Standard ratings are ⅛, ¼, ½, 1, 2, 3, 5 and 10 watts, and higher for wire-wound and chassis-mount resistors.
Ohm's law formulas
| To find | From V and I | From V and R | From I and R | From P and V | From P and I | From P and R |
|---|---|---|---|---|---|---|
| Voltage V | — | — | I × R | — | P ÷ I | √(P × R) |
| Current I | — | V ÷ R | — | P ÷ V | — | √(P ÷ R) |
| Resistance R | V ÷ I | — | — | V² ÷ P | P ÷ I² | — |
| Power P | V × I | V² ÷ R | I² × R | — | — | — |
Unit prefixes
| Prefix | Factor | Example |
|---|---|---|
| µ (micro) | 0.000001 | 250 µA = 0.00025 A |
| m (milli) | 0.001 | 20 mA = 0.02 A |
| k (kilo) | 1,000 | 4.7 kΩ = 4,700 Ω |
| M (mega) | 1,000,000 | 1 MΩ = 1,000,000 Ω |
Standard resistor values (E24, 5%)
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, and the same values × 10, × 100, × 1k and so on. The voltage divider tab suggests the nearest E24 value from the IEC 60063 series.
OhmCalc assumes ideal components and DC (or resistive AC) circuits. For AC loads with a power factor, real power is V × I × PF. Mains voltage can kill: these tools are for learning and low-voltage design.
Frequently asked questions
What wattage resistor do I need?
Calculate the power with P = I² × R and choose a rating at least twice that, for example ½ W for 0.2 W.
Can I use a higher wattage resistor?
Yes. A higher rating only means it can dissipate more heat; the resistance is the same.
Why double the power?
Ratings assume ideal cooling at room temperature. Enclosures, nearby parts and high ambient temperatures reduce how much a resistor can safely dissipate.
Is OhmCalc free?
Yes, with no sign-up or ads.
Are my values saved?
Yes, in this browser only.