オームの法則 計算ツール
電圧・電流・抵抗・電力を即座に算出
Required Parameters
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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
オームの法則の3つの基本公式は？
V = I × R（電圧）、I = V / R（電流）、R = V / I（抵抗）。1827年にゲオルク・オームが発見した基本関係式です。
オームの法則の三角形の使い方は？
Vを上、Iを左下、Rを右下に配置。求めたい変数を隠す：V → I×R、I → V÷R、R → V÷I。
交流回路でも使えますか？
純抵抗負荷のみ有効です。コンデンサやインダクタがある場合はインピーダンス（Z = √(R²+X²)）を使用してください。
電力の計算方法は？
P = V × I、P = I² × R、P = V² / R の3通り。既知の2変数に応じて選択します。
LEDにオームの法則が適用できないのはなぜ？
LEDは非線形デバイスで、順方向電圧が固定されています。LED回路設計には当サイトのLED抵抗計算ツールをご利用ください。
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