Calculators
Standard resistor combination finder
By Bulan Sarkar
Type the resistance you need and pick the series your parts come from. The finder shows the nearest single standard value, the best two resistors in series and the best two in parallel, each with its error in per cent. For example, 720 Ω is 330 Ω + 390 Ω in series, or 1.2 kΩ in parallel with 1.8 kΩ. Both are exact.
You can also paste the values you actually have, and it will search only those. Ask for three parts when two don't get close enough.
720, 1.25k, 4k7 and 47.5k all work.
Blank uses ±5 %. Sets the range column only.
Paste the values in your drawer, separated by commas or new lines. Leave empty to search the whole series from 1 Ω to 10 MΩ.
- Best result
- 330 Ω + 390 Ω
- 720 Ω, exact
- Nearest single part below
- 680 Ω
- −5.556 %
- Nearest single part above
- 820 Ω
- +13.889 %
| Parts | Gives | Error | Range at ±5 % |
|---|---|---|---|
| 330 Ω + 390 Ω | 720 Ω | exact | 684 Ω to 756 Ω |
| 39 Ω + 680 Ω | 719 Ω | −0.139 % | 683 Ω to 755 Ω |
| 33 Ω + 680 Ω | 713 Ω | −0.972 % | 677.4 Ω to 748.6 Ω |
| 47 Ω + 680 Ω | 727 Ω | +0.972 % | 690.6 Ω to 763.4 Ω |
| 150 Ω + 560 Ω | 710 Ω | −1.389 % | 674.5 Ω to 745.5 Ω |
| Parts | Gives | Error | Range at ±5 % |
|---|---|---|---|
| 1.2 kΩ ∥ 1.8 kΩ | 720 Ω | exact | 684 Ω to 756 Ω |
| 820 Ω ∥ 5.6 kΩ | 715.3 Ω | −0.658 % | 679.5 Ω to 751 Ω |
| 1 kΩ ∥ 2.7 kΩ | 729.7 Ω | +1.351 % | 693.2 Ω to 766.2 Ω |
| 820 Ω ∥ 6.8 kΩ | 731.8 Ω | +1.633 % | 695.2 Ω to 768.3 Ω |
| 820 Ω ∥ 4.7 kΩ | 698.2 Ω | −3.029 % | 663.3 Ω to 733.1 Ω |
Searched 85 values. The range column is the worst case, with every part at the same end of its tolerance.
Standard values from IEC 60063 as printed in Vishay's E-series table (document 28372). Checked 9 October 2026.
Why the value you need isn't on the shelf
Resistors come in fixed steps called the E-series. IEC 60063 sets them, and Vishay prints all of them on one sheet. E12 has 12 values per decade, E24 has 24 and E96 has 96. If your calculation lands on 720 Ω, none of those lists has it. You can take the nearest part (680 Ω in E12, −5.556 %) or join two parts to hit it.
Series or parallel?
In series the values add: 330 Ω + 390 Ω = 720 Ω. Use series when the target sits just above a standard value, and let a small resistor top it up.
In parallel the result is the product over the sum: 1.2 kΩ ∥ 1.8 kΩ = (1200 × 1800) / 3000 = 720 Ω. It always comes out below the smaller part, so parallel suits a target just under a standard value: a large resistor across a slightly bigger one trims it down.
With three parts there are two more shapes: one resistor in series with a parallel pair, and a series pair in parallel with a third. The finder tries all four three-part shapes when you ask for three parts.
What tolerance does to a pair
Two ±5 % resistors in series are still a ±5 % pair. If both run 5 % high, the sum runs 5 % high too, so joining parts brings the value closer but makes it no more precise. Vishay's D/CRCW chip resistor datasheet shows which series you can buy at which tolerance: ±5 % parts come in E24 values and ±1 % parts in E24 and E96. If the result has to be better than ±5 %, start with ±1 % parts.
The finder prints this worst-case range next to every result.
When a combination is worth it
- The value sets something: an LM317 output, an op-amp gain, a sensor bridge.
- You are building one board and your kit has only E12 parts.
- A one-off repair where the original value isn't in your drawer.
For an LED resistor, 470 Ω or 510 Ω makes no difference you would notice. Take the nearest single value with the LED resistor calculator and move on.
