Electric Power Calculator (P = U × I)
Calculate electric power P = U × I, voltage or electric current online. Physics calculator with results in watts and kilowatts — free, fast, no registration.
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Ohm's Law Calculator (V = I × R)
Ohm's Law is the cornerstone of electrical engineering and electronics. It describes the relationship between voltage (U), electric current (I) and resistance (R) in any electrical circuit: U = I × R. This simple formula is the starting point for analyzing every DC and AC circuit you will ever encounter. Our calculator lets you instantly find any one of the three quantities when the other two are known, and it automatically computes the electric power P as a bonus. Whether you are a student working through a physics homework assignment, an electronics hobbyist designing a circuit, or a professional electrician checking a domestic installation, this tool covers your needs. Simply choose what you want to calculate — voltage, current or resistance — enter the two known values and hit the button. The result appears in milliseconds, rounded to four significant figures for clarity. Common use cases include: checking the current draw of a 230 V household appliance, sizing a current-limiting resistor for an LED, verifying the fuse rating of a 12 V automotive circuit, or understanding why short circuits are so dangerous (R ≈ 0 means I → ∞).
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How the Ohm's Law calculator works
The calculator applies three rearrangements of Ohm's Law: • Voltage mode: U = I × R, power P = I² × R • Current mode: I = U / R, power P = U² / R • Resistance mode: R = U / I, power P = U × I All results are rounded to four significant figures, balancing accuracy with readability. Division-by-zero is guarded: if resistance equals zero in current mode, or if current equals zero in resistance mode, the calculator shows an error instead of crashing. The power formulas follow directly from Ohm's Law: substituting U = I×R into P = U×I gives P = I²×R; substituting I = U/R gives P = U²/R.
Example: 12 V LED bulb drawing 2 A
A 12 V LED bulb draws 2 A of current. We want to find the resistance and the power consumption. Select mode "Resistance", enter voltage U = 12 V and current I = 2 A: R = U / I = 12 / 2 = 6 Ω P = U × I = 12 × 2 = 24 W The bulb has an internal resistance of 6 ohms and consumes 24 watts of power. Knowing this, you can choose the correct cable cross-section, fuse rating or current-limiting resistor. Second example — adapter for a 230 V socket: if the adapter has a resistance of 46 Ω, the current is I = 230 / 46 = 5 A and the power is P = 230 × 5 = 1 150 W.
Frequently asked questions
What is Ohm's Law?
Ohm's Law states that the electric current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. The formula is U = I × R (or V = I × R), where U is voltage in volts, I is current in amperes and R is resistance in ohms.
How do I calculate voltage?
Use the formula U = I × R. Select "Voltage" mode, then enter the known current (A) and resistance (Ω). Example: a current of 0.5 A through a 100 Ω resistor produces U = 0.5 × 100 = 50 V.
How do I calculate electric current?
Use the formula I = U / R. Select "Current" mode and enter voltage (V) and resistance (Ω). Example: a 230 V supply across a 46 Ω load gives I = 230 / 46 ≈ 5 A.
How do I calculate resistance?
Use the formula R = U / I. Select "Resistance" mode and enter voltage (V) and current (A). Example: 12 V across a component drawing 2 A gives R = 12 / 2 = 6 Ω.
What is electric power and how is it calculated?
Electric power P (in watts, W) is the rate at which electrical energy is transferred. The three equivalent formulas are P = U × I, P = I² × R and P = U² / R. The calculator automatically shows power alongside the main result.
Why is a short circuit dangerous?
A short circuit occurs when two points in a circuit are connected with near-zero resistance. From Ohm's Law, I = U / R — if R ≈ 0, the current becomes extremely large. This causes rapid heating of conductors, melted insulation, fire and equipment damage. Fuses and circuit breakers interrupt the circuit when current exceeds a safe limit.
How does resistance work in series circuits?
In a series circuit, total resistance is the sum of individual resistances: R_total = R1 + R2 + ... + Rn. The same current flows through every element, and the voltage divides proportionally to each resistance. This is used, for example, to create a voltage divider.
How does resistance work in parallel circuits?
In a parallel circuit, conductances add: 1/R_total = 1/R1 + 1/R2 + ... The voltage across each branch is the same, but current divides inversely proportional to resistance. Total resistance is always less than the smallest individual resistor.
Does Ohm's Law apply to all materials?
No. Ohm's Law holds for ohmic conductors (e.g. metals at constant temperature) where resistance is independent of voltage. Non-ohmic devices — diodes, transistors, incandescent bulbs — have a resistance that changes with operating conditions. The calculator assumes an ideal ohmic resistor.
How do I use the Ohm's Law calculator?
Select which quantity you want to find (voltage, current or resistance), then enter the two known values in their respective fields. The calculator instantly displays the computed result and the electric power. All values are rounded to four significant figures.
The calculator is for educational and informational purposes. Ohm's Law applies to ohmic (linear) elements at constant temperature. For non-linear devices (diodes, transistors, incandescent bulbs) resistance varies with current. Always consult a qualified electrician before carrying out work on electrical installations.
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