volts to ohms

Ω = V ÷ I. A voltage across a known current gives a resistance: 12 V at 3 A is 4 Ω. Ohm’s law in its resistance form, R = V/I, which is the basis of every shunt and sensor resistor.

Resistance is what a voltage pushes against: R = V/I. An LED running on 5 V at 20 mA needs a series resistor — and choosing it is exactly this conversion. The same calculation sizes the resistance of heating elements, sensor dividers, and every load that converts voltage into current.

By EnginStack Editorial Team | Reviewed against NIST, BIPM and primary-source documentation About →
volts
amps
— ohms
— —

Authority: NIST SP 811 (SI usage guide) and the 1959 International Yard and Pound Agreement. The coefficient above is a defined value — it does not come from measurement and carries no uncertainty.

volts to ohms

The resistor question is the most common Ohm's law task in electronics: given a supply voltage and a desired current, what resistance limits the current to that value? For an LED, subtract the LED's forward voltage first (roughly 2 V for red, 3 V for white), then apply R = V/I to the remaining voltage. The result for 5 V minus 2 V at 20 mA is 150 Ω — the classic LED resistor. The same math runs heating elements, where resistance is fixed and current follows the voltage applied.

ohms = volts ÷ amps
R = V/I
Exact Ohm's law — the ratio of the voltage and current you provide.

Worked Examples

Example 1: Size a series resistor for an LED at 5 V, 20 mA

5 ÷ 0.02 = 250 Ω

Example 2: Find the resistance of a 12 V, 20 mA path

12 ÷ 0.02 = 600 Ω

Example 3: Find the resistance of a 240 V, 10 A heater

240 ÷ 10 = 24 Ω

Common volts to ohms Values

voltsampsohmsContext
5 V0.02 A250 ΩLED resistor at 5 V (minus 2 V forward = 150 Ω).
12 V0.02 A600 ΩLED resistor at 12 V.
120 V15 A8 ΩUS branch circuit load.
240 V10 A24 ΩDryer heating element.
3.3 V0.005 A660 ΩSensor pull-up at 3.3 V.

Engineering Context

Volts-to-ohms is one corner of the four-way Ohm's law family with volts to amps and amps to volts. The LED resistor angle connects to the electric hub's current pages, and the heating-element angle connects to the energy family — resistance at fixed voltage is how a heater's wattage is determined.

Choosing an LED series resistor is the classic use of this division, and the classic mistake is to divide the whole supply voltage by the target current. A diode holds a roughly constant forward voltage while it conducts, so the resistor only sees the difference between the supply and that forward drop. For a red indicator with a forward voltage near two volts, run from five volts at twenty milliamperes, the correct resistor is five minus two, divided by 0.02, which is one hundred and fifty ohms, not two hundred and fifty. Using the supply voltage directly under-runs the diode and can push the current past its rating, which is how indicators fail in the field.

More: Ω to V · V to A · A to V · Electric Guide · Electric Hub

Related Unit Converters

ohms to volts volts to amps amps to volts Electric Conversion Guide Electric Converters

Frequently Asked Questions

How do I convert volts to ohms?

Divide volts by amps: R = V/I. A 5 V circuit pushing 20 mA has an effective resistance of 250 Ω. If you know watts instead, R = V²/P.

How do I choose an LED resistor?

Subtract the LED's forward voltage (about 2 V for red, 3 V for white) from your supply, then divide by the desired current. For 5 V and 20 mA: (5 − 2) / 0.02 = 150 Ω.

Why is Ohm's law called a law?

Because it is exact for ohmic materials (resistors, wire, heating elements) at a fixed temperature: current through them is precisely proportional to voltage. That proportionality is the law, and R = V/I is its statement.