Electric Field Strength Calculator: V/m and N/C
Electric field strength (E) quantifies the force a field exerts on a unit positive charge at a given point in space. The SI unit is volts per metre (V/m), which is physically identical to newtons per coulomb (N/C): both express the same dimension. Electric fields appear in almost every area of physics and engineering, from the fields inside capacitors and near high-voltage power lines to the RF fields radiated by antennas and the breakdown fields that produce lightning. This calculator offers two methods. The voltage and distance method calculates the uniform field between two parallel plates (or along a simple gap): E = V / d. The point charge method applies Coulomb's law to find the field at a distance r from a charge Q: E = k * Q / r², where k = 8.9875517923 x 10^9 N m^2/C^2. Both methods output in V/m (= N/C) and show the force on a 1 microcoulomb test charge.
Method 1: Voltage and Distance (E = V / d)
Method 2: Point Charge (E = kQ / r²)
Coulomb's constant k = 8.9875517923 × 10&sup9; N·m²/C²
Unit equivalences: V/m and N/C are identical
1 V/m = 1 N/C. Both units appear in standards and datasheets; they are interchangeable.
Electric field unit conversion factors
All factors relative to V/m (= N/C).
| Unit | Symbol | V/m equivalent |
|---|---|---|
| Volt per metre | V/m (= N/C) | 1 |
| Kilovolt per metre | kV/m | 1,000 |
| Megavolt per metre | MV/m | 1,000,000 |
| Volt per centimetre | V/cm | 100 |
| Volt per millimetre | V/mm | 1,000 |
| Millivolt per metre | mV/m | 0.001 |
Reference field strengths
| Source | Approximate field strength |
|---|---|
| FM radio signal at receiver | ~0.01 V/m |
| Household power outlet (near plug) | ~1 kV/m |
| 1 µC charge at 1 m distance | ~8,988 V/m |
| Inside a microwave oven | ~1 MV/m |
| Air breakdown (lightning precursor) | ~3 MV/m |
Frequently asked questions
What is electric field strength?
Electric field strength (E) is a measure of the force per unit positive charge at a point in space, expressed in volts per metre (V/m) in the SI system. It describes how strongly a region of space can accelerate a charged particle. Electric fields arise from charges and from changing magnetic fields (Faraday induction). The direction of the field points from positive to negative charges, representing the direction a positive test charge would be pushed.
Are V/m and N/C the same unit?
Yes. One volt per metre (V/m) is physically identical to one newton per coulomb (N/C). This equivalence follows from the definitions: 1 V = 1 J/C and 1 J = 1 N·m, so 1 V/m = 1 J/(C·m) = 1 N·m/(C·m) = 1 N/C. V/m is the more common notation in electrical engineering, while N/C is preferred in physics textbooks to emphasise the force interpretation. Both notations appear in official standards.
How does electric field strength change with distance from a point charge?
For a point charge Q, the electric field at distance r is given by Coulomb's law: E = k * Q / r², where k = 8.9875517923 × 10⁹ N·m²/C² is Coulomb's constant (equal to 1/(4πε₀)). Because of the r² term, the field falls off with the square of the distance: doubling the distance reduces the field to one quarter. This inverse-square law applies in free space; nearby conductors or dielectric materials modify the field distribution.
What electric field strength causes air breakdown (lightning)?
The dielectric strength of dry air at standard pressure is approximately 3 MV/m (3,000,000 V/m or 30 kV/cm). When the electric field exceeds this threshold, air molecules become ionised and the air conducts electricity, producing a spark or lightning arc. In practice, lightning precursors form in thunderclouds when local field enhancement around raindrops and ice crystals exceeds this value. Humidity, pressure, and electrode geometry all affect the actual breakdown threshold.
How is electric field strength measured in practice?
Electric field strength is measured with field mills (rotating shutter sensors) for quasi-static DC fields, and with broadband electric field probes or dipole antennas for AC and RF fields. Standards such as IEC 61000-4-3 specify measurement procedures for radiated electromagnetic immunity testing. NIST maintains calibration standards for electric field probes. Measurements near high-voltage power lines typically find fields of 0.1 to 10 kV/m at ground level directly below the line.
Official sources
- NIST CODATA: Permittivity of free space and Coulomb's constant values. NIST.
- IEC 61000-4-3: Electromagnetic Compatibility, Radiated RF Electromagnetic Field Immunity Test. IEC.
- NIST Special Publication 330: The International System of Units (SI).
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.