Ohm's Law Calculator
Solve for voltage, current, resistance or power from any two known values. Includes LED dropping resistor calculator and series/parallel resistance combination tool — the electronics toolkit most Ohm's Law calculators skip.
Ohm's Law Calculator Tool
Select two values you know, enter them, and all four quantities are calculated instantly.
Calculate the dropping resistor needed to protect an LED. Formula: R = (Vs – Vf) / If
Series: R_total = R1 + R2 + … — same current through each resistor.
LED resistor calculator and series/parallel — features most tools skip
Most Ohm's Law calculators solve only the basic four-variable problem. This tool adds an LED dropping resistor calculator with E24 standard value rounding, and a series/parallel resistance combination tool — all in one page.
How to use the Ohm's Law calculator
How this Ohm's Law calculator compares
| Feature | LazyTools ✦ | OmniCalculator | RapidTables | CircuitBread |
|---|---|---|---|---|
| Solve V, I, R, P from any 2 | ✔ All 6 combos | ✔ Yes | ✔ Yes | ✔ Yes |
| Unit scaling (mA, kΩ, mW…) | ✔ Per field | ✔ Yes | Manual | Manual |
| LED dropping resistor | ✔ + E24 standard | No | No | ✔ Yes |
| E24 standard value rounding | ✔ Auto | No | No | No |
| Series/parallel resistance | ✔ Up to 8 resistors | ✔ Separate tool | No | No |
| Formula wheel diagram | ✔ Inline SVG | No | ✔ Image | No |
| Power rating warning (LED) | ✔ 0.25/0.5/1W | No | No | No |
| No ads blocking the tool | ✔ Clean layout | Ads present | Heavy ads | Ads present |
Ohm's Law formulas — all 12 combinations
| Solve for | Formula | Given |
|---|---|---|
| Voltage (V) | V = I × R | Current and Resistance |
| Voltage (V) | V = P / I | Power and Current |
| Voltage (V) | V = √(P × R) | Power and Resistance |
| Current (I) | I = V / R | Voltage and Resistance |
| Current (I) | I = P / V | Power and Voltage |
| Current (I) | I = √(P / R) | Power and Resistance |
| Resistance (R) | R = V / I | Voltage and Current |
| Resistance (R) | R = V² / P | Voltage and Power |
| Resistance (R) | R = P / I² | Power and Current |
| Power (P) | P = V × I | Voltage and Current |
| Power (P) | P = I² × R | Current and Resistance |
| Power (P) | P = V² / R | Voltage and Resistance |
Ohm's Law Explained — Formulas, Units and Practical Applications
Ohm's Law is the cornerstone of electronics. Established by German physicist Georg Simon Ohm in 1827, it describes the relationship between voltage, current and resistance in an electrical conductor: the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. Expressed as V = I × R, this single equation — and its algebraic rearrangements — underlies the analysis of virtually every resistive electrical circuit, from a simple LED on a breadboard to industrial power distribution systems.
The four quantities and their units
Voltage (V) is the electrical potential difference between two points, measured in volts (V). It is the "pressure" that drives current through a circuit. Common sources include batteries (1.5V AA, 9V PP3, 12V car battery), USB power (5V), and mains supplies (120V or 230V). Current (I) is the rate of flow of electric charge, measured in amperes or amps (A). Household circuits run at 10–30A; electronics work at milliamps (1mA = 0.001A) or microamps. Resistance (R) is the opposition to current flow, measured in ohms (Ω). Resistors range from fractions of an ohm (wire resistance) to megohms (1MΩ = 1,000,000Ω) in high-voltage applications. Power (P) is the rate of energy conversion, measured in watts (W). It equals V × I, or equivalently I² × R, or V² / R.
The power triangle — extending Ohm's Law
Ohm's original law covers V, I and R. Combining it with the power definition (P = V × I) gives a family of 12 formulas covering all combinations of the four quantities. These are shown in the formula wheel above. The key insight is that knowing any two of the four quantities allows you to calculate the other two without needing additional information about the circuit. This makes Ohm's Law exceptionally versatile: you can, for example, calculate the resistance of an appliance from its power rating and supply voltage, without ever measuring the current directly.
LED dropping resistors — the most common practical application
One of the most frequent uses of Ohm's Law in hobby and professional electronics is calculating the correct series resistor for an LED circuit. An LED is not a resistor — it has a characteristic forward voltage (Vf) below which it will not conduct, and above which it will conduct with very low resistance. Without a current-limiting resistor in series, connecting an LED directly to a power supply causes excessive current and immediate failure. The dropping resistor formula is R = (Vs - Vf) / If, where Vs is the supply voltage, Vf is the LED forward voltage and If is the desired forward current. Typical values: Vf = 2.0V (red/orange/yellow), Vf = 3.2V (blue/green/white), If = 20mA for standard through-hole LEDs. After calculating the exact resistance, round up to the nearest standard E24 series value and check the power dissipation: P = (Vs - Vf) × If. If this exceeds 0.25W, use a 0.5W or 1W rated resistor.
Series and parallel resistance
Resistors combined in series add directly: R_total = R1 + R2 + R3. The same current flows through each, and the voltage splits proportionally. Use series combinations to achieve resistance values not available as standard parts, or to divide voltage in a known ratio. Resistors in parallel follow the reciprocal rule: 1/R_total = 1/R1 + 1/R2 + ... The total resistance is always less than the smallest individual value. Use parallel combinations to reduce resistance or to increase current-handling capacity. For exactly two resistors in parallel, the useful shortcut is R_total = (R1 × R2) / (R1 + R2).
Standard E24 resistor values
Resistors are manufactured in standard values from the E-series. The E24 series provides 24 values per decade with approximately 5% steps: 1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1 (multiplied by a power of 10). When the LED resistor calculator gives you 137Ω, the nearest E24 standard values are 130Ω and 150Ω. For an LED circuit, always round up (to 150Ω in this case) to protect the LED from excess current — a slightly higher resistance reduces brightness marginally but extends the LED's life significantly.
Power dissipation and resistor ratings
Every resistor has a maximum power rating, typically 0.125W (⅛W), 0.25W (¼W), 0.5W (½W) or 1W for common through-hole types. The power a resistor must dissipate is P = I² × R. For safety, always use a resistor rated at least twice the calculated dissipation: if a resistor must dissipate 0.15W, use a 0.5W part. Exceeding the rating causes the resistor to overheat, change value, and eventually fail open or cause a fire. In LED circuits the power dissipated in the dropping resistor equals (Vs - Vf) × If — for a 5V supply with a 2V LED at 20mA this is (5-2) × 0.02 = 0.06W, comfortably within a 0.25W resistor rating.