Ohm Rechner — Ohmsches Gesetz (V=IR)
Spannung, Strom, Widerstand und Leistung berechnen
Required Parameters
Waiting for input data...
Quick Answer
Ohm's Law: V = I × R. Enter any two values to solve voltage, current, resistance, or power (P = V × I) for DC resistive circuits.
Ohms Calculator
Use this free ohms calculator to solve Ohm's law instantly: enter any two of voltage (V), current (I), and resistance (R) — the tool computes the third plus power (P) when relevant.
Quick start: Pick what you want to find (Voltage, Current, Resistance, or Power), enter the other two values, and read the result.
Ohms calculator formulas
| Find | Formula |
|---|---|
| Voltage | V = I × R |
| Current | I = V / R |
| Resistance | R = V / I |
| Power | P = V × I = I²R = V²/R |
The Ohm's law triangle
Cover the value you want:
- V covered → multiply I × R
- I covered → divide V ÷ R
- R covered → divide V ÷ I
Worked example
12 V supply, 470 Ω resistor:
- I = 12 / 470 = 25.5 mA
- P = 12 × 0.0255 = 0.31 W → use a 1/2 W resistor minimum
Unit tips
- mA → A: divide by 1,000 (250 mA = 0.25 A)
- kΩ → Ω: multiply by 1,000 (4.7 kΩ = 4700 Ω)
- Engineering shorthand: 4k7 = 4700 Ω, 10u = 10 µA
When Ohm's law does not apply
- LEDs and diodes — non-linear Vf, not constant R → LED resistor calculator
- AC with L or C — use impedance Z, not R → Reactance calculator
Related tools
- Ohm's Law Calculator — same engine, Ohm's law focused entry
- Voltage divider
- Parallel resistor calculator
- Electrical power calculator
Design Notes
Ohm's Law applies precisely to Ohmic materials where resistance remains constant regardless of current or voltage. In practical engineering, resistors heat up when dissipating power (P = I²R), which can alter their resistance (Temperature Coefficient of Resistance). Always verify that your calculated power dissipation is below the resistor's actual wattage rating (typically leave a 50% margin).
Common Mistakes
- 1
Mixing up units: Forgetting to convert milli-amps (mA) to Amps (A) before calculating.
- 2
Ignoring power dissipation: A 1/4W resistor will burn up if your calculation yields 0.5W, even if the resistance is correct.
- 3
Applying it to non-linear components: Ohm's law does not directly apply to diodes, LEDs, or transistors which have dynamic resistance.
Engineering Handbox
1. Identify knowns: V = 12, R = 470 2. Use Formula: I = V / R 3. Calculate: I = 12 / 470 = 0.02553 A (25.53 mA) 4. Calculate Power: P = 12 × 0.02553 = 0.306 W
Knowledge Base
Was sind die 3 Hauptformeln des Ohmschen Gesetzes?
U = I × R (Spannung), I = U / R (Strom) und R = U / I (Widerstand). Diese drei Formeln basieren auf der 1827 von Georg Ohm entdeckten Beziehung.
Wie benutze ich das Ohm-Dreieck?
Platzieren Sie U oben, I links unten und R rechts unten. Decken Sie die gesuchte Größe ab: U abgedeckt → I × R multiplizieren. I abgedeckt → U ÷ R. R abgedeckt → U ÷ I.
Kann ich den Rechner für Wechselstrom nutzen?
Nur für rein ohmsche Lasten. Bei Induktivitäten oder Kapazitäten müssen Sie die Impedanz (Z) verwenden, wobei Z = √(R² + X²).
Wie berechne ich die Leistung?
P = U × I, P = I² × R oder P = U² / R. Wählen Sie die Formel basierend auf Ihren bekannten Werten.
Warum gilt das Ohmsche Gesetz nicht für LEDs?
LEDs sind nichtlineare Bauteile mit fester Durchlassspannung. Ihr Widerstand ändert sich mit Strom und Spannung. Verwenden Sie unseren LED-Vorwiderstandsrechner.
Related Engineering Tools
Voltage Divider Calculator
Loaded and unloaded divider output voltage
Current Divider Calculator
Branch current for 2+ parallel resistors
Parallel Resistor Calculator
Equivalent resistance for 2–10 resistors in parallel or series
LED Series Resistor Calculator
Drop the excess voltage safely
Electrical Power Calculator
Calculate watts, voltage, current, or resistance