: m2 kg s-3 A-2. Datum qty.ohm.base-units
- NIST SP 811, web page updated 2025-08-18 edition, clause Table 3. In the NIST Guide to the SI, the volt, symbol V, is the SI coherent derived unit of electric potential difference and is expressed in other SI units as the watt per ampere.Value: W/A. Datum qty.ohm.volt
- NIST SI Units, web page edition. NIST lists the ampere, symbol A, as the SI base unit of electric current.Value: A. Datum qty.ohm.ampere
- NIST SP 811, web page updated 2025-08-18 edition, clause Table 3. In the NIST Guide to the SI, the siemens, symbol S, is the SI coherent derived unit of electric conductance and is expressed in other SI units as the ampere per volt, the reciprocal of the ohm.Value: A/V. Datum qty.ohm.siemens
Data records
- NIST lists the ampere, symbol A, as the SI base unit of electric current. Value: A.
- Expressed in SI base units, the ohm equals the square meter times the kilogram divided by the cube of the second and the square of the ampere. Value: m2 kg s-3 A-2.
- In the NIST Guide to the SI, the ohm, symbol Ω, is the SI coherent derived unit of electric resistance and is expressed in other SI units as the volt per ampere. Value: V/A.
- Applying the SI prefix mega, symbol M, which denotes the factor one million, to the ohm gives the megohm, equal to one million ohms. Value: 1,000,000 Ω.
- Applying the SI prefix micro, symbol μ, which denotes the factor one millionth, to the ohm gives the microhm, equal to one millionth of an ohm. Value: 0.000001 Ω.
- In the NIST Guide to the SI, the siemens, symbol S, is the SI coherent derived unit of electric conductance and is expressed in other SI units as the ampere per volt, the reciprocal of the ohm. Value: A/V.
- In the NIST Guide to the SI, the volt, symbol V, is the SI coherent derived unit of electric potential difference and is expressed in other SI units as the watt per ampere. Value: W/A.
Sources
- Electrical Testing Reference editorial taxonomy, Electrical Testing Reference editorial taxonomy of test methods, 2026-10 edition, Electrical Testing Reference. Held as: Own observation.
- IEEE 43-2013, Recommended Practice for Testing Insulation Resistance of Electric Machinery, 2013 edition, IEEE. Cited: clause 9.2. Held as: Purchased single-user copy. Publisher page
- NIST SI Units, SI Units, web page edition, NIST. Held as: Government work. Publisher page
- NIST SP 811, NIST Guide to the SI, Chapter 4: The Two Classes of SI Units and the SI Prefixes, web page updated 2025-08-18 edition, NIST. Cited: clause Table 3; clause Table 5. Held as: Government work. Publisher page
Obtaining the documents
- IEEE 43-2013: publisher catalog or sales page
Changelog (19 entries)
| Date | Revision | Change | Reason | By |
|---|---|---|---|---|
| 2026-10-13 | created (instr.low-resistance-ohmmeter.definition) | editorial definition of general physics (Task 10 exemplars) | agent | |
| 2026-10-13 | reviewed (instr.low-resistance-ohmmeter.definition) | fresh-context review task-10-exemplars-review.md + fix round 1 | agent | |
| 2026-10-13 | 1 | created | hand-written exemplar for the article generator (Task 10) | agent |
| 2026-10-13 | created (qty.ohm.ampere) | read from the government page (Task 10 exemplars) | agent | |
| 2026-10-13 | reviewed (qty.ohm.ampere) | fresh-context review task-10-exemplars-review.md + fix round 1 | agent | |
| 2026-10-13 | created (qty.ohm.base-units) | read from the government page (Task 10 exemplars) | agent | |
| 2026-10-13 | reviewed (qty.ohm.base-units) | fresh-context review task-10-exemplars-review.md + fix round 1 | agent | |
| 2026-10-13 | created (qty.ohm.definition) | read from the government page (Task 10 exemplars) | agent | |
| 2026-10-13 | reviewed (qty.ohm.definition) | fresh-context review task-10-exemplars-review.md + fix round 1 | agent | |
| 2026-10-13 | created (qty.ohm.megohm) | read from the government page (Task 10 exemplars) | agent | |
| 2026-10-13 | reviewed (qty.ohm.megohm) | fresh-context review task-10-exemplars-review.md + fix round 1 | agent | |
| 2026-10-13 | created (qty.ohm.microhm) | read from the government page (Task 10 exemplars) | agent | |
| 2026-10-13 | reviewed (qty.ohm.microhm) | fresh-context review task-10-exemplars-review.md + fix round 1 | agent | |
| 2026-10-13 | created (qty.ohm.siemens) | read from the government page (Task 10 exemplars) | agent | |
| 2026-10-13 | reviewed (qty.ohm.siemens) | fresh-context review task-10-exemplars-review.md + fix round 1 | agent | |
| 2026-10-13 | created (qty.ohm.volt) | read from the government page (Task 10 exemplars) | agent | |
| 2026-10-13 | reviewed (qty.ohm.volt) | fresh-context review task-10-exemplars-review.md + fix round 1 | agent | |
| 2026-10-13 | created (tests.insulation-resistance.megohm-unit) | method fact restated from the licensed-copy digest (Task 10 exemplars) | agent | |
| 2026-10-13 | reviewed (tests.insulation-resistance.megohm-unit) | fresh-context review task-10-exemplars-review.md + fix round 1 | agent |
Cite this page
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Permanent revision URL: https://www.electricaltestingreference.com/quantities/ohm/rev/1/. Current version: https://www.electricaltestingreference.com/quantities/ohm/.
Plain
"Ohm." Electrical Testing Reference. Revision 1, 2026-10-13. https://www.electricaltestingreference.com/quantities/ohm/rev/1/.
Legal (Bluebook, internet source)
Electrical Testing Reference, Ohm (rev. 1, Oct. 13, 2026), https://www.electricaltestingreference.com/quantities/ohm/rev/1/.
BibTeX
@misc{etr-quantities-ohm-rev1,
title = {Ohm},
howpublished = {\url{https://www.electricaltestingreference.com/quantities/ohm/rev/1/}},
organization = {Electrical Testing Reference},
publisher = {Electrical Testing Reference},
year = {2026},
note = {Revision 1; last substantive change 2026-10-13}
} Wikipedia
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