Resistance Calculator: Easily Compute Ohm’s Law Values Online
Enter voltage and current, press “Calculate”, and the tool instantly returns resistance with two-decimal precision—saving you the manual step of dividing volts by amps. Home outlets deliver 230 V RMS in most regions (IEC 60038), so a 0.23 A load shows about 1 kΩ on this calculator.
Resistance Calculator
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How to use the tool
Current (A) – type the circuit current, e.g., 2.20 A or 0.33 A.
Voltage (V) – enter the measured voltage, e.g., 9.00 V or 5.00 V.
Press “Calculate”; the result appears below the form with two-decimal accuracy.
Formulas
Ohm’s law links voltage V, current I and resistance R:
Standard mains voltage: 230 V ±10 % in Europe (IEC 60038).
Common speaker impedance: 4-8 Ω (Audio Engineering Society, 2015).
E12 resistor series tolerances: ±10 % (IEC 60062).
1/4 W 1 kΩ resistor price ≈ $0.02 each in bulk (Mouser, 2024 catalogue).
Maximum safe touch voltage: 50 V AC (NFPA 70E, 2021).
What is electrical resistance?
Electrical resistance is a material’s opposition to current flow, measured in ohms (Ω) (Horowitz & Hill, 2015).
Why divide volts by amps?
Dividing voltage by current isolates resistance because Ohm proved their proportionality in 1827 (Ohm, 1827).
How accurate is the calculator?
It shows two-decimal resolution; measurement accuracy still depends on your meter’s ±1 % typical error (Fluke 87-V datasheet, 2022).
Can I enter milliamps?
Yes. Convert 250 mA to 0.25 A before entry to keep units consistent (NIST SP811).
What happens with zero current?
The form blocks zero because dividing by zero is undefined and yields infinite resistance (IEEE Std 100-2020).
Is 1 kΩ safe for Arduino I/O?
Yes; 1 kΩ limits current to 5 mA at 5 V, below the 20 mA pin rating (Arduino Uno Rev3 datasheet, 2023).
How do I size a power resistor?
Use P = V²/R; a 12 V drop across 10 Ω dissipates 14.4 W, so pick ≥25 W package (Vishay PR02 datasheet, 2021).
Expert tip on derating
“Operate power resistors below 70 % of rated wattage for long-term reliability” (Vishay PR02 datasheet, 2021).
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