Introduction
This resistor calculator helps you find resistor values fast. Pick the color bands on a resistor, and it tells you the resistance in ohms, plus the tolerance and the smallest and largest value it can be. You can also work backwards: type a value, and it shows you which color bands to look for.
The tool does more than color codes. It reads SMD chip resistor codes, like 103, 4R7, or 01C, and it can turn a value back into a printed code. It adds up resistors in series, and works out the total for resistors in parallel. It also finds the resistance of a wire from its length, diameter, and material.
Need a real part you can buy? The standard value finder checks the E6, E12, E24, E48, E96, and E192 series and shows the closest match to your target. Every answer comes with clear steps, so you can see how the math works and check it yourself.
How to Use Our Resistor Calculator
Pick the tool you need, type in your resistor details, and press Calculate. This resistor calculator gives you the resistance value, tolerance range, color bands, SMD codes, total series or parallel resistance, wire resistance, and the closest standard E-series values, with step-by-step math.
Number of Bands: Choose how many color bands are printed on your resistor: 3, 4, 5, or 6.
Show Resistance In: Pick the unit for the answer, like Ω, kΩ, or MΩ. Leave it on Auto and the tool picks the best fit.
Band 1 to Band 6: Pick the color of each band, reading from the left edge of the resistor. Digit bands, the multiplier band, the tolerance band, and the temperature band each show their own color list. For a dedicated band-reading tool, see the Resistor Color Code Calculator.
Resistance Value (Reverse Lookup): Type the resistance you want, like 4.7, to see which color bands you need.
Unit (Reverse Lookup): Choose Ω, kΩ, MΩ, or GΩ for the value you typed.
Band Count (Reverse Lookup): Pick 4 bands for 2 digits or 5 bands for 3 digits.
Preferred Tolerance: Pick how close the resistor must be to its marked value, such as ±5% (Gold).
Printed SMD Code: Type the code printed on the small chip resistor, like 103, 1002, 4R7, or 01C.
Resistance Value and Unit (SMD Encode): On the Encode tab, type the resistance and pick its unit to get the printed code.
Code Format: Choose 3-digit, 4-digit, or EIA-96 for the code you want back.
Resistor Values (Parallel): Type two or more values with commas, like 10, 2, 38. All values must be more than zero. The stand-alone Parallel Resistor Calculator covers this case in more depth.
Unit of Entered Values (Parallel): Pick the unit used for every value in your list.
Show Total In (Parallel): Pick the unit for the total parallel resistance.
Resistor Values (Series): Type two or more values with commas, like 10, 22, 47, 100.
Unit of Entered Values (Series): Pick the unit used for every value in your series list.
Show Total In (Series): Pick the unit for the total series resistance.
Conductor Length: Type the wire length and pick a unit like ft, m, or km.
Conductor Diameter: Type the wire diameter and pick a unit like mm, in, or mil.
Material Preset: Choose the wire metal, such as copper or aluminium, or choose Custom.
Conductivity: This fills in from the preset. Type your own number and pick S/m, MS/m, or %IACS if you chose Custom.
Show Resistance In and Show Area In: Pick the units for the wire resistance and the cross-section area.
Target Resistance and Unit (E-Series): Type the value you want and pick Ω, kΩ, or MΩ.
Show Values In (E-Series): Pick the unit for the standard values in the results table.
E-Series to Search: Check the series you want to compare, such as E12, E24, or E96. Check at least one.
What This Resistor Calculator Covers
A resistor is a small part that slows down the flow of electric current. We measure how much it slows current in ohms (Ω). Ohm's law ties it all together: V = I × R. If you know two of them, you can find the third with the Ohms Law Calculator.
Resistor Color Codes
Most through-hole resistors are too small to print numbers on, so makers paint colored rings, called bands, around the body. Each color stands for a number. Read the bands from the side where they are packed closest together.
- First bands give the digits (two digits for 3- and 4-band parts, three digits for 5- and 6-band parts).
- Next band is the multiplier. It tells you how many zeros to add.
