Resistance equals voltage divided by current, R = V ÷ I. If you know power instead, use R = V² ÷ P or R = P ÷ I². For example, a 60 W lamp on a 120 V supply has a working resistance of 120² ÷ 60 = 240 Ω and draws 0.5 A.
Hobbyists sizing a resistor, technicians checking a heating element and students working circuit problems all hit the same question: what resistance does this load present? This calculator takes any two of voltage, current and power and returns the resistance in ohms, along with the missing quantity, the power the part dissipates and its conductance in siemens. The working is printed below the result so each figure can be traced back to Ohm's law or the power relation. One limit to keep in mind: the numbers describe a load at the operating point you enter. Filaments, thermistors and motors change resistance with temperature and speed, so a cold multimeter reading of a bulb will be far lower than the value computed from its rated wattage.
Choose voltage and current, voltage and power, or power and current. Only the two matching fields are shown.
Type volts, amps or watts. Decimals are fine, so 0.02 A works for a 20 mA LED current.
The result shows ohms, then the third quantity, dissipated power and conductance, with the formula used.
Result Resistance (Ω): 6
Voltage (V): 12
Current (A): 2
Power dissipated (W): 24
Conductance (S): 0.166667
A 12 V source pushing 2 A through a load implies 6 Ω, and the load turns 24 W into heat.
Result Resistance (Ω): 240
Voltage (V): 120
Current (A): 0.5
Power dissipated (W): 60
Conductance (S): 0.004167
The rated wattage and supply voltage give the filament's resistance while lit, 240 Ω, drawing half an amp.
Result Resistance (Ω): 40
Voltage (V): 20
Current (A): 0.5
Power dissipated (W): 10
Conductance (S): 0.025
A part dissipating 10 W while carrying 0.5 A has 40 Ω of resistance and 20 V across it.
R = V ÷ I = V² ÷ P = P ÷ I²; G = 1 ÷ R
RVIPGOhm's law, V = I × R, links the three electrical quantities for a resistive load. Combining it with the power relation P = V × I gives two more forms: substituting I = V ÷ R yields P = V² ÷ R, and substituting V = I × R yields P = I² × R. Rearranging each for R gives the three formulas above. Once R is known the calculator back-solves the missing quantity, so all of V, I and P are shown, and reports conductance as the reciprocal of resistance.
Subtract the LED's forward voltage from the supply, then divide by the target current. With a 2 V red LED at 20 mA on 5 V, enter 3 V and 0.02 A to get 150 Ω, dissipating 0.06 W, so a quarter-watt part is comfortable.
Tungsten's resistance rises sharply with temperature. A 60 W, 120 V lamp works out at 240 Ω when lit, but a meter on the cold filament typically reads around a tenth of that. The rated-power figure describes the lamp while it is running.
Conductance is the reciprocal of resistance, measured in siemens (S). A 6 Ω resistor has a conductance of about 0.1667 S. It is convenient when adding parallel branches, because conductances in parallel simply add.
Any pair gives the same answer for an ideal resistor. If your measurements disagree, the voltage and current pair is usually the most direct, since power ratings on labels are often nominal rather than measured.
For purely resistive loads such as kettles, heaters and incandescent lamps, use RMS voltage and current and the result is correct. Motors, transformers and electronic supplies have reactance and power factor, so V ÷ I gives impedance rather than pure resistance.
Look at the dissipated power shown in the results and pick a part rated for at least double that figure. A resistor burning 0.3 W should be a 1 W or at least ½ W type, not the common ¼ W size.
Using this tool in an assignment, article or report? Copy a citation, or a link for your website.
Tools Directory Editorial Team. (2026, October 27). Resistance Calculator. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/electronics-physics/resistors-and-capacitors/resistance-calculator/
Tools Directory Editorial Team. “Resistance Calculator.” Tools Directory, 27 Oct. 2026, https://tools.milunir.com/electronics-physics/resistors-and-capacitors/resistance-calculator/. Accessed 27 Oct. 2026.
Tools Directory Editorial Team. “Resistance Calculator.” Tools Directory. Last modified October 27, 2026. Accessed October 27, 2026. https://tools.milunir.com/electronics-physics/resistors-and-capacitors/resistance-calculator/.
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