What is an ohm in simple terms?

An ohm is the SI unit of electrical resistance. One ohm is the resistance that lets exactly one ampere of current flow when one volt is applied across it, so 1 ohm = 1 volt per ampere (1 V/A).

What is the symbol for the ohm?

The symbol for the ohm is the Greek capital letter omega (Ω). Kilohms are written kΩ and megohms MΩ. The unit is sometimes spelled out as 'ohm' where the Greek character is unavailable.

Ohm's Law states that resistance equals voltage divided by current: R = V / I. It can also be written as V = I × R or I = V / R, letting you solve for any one of the three quantities when you know the other two.

What is the difference between an ohm and a siemens?

The ohm measures resistance (opposition to current), while the siemens measures conductance (how easily current flows). They are reciprocals: conductance in siemens equals 1 divided by the resistance in ohms.

Who was the ohm named after?

The ohm is named after Georg Simon Ohm (1789-1854), the German physicist who published the relationship between voltage, current, and resistance in 1827.

What Is an Ohm? Resistance, Ohm's Law & Examples

Definition

An ohm is the SI unit of electrical resistance. Its symbol is the Greek capital letter omega: Ω. Resistance describes how strongly a material or component opposes the flow of electric current.

One ohm is defined as the resistance that allows exactly one ampere of current to flow when one volt is applied across it:

1 Ω = 1 V/A (one volt per ampere)

In SI base units, the ohm works out to kg·m²·s−³·A−². A larger resistance value means less current flows for a given voltage, which is why resistors are used to limit and control current in a circuit.

Ohm's Law

The ohm is most useful through Ohm's Law, which links resistance (R), voltage (V), and current (I):

R = V ÷ I (also written V = I × R, or I = V ÷ R)

Worked example: A small lamp draws 0.5 amperes when connected to a 6-volt battery. Its resistance is R = 6 V ÷ 0.5 A = 12 Ω. If you instead know the resistance and voltage, you can find the current: a 240 Ω element on a 120 V supply draws I = 120 ÷ 240 = 0.5 A.

The inverse of resistance is conductance, measured in siemens (S). A component with 4 Ω of resistance has a conductance of 1 ÷ 4 = 0.25 S. Resistance answers "how much does this oppose current?" while conductance answers "how easily does current pass?"

Everyday Examples

Resistance values span an enormous range, from near-zero in good conductors to billions of ohms in insulators.

Item	Approximate Resistance
Short copper wire	under 0.01 Ω
Common circuit resistor	100 Ω to 1 MΩ
Incandescent bulb filament (hot)	~240 Ω
Toaster heating element	10 – 20 Ω
Human body (dry skin)	~100,000 Ω
Human body (wet skin)	~1,000 Ω
Rubber or glass insulator	over 1,000,000,000 Ω

Common Multiples

Because resistances vary so widely, SI prefixes keep the numbers manageable. The most common are listed below.

Name	Symbol	Value	Typical Use
Milliohm	mΩ	0.001 Ω	Cables, shunts, contacts
Ohm	Ω	1 Ω	Heating elements, low-value resistors
Kilohm	kΩ	1,000 Ω	General electronics
Megohm	MΩ	1,000,000 Ω	Insulation, high-impedance inputs

History

The ohm is named after Georg Simon Ohm (1789–1854), a German physicist and mathematician. In 1827 he published The Galvanic Circuit Investigated Mathematically, describing the proportional relationship between voltage and current that now bears his name. His work was initially dismissed but later recognized as foundational to electrical science.

The unit was adopted by the British Association for the Advancement of Science in the 1860s and formalized internationally in the following decades. Today the ohm is one of the SI derived units and is defined precisely in terms of the volt and the ampere.

Need to switch between milliohms, ohms, kilohms, and megohms? Use our resistance converter for instant, precise results.