Resistor Calculator – Ohm’s Law, Series, Parallel And LED Design
This resistor calculator brings together several small tools that electronics hobbyists, students and engineers use every day. You can solve Ohm’s Law for voltage, current, resistance and power, combine resistors in series or parallel, decode resistor color bands, calculate tolerance limits, size LED resistors and design simple voltage dividers without changing pages.
Ohm’s Law:ationship Between Voltage, Current, Resistance And Power
Ohm’s Law expresses how voltage, current and resistanceate in a circuit:
- V = I × R (voltage in volts)
- I = V ÷ R (current in amps)
- R = V ÷ I (resistance in ohms)
- P = V × I or P = I² × R or P = V² ÷ R (power in watts)
In the Ohm’s Law tab you type any two known quantities and the calculator derives the other two. This is useful when selecting resistor values, checking power dissipation or understanding how a change in one variable affects the rest of the circuit.
Series And Parallel Resistor Networks
Real circuits often use more than one resistor. The total resistance depends on how the parts are connected:
- Series: Rtotal = R₁ + R₂ + R₃ + …
- Parallel: 1 ÷ Rtotal = 1 ÷ R₁ + 1 ÷ R₂ + 1 ÷ R₃ + …
The series tab simply adds up to six resistors and shows the total. The parallel tab uses the reciprocal formula and also displays equivalent conductance so you can see how adding extra parallel paths lowers overall resistance and increases current capacity.
Resistor Color Codes, Tolerance And LED Resistors
Many through-hole resistors use color bands to show their value and tolerance. The color code tab supports 4-band, 5-band and 6-band resistors. Choose the number of bands, set each color and the calculator returns the nominal resistance, tolerance percentage, minimum and maximum possible values and temperature coefficient when available.
The tolerance tab also works the other way around: enter a nominal value and tolerance to see the resistance window. This is helpful when you are checking measurements from a multimeter or verifying that a particular value is suitable across temperature and production variation.
The LED tab sizes a series resistor for one or more diodes. You enter the supply voltage, LED forward voltage, desired current and count in series. The tool calculates the resistor value, voltage drop across the part, power dissipation and a suggested minimum wattage rating.
Voltage Divider Design Andated Tools
A two-resistor voltage divider is one of the simplest and most useful circuits. It scales a higher voltage down to a lower level. The divider tab takes input voltage and two resistor values and returns output voltage, total current and power in each resistor so you can check that they are within safe ratings.
When you work with low-voltage digital or analog circuits, you can combine this page with other tools on this site. For example, a Voltage Drop Calculator helps you see how long wires affect supply rails, and a Dedicated Ohm’s Law Calculator can be useful when you want a focused view for quick checks.
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Ohm’s Law & Resistor Tools
Frequently Asked Questions
Quick answers to common questions Ohm’s Law, resistor color codes, LED resistor sizing, and standard electrical calculations.
You need to enter at least two quantities out of voltage, current, resistance, and power. The calculator then solves the remaining values using the standard Ohm’s Lawationships. If you enter more than two values and they do not match consistently, the tool uses the first consistent pair and recalculates the rest.
All values are expressed in standard SI base units: volts (V) for voltage, amps (A) for current, ohms (Ω) for resistance, and watts (W) for power. You may mentally convert to kilohms or milliamps as needed, or enter base-unit equivalents directly into the fields.
The resistor color code tab is designed for quick decoding and education. For safety-critical, high-power, or precision circuits, always confirm with manufacturer part numbers, datasheets, and measurements from calibrated instruments before selecting final components.
The LED resistor tab uses ideal formulas based on the forward voltage and current you enter. Real LEDs vary with temperature, binning, and manufacturing tolerances. For practical safety, many designers choose a slightly higher resistor value than the ideal calculation when protecting indicator LEDs and similar light sources.