Home / Utilities / Electrical installations
Any two of V, I, R and P give the other two. It also handles series and parallel resistors, voltage dividers, LED resistors, the color code and single-phase power.
Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.
Each quarter of the wheel gives one quantity (V, I, R or P) from two others. The formulas used in your calculation are highlighted.
How to read: the center shows the quantity you want; the outer ring holds the three formulas that give it. The two colored slices are the formulas used with your data; the highlighted letters in the center are the known quantities.
Power dissipated in the same resistor for voltages from zero to above the current one, with the chosen rating.
How to read: the curve shows that power grows with the square of voltage: doubling voltage quadruples the heat. The horizontal line is the suggested resistor rating; the vertical line is your voltage. Where the curve crosses the horizontal line is the highest voltage the resistor can take.
Power dissipated in each resistor at the applied voltage, and the minimum suggested rating (with the margin).
How to read: each bar is one resistor, in list order. The label shows the power it dissipates and, after the arrow, the smallest standard rating that handles it with the margin. The longest bar is the resistor that runs hottest.
Key: block = line of the list (mixed mode); V = voltage across the resistor; I = current through it; P = V × I, the heat produced; rating = smallest standard power with the margin.
Divider output voltage for loads from 1/10 to 1,000 times R2 (log scale), compared with the unloaded output.
How to read: from left to right the load gets “lighter” (higher resistance) and the output rises to the dashed “no load” line. The dot is your load. The further below the dashed line, the bigger the error: a load of at least 10 × R2 keeps the curve close to it.
LED current with the chosen standard resistor, for supply voltage from 75% to 125% of nominal.
How to read: the bottom axis is the supply voltage and the vertical axis the LED current. The horizontal line is the desired current and the vertical line the nominal supply. A very steep line means little voltage is left across the resistor and brightness changes a lot with the supply: use fewer LEDs in series.
Real (P), reactive (Q) and apparent (S) power of the single-phase load, drawn to scale.
How to read: the base is real power, which does work and is billed; the vertical side is reactive power, which just flows back and forth; the hypotenuse is apparent power, which sets the current. The flatter the triangle (small angle φ), the closer the power factor is to 1.
Drawing of the resistor with the bands in reading order and the meaning of each one.
How to read: read from left to right, starting at the end where the bands are bunched together. The first bands are the digits, the next one is the multiplier (how many zeros) and the separate band on the right is the tolerance.
Value of each color as digit, multiplier and tolerance.
Take this page’s results to another calculator with the fields already filled in.
The calculation report gathers this page's inputs, results, equations, assumptions and references, with fields for project, engineer and revision. The full report also includes the charts and interpretations, in color. In the print dialog, choose Save as PDF.
That in a resistor, voltage equals resistance times current: V = R × I. If you know two of these quantities, you can work out the third. Example: 12 V across a 220 Ω resistor drives 12 ÷ 220 = 54.5 mA. Together with power P = V × I, Ohm’s wheel gathers all 12 formulas.
Use P = V × I, P = V²/R or P = I² × R, depending on what you know. In the example, 12 V across 220 Ω gives 12² ÷ 220 = 0.655 W. Pick a resistor rated at least twice that: here, 2 W. A 1/4 W or 1/2 W part would burn out.
Add the reciprocals and invert the result: 1/Req = 1/R1 + 1/R2 + … For two resistors, Req = R1 × R2 ÷ (R1 + R2). Example: 2.2 kΩ in parallel with 3.3 kΩ gives 1.32 kΩ, less than the smaller one. In series, just add them.
Subtract the LED voltages from the supply and divide by the current: R = (Vsupply − n × Vf) ÷ If. With 24 V, 3 red LEDs of 2.0 V and 20 mA: (24 − 6) ÷ 0.020 = 900 Ω; the next E24 value up is 910 Ω, which passes 19.8 mA and dissipates 0.36 W: use a 1 W resistor.
Two resistors in series split the voltage in proportion to their resistances: Vout = Vin × R2 ÷ (R1 + R2). With 24 V, R1 = 10 kΩ and R2 = 4.7 kΩ, the output is 7.67 V unloaded. A 100 kΩ load pulls it down to 7.44 V (−3.1%), because it sits in parallel with R2.
Start at the end where the bands are bunched together. With 4 bands: the first two are digits, the third is the multiplier and the fourth the tolerance. Yellow (4), violet (7), red (×100) and gold (±5%) = 47 × 100 = 4.7 kΩ ±5%. With 5 bands, there are three digits before the multiplier.
It does for pure resistance. With motors, ballasts or electronic power supplies, current and voltage are out of phase and power factor comes in: P = V × I × PF. A 1,500 W load at PF 0.80 on 120 V draws 15.6 A, not 12.5 A. For three-phase loads, use the three-phase power and current calculator.
Important notice: Indus Metrics tools produce estimates and pre-sizing results from the data you enter and from published engineering methods. They do not replace design, calculations or reports signed by a licensed professional engineer (PE), nor verification against the applicable codes, standards and regulations, such as OSHA rules and the ASME codes. Material and fuel properties vary: check manufacturer data or your own process analyses before deciding. By using this site you agree to the Terms of Use.
© 2026 Indus Metrics. Found an error? Write to contact@indusmetrics.com.