Ohm's law calculator
Calculate voltage, current, resistance and power using Ohm's law - enter the two known values.
Calculations are based on Estonia's tax rates in force for 2026. Results are informational.
Last updated: 2026-06-03
What is Ohm's law?
Ohm's law relates voltage, current and resistance: U = I × R. Power is P = U × I. Knowing two values, you can find the rest.
How it is calculated
Formula
Ohm's law U = I × R (voltage = current × resistance). Power P = U × I. Given any two values, the rest are derived.
Example
U = 12 V, R = 4 Ω → I = 3 A, P = 36 W.
Example 1: current and resistance of a kettle
A kettle's rating plate says 230 V and 2000 W. The calculator takes two of voltage, current and resistance, not power, so find the current first: I = P ÷ U = 2000 ÷ 230 = 8.70 A.
The resistance follows from R = U ÷ I, or directly R = U² ÷ P = 230 × 230 ÷ 2000 = 26.45 Ω. Check it: choose the V, Ω mode and enter 230 V and 26.45 Ω. The calculator shows a current of 8.696 A and power of 2000 W, so the numbers agree.
A real heating element's resistance depends on temperature and is usually somewhat lower when cold. The resistance worked out from the rating plate applies to the appliance while it is running.
Example 2: a series resistor for an LED
An LED is connected to a 5 V supply and needs 20 mA. A red LED typically drops around 2 V; the exact value is in its datasheet. That leaves 5 - 2 = 3 V across the resistor.
Choose the V, A mode and enter 3 V and 0.02 A (20 mA). The calculator gives a resistance of 150 Ω and 0.06 W dissipated in the resistor. If no resistor with exactly that value is available, pick the nearest higher value, and the current will be slightly lower.
The resistor's power rating must exceed the calculated power so it does not overheat. At 0.06 W an ordinary low-power resistor is fine.
The milliamp and kilohm trap
In electronics, currents are often in milliamps and resistances in kilohms. Enter 9 V and 4700 Ω and the calculator shows a current of 0.002 A, because the result is rounded to three decimals. The actual current is 1.915 mA, and some of that detail is lost to rounding.
The fix is to use a consistent set of units: volts, kilohms and milliamps. Enter 9 V and 4.7 (kΩ), and the current field shows 1.915, which is in milliamps, while the power field shows 17.234, which is in milliwatts. Small values stay readable that way.
Example 3: series and parallel circuits
Resistors of 100 Ω and 220 Ω in series give a total of 320 Ω. On a 12 V supply the current is 12 ÷ 320 = 0.0375 A, or 37.5 mA, and the total power is 0.45 W. Find the voltage across each resistor with the A, Ω mode: 0.0375 A and 220 Ω give 8.25 V, leaving 3.75 V across the 100 Ω resistor. Together that is 12 V again.
The same resistors in parallel have a total resistance of 1 ÷ (1/100 + 1/220) = 68.75 Ω. At 12 V the total current is about 0.175 A and the power about 2.09 W. A parallel combination is always lower than the smallest single resistor, which makes a handy check.
Symbols and limitations
Textbooks in Estonia usually write voltage as U, while the calculator field is labelled V. Both mean the same quantity, measured in volts.
- Ohm's law holds well for metal resistors. Current through diodes, LEDs and filament bulbs does not change in proportion to voltage, so the calculation is only approximate for them.
- For AC motors and other inductive loads, P = U × I is not the real power, because the power factor comes into play.
- If the resistance is 0, the calculator shows a current of 0. In reality zero resistance means a short circuit and a very large current.
- Calculations for mains appliances are educational. Electrical work on 230 V circuits should be done by a qualified electrician.
Frequently asked questions
What is Ohm's law?+
Ohm's law relates voltage, current and resistance: U = I × R. Given any two, you can compute the third.
How do I calculate power?+
Power is P = U × I, that is voltage multiplied by current.