Ohm's law calculator
Any two of voltage, current, resistance and power give the other two.
V = I × R and P = V × I connect all four. Rather than showing four fields, this asks which two you know — four open fields would let you enter values that contradict each other, with no way to say which to believe. Resistor colour bands are here too.
What you know
What you enter is saved in this browser, so it is still here next time
I = V ÷ R · P = V × I
All four
Power P
24 W
All four
- Voltage V
- 12 V
- Current I
- 2 A
- Resistance R
- 6 Ω
V = I × R and P = V × I tie all four together, so any two give the rest.
Assumes DC and a resistive load. AC adds power factor and reactance, which these equations do not cover.
Resistor colour bands (4-band)
Resistance
1,000 Ω
- Tolerance
- ±5%
- Actual resistance range
- 950 Ω ~ 1,050 Ω
Each of the six pairs uses a different formula
Choose the two you know and the other two come from the formula for that pair. With voltage and resistance, I = V ÷ R gives the current first and P = V × I the power — 12 V across 6 Ω is 2 A and 24 W. Voltage and current gives R = V ÷ I; current and resistance gives V = I × R. Pairs involving power take one more step: with power and voltage, I = P ÷ V gives the current and then R = V ÷ I the resistance. With power and resistance, I = √(P ÷ R) comes first. All four values are shown to four decimal places.
Zero and negative inputs produce no answer
Put a voltage across 0 Ω and the formula says infinite current. This page leaves a 0 in that place and reports that it cannot be solved. The same applies when finding resistance from 0 A. Negatives are filtered too — −12 V with 2 A would give −6 Ω, and no circuit has negative resistance, so it is not treated as an answer. Negative power in the power-and-resistance pair puts a negative number under the square root, which cannot be solved either. A result is accepted only when all four values are finite and the resistance is zero or above.
Bands: two digits, then a power of ten
Resistance = (first × 10 + second) × 10^third. Brown, black, red is (1 × 10 + 0) × 10² = 1,000 Ω, or 1 kΩ; yellow, violet, orange is 47 × 10³ = 47 kΩ. The third band uses the same ten colours, from black (×1) to white (×10⁹). Gold and silver (×0.1 and ×0.01) are not in the list, so values under 10 Ω cannot be built from this table, and the fourth band is tolerance: it does not change the value but sets the range — gold is ±5%, so a 1 kΩ part lies between 950 and 1,050 Ω.
Direct current, and purely resistive loads only
Units are fixed at V, A, Ω and W with no prefixes. Enter 4.7 kΩ as 4700 and 20 mA as 0.02. Alternating current is not covered — with coils and capacitors, impedance replaces resistance and a power factor multiplies the power, and the formulas here have no place for either. Resistance changing with temperature, the internal resistance of the supply and the resistance of the wiring are left out too. It is V = IR exactly as in the textbook.
Common questions
QHow is resistance found from power?
Rearranging P = I²R gives I = √(P/R) for the current, and V = IR follows. That relationship is why power and resistance alone are enough.
QHow many colour bands?
Four-band resistors — two digit bands, a multiplier and a tolerance band. Tolerance does not change the nominal value; it sets the range the real resistance falls in. Five-band parts have three digit bands for greater precision and are read differently.
QWhat if I enter zero for resistance or current?
Any combination that would divide by zero reports that it cannot be solved, and so does any input that would give an infinite value or a negative resistance. A theoretical short circuit implies infinite current; in practice the wire and the supply have resistance of their own, so it does not.
QWhat current does a 1,500 W heater draw at 220 V?
Choose power and voltage, enter 1500 and 220, and you get I = 1,500 ÷ 220 = 6.8182 A and R = 220 ÷ 6.8182 = 32.27 Ω. Two such heaters on one 16 A breaker draw 13.6 A and still fit; three draw 20.5 A and trip it. A heater is close to a purely resistive load, one of the few cases where this formula matches reality well.
QHow do I enter kΩ or mA?
Expand to the base unit. 4.7 kΩ is 4700, 2.2 MΩ is 2200000, 20 mA is 0.02, 350 μA is 0.00035. Decimals are accepted; prefixes are not. Results also come out in base units only, so 0.0042 A has to be read as 4.2 mA yourself.