Resistor Calculator
Calculate series/parallel resistance, decode color codes, and voltage dividers
Enter resistor values separated by commas (e.g., 100, 220, 1k, 4.7k)
Quick Reference:
- Series: Total resistance increases (add all values)
- Parallel: Total resistance decreases (always less than smallest)
- E12 Series: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82
- Color Order: Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Gray, White
Understanding Resistors
Resistors are fundamental electronic components that limit current flow in a circuit. They're characterized by their resistance value (measured in ohms, Ω), power rating (watts), and tolerance (percentage variation).
Series Resistors
When resistors are connected in series (end-to-end), their resistances add up: Rtotal = R1 + R2 + R3 + ...The same current flows through all resistors, but voltage divides proportionally.
Parallel Resistors
When resistors are connected in parallel (same two nodes), the total resistance is always less than the smallest individual resistor:1/Rtotal = 1/R1 + 1/R2 + 1/R3 + ...For two resistors: Rtotal = (R1 × R2) / (R1 + R2).
Resistor Color Code
Through-hole resistors use colored bands to indicate their value:
- 4-band: 2 digits + multiplier + tolerance
- 5-band: 3 digits + multiplier + tolerance
- Color mnemonic: Black Brown Red Orange Yellow Green Blue Violet Gray White (0-9)
SMD Resistor Codes
Surface-mount resistors use numeric codes:
- 3-digit: 103 = 10 × 10³ = 10kΩ
- 4-digit: 4702 = 470 × 10² = 47kΩ
- R notation: 4R7 = 4.7Ω, R47 = 0.47Ω
Voltage Dividers
A voltage divider uses two resistors to produce a lower output voltage from a higher input voltage. The formula is:Vout = Vin × R2 / (R1 + R2)This is commonly used for sensor interfaces and reference voltages.
Standard Resistor Values (E12 Series)
The E12 series provides 12 values per decade with 10% tolerance: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 (then 100, 120, 150... and so on)
Worked Example: Reading a 4-Band Color Code
A resistor banded yellow, violet, red, gold decodes as: yellow = 4, violet = 7, red multiplier = ×100, gold = ±5% tolerance. So the value is 47 × 100 = 4.7 kΩ ±5%, meaning the actual resistance lies between 4,465 Ω and 4,935 Ω. Running the same digits the other way, a 3-digit SMD code of "472" is also 4.7 kΩ (47 × 10²).
Worked Example: Parallel Combination
Two resistors of 1 kΩ and 4.7 kΩ in parallel give Rtotal = (1000 × 4700) / (1000 + 4700) ≈ 824.6 Ω — always lower than the smallest branch. This trick is often used to hit a value that is not available as a single standard E12 part.
Frequently Asked Questions
What is the difference between series and parallel resistance?+
In series, resistances simply add (Rtotal = R1 + R2 + ...), so the total is always larger than any single resistor. In parallel, the reciprocals add (1/Rtotal = 1/R1 + 1/R2 + ...), so the total is always smaller than the smallest branch. This calculator handles any number of resistors in either configuration.
How do I read a 4-band resistor color code?+
The first two bands are digits, the third is a power-of-ten multiplier, and the fourth is tolerance (gold = 5%, silver = 10%). For example, brown-black-orange-gold is 1 and 0 followed by ×1,000, giving 10 kΩ ±5%. The color-to-digit order is black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, gray 8, white 9.
How do SMD resistor codes like 103 or 4R7 work?+
In a 3-digit code the first two digits are the value and the third is the number of zeros: 103 means 10 followed by three zeros, i.e. 10 kΩ. Four-digit codes work the same with three significant digits (4702 = 47 kΩ). The letter R marks a decimal point, so 4R7 is 4.7 Ω and R47 is 0.47 Ω.
How do I choose resistor values for a voltage divider?+
The output is Vout = Vin × R2 / (R1 + R2), so the ratio of R2 to the total sets the division. To halve 5 V, use equal resistors, such as two 10 kΩ parts. Keep the total resistance high enough to limit wasted current but low enough that the load does not sag the output — 1 kΩ to 100 kΩ covers most sensor and reference applications.
Is this resistor calculator free and does it work offline in the browser?+
Yes. All five modes — series, parallel, color code, SMD, and voltage divider — are free, require no sign-up, and compute entirely in your browser with nothing sent to a server. It works on mobile, which is convenient at the workbench.
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