- Tolerance band tells you how far off the real value can be. Gold means ±5%, silver means ±10%. No band means ±20%.
- Sixth band (if there is one) is the temperature coefficient in ppm/°C. It shows how much the value drifts when the part heats up.
Example: brown, black, red, gold = 1, 0, ×100, ±5% = 1,000 Ω (1 kΩ) ±5%. That means the true value sits between 950 Ω and 1,050 Ω.
Color Chart
| Color | Digit | Multiplier | Tolerance |
|---|---|---|---|
| Black | 0 | ×1 | — |
| Brown | 1 | ×10 | ±1% |
| Red | 2 | ×100 | ±2% |
| Orange | 3 | ×1,000 | — |
| Yellow | 4 | ×10,000 | — |
| Green | 5 | ×100,000 | ±0.5% |
| Blue | 6 | ×1,000,000 | ±0.25% |
| Violet | 7 | ×10,000,000 | ±0.1% |
| Grey | 8 | ×100,000,000 | ±0.05% |
| White | 9 | ×1,000,000,000 | — |
| Gold | — | ×0.1 | ±5% |
| Silver | — | ×0.01 | ±10% |
SMD Resistor Codes
Surface-mount (SMD) resistors are flat chips with printed numbers instead of bands. There are four common styles:
- 3-digit: 103 = 10 with 3 zeros = 10,000 Ω (10 kΩ).
- 4-digit: 1002 = 100 with 2 zeros = 10,000 Ω. Used for tighter 1% parts.
- R-notation: the letter R is the decimal point. 4R7 = 4.7 Ω.
- EIA-96: two digits point to a value in the E96 list, and the letter is the multiplier. 01C = 100 × 100 = 10,000 Ω.
Resistors in Series and Parallel
When resistors are wired in a line, one after the other, they are in series. Just add them up: Rtotal = R1 + R2 + R3… The total is always bigger than the biggest single resistor. Two resistors in series across a supply also make a simple divider, which you can size with the Voltage Divider Calculator.
When they sit side by side across the same two points, they are in parallel. Add the reciprocals: 1/Rtotal = 1/R1 + 1/R2 + 1/R3… The total is always smaller than the smallest single resistor, because current now has more paths to take. In AC circuits with capacitors or coils, use the Impedance Calculator instead, and the Capacitor Calculator or RC Filter Calculator for filter work.
Resistance of a Wire
Every wire has some resistance. It depends on how long the wire is, how thick it is, and what metal it is made of:
R = L ÷ (A × σ)
- L = length in meters. Longer wire means more resistance.
- A = cross-section area in square meters. For a round wire, A = π(d/2)². Thicker wire means less resistance.
- σ = conductivity in S/m. Silver and copper conduct best, then gold, then aluminum.
This is why long extension cords and thin wires get warm and drop voltage. For cable runs, pair this with the Wire Resistance Calculator, the Voltage Drop Calculator, and the Wire Size Calculator. On printed boards, the Trace Width Calculator does the same job for copper traces.
E-Series Standard Values
Factories do not make every possible resistor value. They make a set list called the E-series. E6 has 6 values per decade, E12 has 12, and so on up to E192. Loose parts (±10% or ±20%) use the small lists like E6 and E12. Precision parts (±1% or better) use E48, E96, and E192. If your math gives an odd number like 3,842 Ω, you pick the closest standard value you can actually buy. When you are sizing a series resistor for an indicator, the LED Resistor Calculator picks the value for you.
Things Worth Remembering
- Tolerance matters. A 100 kΩ ±5% part can really be anywhere from 95 kΩ to 105 kΩ.
- Check the power rating too. Power is P = I²R. Too much power burns the part. The Power Calculator, Amp Calculator, and Watts to Amps Calculator help you check current and heat.
- Always double-check a color reading with a multimeter. Bad lighting and faded bands cause mistakes, especially with brown, red, and orange.
- Old bands can look gold when they are really yellow, which shifts the value by a lot.
- Running the numbers for a whole build? The Power Consumption Calculator and Electricity Cost Calculator show what the finished circuit costs to run.