How to use the resistor combination finder
- List the resistors you own, in ohms, separated by commas or new lines. Write 220 x2 if you have two of that value. Prefixes work: 4.7k, 1M.
- Type the value you need, for example 1.2k.
- Choose series only, parallel only or both, and whether to use up to one, two or three parts.
- Read the ten closest combinations. The headline shows the best, and the table shows each value and its difference from the target.
Formula
series: R = R1 + R2 (+ R3)
parallel: 1 / R = 1 / R1 + 1 / R2 (+ 1 / R3)
difference = (R - target) / target
Every choice of parts from your stock is tried, never using a value more often than you own it. Ties are listed with fewer parts first. The search covers all-series and all-parallel groups, not networks that mix the two.
Making 1.2k from a small drawer
The default stock is 100, 220 (two of them), 330, 470, 1k, 1.5k, 2.2k, 4.7k and 10k ohms, and the target is 1.2k. The closest pair is 220 in series with 1k, which makes 1220 ohms, 1.667% above the target.
The next best are 1.5k in parallel with 4.7k at 1137.1 ohms (-5.242%) and 100 in series with 1k at 1100 ohms (-8.333%). Allowing three parts adds 2.2k, 4.7k and 10k in parallel at 1303.25 ohms, but it does not beat the best pair.
Using the result
A combination that lands within the tolerance of your resistors is as good as an exact match: two 5% parts in series have an error of at most 5% of the total. If you need a closer value, allow three parts or buy a value from a finer series.
Check the power each part will carry. A series pair shares the current, so each part dissipates its own share, and a parallel pair shares the voltage. This page looks at resistance only; for the power in a single resistor, use the Ohm's Law Calculator.
Reading resistor markings
To read the value of a resistor from its color bands, use the Resistor Color Code Calculator, and for surface mount markings the SMD Resistor Code Calculator. To pick a resistor for an LED from a supply voltage, the LED Resistor Calculator also suggests the next standard value.
More science and electronics tools
For other science and electronics questions, try the Room Modes Calculator, pH Calculator and RLC Circuit Calculator.
Assumptions and limits
- Values are in ohms and are treated as exact. Real parts have a tolerance.
- A value is used no more often than the count you gave. Without a count, you own one.
- Only all-series and all-parallel groups of up to three parts are tried.
- With three parts, up to 40 different values are supported.
Frequently asked questions
How do I enter several resistors of the same value?
Write the value followed by x and the count, such as 220 x3. You can also repeat the value on separate lines, and the counts are added.
Why does parallel always give a smaller value than any single part?
Each added path gives the current another way through, so the total resistance falls below the smallest part. Two equal resistors in parallel give half of one.
Can the finder mix series and parallel in one network?
No. It looks at groups wired all in series or all in parallel. A mixed network can reach more values, but it is not searched here.
What do the prefixes mean?
k is thousands, M is millions and m, u, n and p are milli, micro, nano and pico. For resistors you mostly use k and M, and a plain number is in ohms.