Worked examples
1. An LM317 set to 5.0 V
TI's LM317 datasheet gives VOUT = 1.25 V × (1 + R2/R1), with R1 = 240 Ω in the typical circuit. For 5.0 V, R2 = 240 × (5 / 1.25 − 1) = 720 Ω.
From E12, the nearest single part is 680 Ω (−5.556 %), which gives 4.79 V. In series, 330 Ω + 390 Ω = 720 Ω (exact). In parallel, 1.2 kΩ ∥ 1.8 kΩ = 720 Ω (exact).
From E24 the nearest single part is 750 Ω (+4.167 %, 5.16 V). E24 also has 360 Ω + 360 Ω, 160 Ω + 560 Ω, 100 Ω + 620 Ω as exact series pairs; the finder lists 330 Ω + 390 Ω first because both parts are also E12 values, so you are more likely to have them. The LM317 calculator covers the rest of the regulator design.
2. 7.5 kΩ from an E12 kit
6.8 kΩ and 8.2 kΩ are both 9.33 % away. In series, 680 Ω + 6.8 kΩ = 7.48 kΩ (−0.267 %). In parallel, 15 kΩ ∥ 15 kΩ = 7.5 kΩ (exact). Two equal parts in parallel always give half the value.
3. 2 kΩ with only E6 values
The nearest single part is 2.2 kΩ (+10.000 %). In series, 1 kΩ + 1 kΩ (exact). In parallel, 2.2 kΩ ∥ 22 kΩ = 2 kΩ (exact).
4. 1.25 kΩ from E12, where series wins
The nearest single part is 1.2 kΩ (−4.000 %). In series, 47 Ω + 1.2 kΩ = 1.247 kΩ (−0.240 %). The best parallel pair, 1.5 kΩ ∥ 8.2 kΩ = 1.268 kΩ, is further off at +1.443 %. Allow three parts and 150 Ω + (2.2 kΩ ∥ 2.2 kΩ) hits 1.25 kΩ exactly.
Tips and tricks
- Use equal pairs. Two equal parts in series double the value, and in parallel they halve it, so your strip of 10 kΩ is also a 20 kΩ and a 5 kΩ strip.
- In parallel, put the big resistor across the smaller one. The big one only nudges the value, so its tolerance hardly matters.
- Measure first. If your 680 Ω reads 690 Ω on the meter, type 690 into the parts box instead.
- On a PCB, leave footprints for two parts in series where a value matters. If the single part turns out close enough, fit a zero-ohm link in the second footprint.
- Check the power in each part. In series the current is shared and the voltage splits by value. In parallel the voltage is shared and the smaller resistor takes more current.
What we would do
For one build from a hobby kit, run the finder on E12 with two parts and take the best series or parallel pair under 0.5 % error. If the circuit sets a voltage or a gain, buy ±1 % E96 parts instead: joining 5 % parts brings the value closer but makes it no more accurate.
Questions people ask
How do I make a resistor value I don't have?
Join two standard values. In series they add; in parallel the result is R1 × R2 / (R1 + R2). The finder tries every pair in the series you pick and lists the closest.
Is series or parallel more accurate?
Neither in general; it depends on the target. Series suits a target just above a standard value and parallel one just below. The finder tries both.
Does joining two 5 % resistors give 2.5 %?
No. In the worst case both parts run high or both run low, so the pair can still be 5 % off. Random errors do partly cancel on average, but a design has to allow for the worst case.
What are E12 and E24?
Sets of standard values per decade, defined in IEC 60063: 12 values in E12 and 24 in E24. E24 is the usual range for ±5 % parts and E96 for ±1 % parts.
Can I use two resistors in parallel to get a higher power rating?
Yes, if they are equal. Two equal resistors in parallel share the current evenly, so each takes half the power. With unequal values the smaller one takes more, so check each part on its own.
Related calculators
Read the parts in your drawer with the resistor colour code calculator or the SMD resistor code calculator. Odd values often come out of the voltage divider calculator and the LM317 calculator. For the whole job of picking a resistor, from value to power rating to marking, follow the choosing a resistor system.
Sources: Vishay, Standard Series Values in a Decade for Resistances and Capacitances, E3 to E192 (IEC 60063), document 28372, p. 1; Vishay, D/CRCW e3 thick film chip resistors, document 20035 (revised 14 April 2026), p. 1, E-series column; Texas Instruments, LM317 datasheet SLVS044Z (revised April 2025), p. 1 and eq. 1. Checked 9 October 2026.
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