Ohm’s Law Calculator

Enter voltage across a resistive load in volts (V) and current through it in amperes (A). This page calculates resistance R = V ÷ I in ohms (Ω) and power P = V × I in watts (W). It does not solve for an unknown voltage or current.

Assumptions & limits

• Both inputs must be finite and strictly greater than zero. Negative signed values and zero are not supported by this version. There is no fixed upper input limit, but results that overflow or underflow the positive numeric range are rejected.
• Enter current in A, not mA: 15 mA = 0.015 A. Use voltage and current for the same load and operating condition.
• Interpret the outputs using a steady DC, ohmic-load model. Resistance is treated as constant at the entered operating point. The calculator does not model temperature changes, AC phase, reactance or a component power rating.

9 V across a load carrying 0.015 A (15 mA).

1. Convert the current if needed: 15 mA ÷ 1000 = 0.015 A.
2. Resistance = 9 ÷ 0.015 = 600 Ω.
3. Power = 9 × 0.015 = 0.135 W.

600 Ω resistance and 0.135 W power.

At 9 V and 0 A the tool shows an input error and no result. R = V/I would require division by zero; it does not report an infinite resistance or model an open circuit. Zero voltage is also rejected, even with positive current.

Resistance is the V/I ratio at the entered operating point; power is the electrical power absorbed under the resistive-load assumption. A single pair of values does not demonstrate that a real device has constant resistance. Calculated watts are not a recommended resistor wattage rating because this tool has no rating or thermal model.

• Units volt, ampere, ohm and watt, and the milli prefix used to convert mA to A. The International System of Units (SI), 9th edition
• Ohm’s law V = IR and electrical power P = VI for the resistive model; rearranging gives the resistance and power in the example. IEC 60050 — International Electrotechnical Vocabulary (Electropedia)

The positive-only input domain and omitted AC/thermal calculations are implementation limits. The synthetic example follows the stated equations and does not specify a component rating.