Ohm's Law Calculator | TechCalcs
Enter any two of V (voltage), I (current), or R (resistance) to calculate the third. Based on Ohm's Law V=IR, designed for electronics circuit design.
Calculate voltage, current, or resistance from two known values using Ohm's Law (V=IR). Free online tool for electronics engineers.
Ohm's Law Calculator | TechCalcs
V = I × R
How to Use
Select a tab for the quantity you want to find, enter the other two values, then press Calculate.
- Choose which value to find — Voltage (V), Current (I), or Resistance (R) — by clicking the corresponding tab
- Fill in the two known values; use the unit dropdown next to each field to choose the unit (e.g., mA, kΩ)
- Press the Calculate button (or press Enter) to see the result
The result unit can be changed with the dropdown next to the result field (e.g., A → mA).
How the Calculation Works
Ohm's Law is V = I × R. Knowing any two of the three quantities lets you solve for the third.
- Find voltage: V = I × R (e.g., 0.5 A × 100 Ω = 50 V)
- Find current: I = V / R (e.g., 12 V ÷ 100 Ω = 0.12 A)
- Find resistance: R = V / I (e.g., 5 V ÷ 0.02 A = 250 Ω)
Results are shown to 4 significant figures (trailing zeros omitted). When selecting components, choose the nearest standard E-series value.
Quick Reference: Common Resistor Values
Common supply voltage and current combinations used in microcontroller and electronics projects, with the corresponding resistor value needed.
| Supply Voltage | Forward Voltage Drop (Vf) | Target Current | Required Resistance (approx.) | Typical Use |
|---|---|---|---|---|
| 5V | 0V | 20mA | 250Ω | Arduino generic |
| 5V | 2V | 20mA | 150Ω | Red / Yellow LED |
| 5V | 3.2V | 20mA | 90Ω | Blue / White LED |
| 3.3V | 2V | 20mA | 65Ω | ESP32 + Red LED |
| 12V | 2V | 20mA | 500Ω | Automotive / Industrial |
These are approximate values. When selecting real components, choose the nearest E24 or E96 standard value and verify that the resistor's power rating is sufficient.
What Is Ohm's Law?
Ohm's Law was formulated in 1827 by German physicist Georg Simon Ohm. It states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance.
Think of voltage as water pressure, current as flow rate, and resistance as pipe diameter. Higher pressure drives more flow; a narrower pipe restricts it.
In electronics, Ohm's Law comes up in nearly every design decision — choosing resistor values, estimating current draw, calculating voltage drop across wiring, and more.
When to Use This Tool
Electronics: Choosing a resistor for an LED
Running an LED from a 5 V supply with a target current of 20 mA? If the LED forward voltage is 2 V, the voltage across the resistor is 3 V. Select "Find Resistance", enter V = 3 and I = 0.02, and the tool returns R = 150 Ω.
Electrical engineering: Checking voltage drop in a cable
Long cables have measurable resistance. With the cable resistance (Ω) and expected current (A), V = I × R gives the voltage drop. Use this to confirm that equipment at the far end still receives adequate voltage.
Studying: Verifying textbook examples
When memorizing formulas, plugging in numbers and checking the result helps lock in understanding. Solve by hand first, then use the tool as a quick sanity check.
Frequently Asked Questions
- What is Ohm's Law?
- Ohm's Law states V = I × R, describing the proportional relationship between voltage (V), current (I), and resistance (R). It was discovered by Georg Ohm in 1827.
- Can I enter current in mA?
- Use the unit selector next to the input field to switch to mA, then enter the value directly.
- Does zero resistance cause an error?
- Yes, to prevent division by zero. Zero resistance means a short circuit, which has no practical meaning in most circuit calculations.
- Can I calculate power (W) too?
- Power calculation (P = V × I) is not included in this version. For power dissipation, check the LED Resistor Calculator. LED Resistor Calculator
- Does Ohm's Law apply to all components?
- No. Non-linear components such as diodes and transistors do not have a proportional V-I relationship, so Ohm's Law does not apply to them. Components that do obey Ohm's Law (e.g. metal resistors) are called "ohmic" or linear elements.
- How do I enter values in kΩ or MΩ?
- Use the unit dropdown next to the resistance input field to select kΩ or MΩ, then enter the number directly. For example, for 10 kΩ, select kΩ and type 10.
Calculate LED Current-Limiting Resistors
Once you know your resistor value, you can also calculate the current-limiting resistor needed for LED circuits. Just enter supply voltage, forward voltage, and current.
Related Tool: Resistor Color Code Calculator
Once you have a resistor value, use the color code calculator to identify the matching resistor by its color bands.