Ohmforge Resistor Network Calculator

What it is for

Sooner or later a calculation asks for a resistance you cannot buy: 2.15 kΩ for a filter, 1234 Ω for a current shunt, 12.5 Ω for a dummy load. The usual fix is two or three standard parts in series or parallel, but finding the best pair by hand means a lot of trial and error. The other everyday job is the reverse: you have a network on the bench or in a schematic and want its total resistance and the power each part dissipates.

The Find task searches the chosen series (E6 to E192) for single parts, pairs and triples in six shapes: a + b, a ∥ b, a + b + c, a ∥ b ∥ c, (a + b) ∥ c and (a ∥ b) + c. The Evaluate task reads an expression such as 100 + (220 || 330) and applies a voltage across it.

Results are nominal. Tolerance and temperature drift are not part of the error figure.

Worked example

You need 2.15 kΩ and keep E24 resistors. Settings: Find, target 2k15, E24, at most 3 resistors.

The best result is 150 + 2k = 2.15 kΩ, an exact match with two parts. The table "Best result per number of resistors" shows what each extra part buys: a single 2.2k is +2.33 % off, two parts are exact, three parts are exact too ((1.1k ∥ 1.1k) + 1.6k). Other exact pairs follow, such as 4.3k ∥ 4.3k, so you can pick whatever is in your drawer.

For Evaluate, enter 100 + (220 || 330) at 10 V. The network is 232 Ω and draws 43.1 mA (431 mW in total). R1 (100 Ω) takes 4.31 V and dissipates 185.8 mW; R2 and R3 share 5.69 V with 147.1 mW and 98.1 mW. The power callout warns when a part goes above 0.2 W, close to the limit of a common 0.25 W resistor.

How to read the result

• Best combination with its value and signed error.
• Best result per number of resistors: the top single, pair and triple, so you can decide if a third part is worth it.
• Closest combinations: the ten best overall, ranked by error, then by fewer parts. ∥ means parallel.
• In Evaluate mode, Names maps R1, R2 … to the values in reading order, and the table gives voltage, current and power per resistor.

Tips and common mistakes

• A 0.01 % combination built from 5 % parts is still a 5 % resistor. For precision, use the E96 series with 1 % parts or measure the parts first.
• Two equal resistors in parallel give exactly half: 4.3k ∥ 4.3k = 2.15k is often the tidiest solution.
• Parallel binds tighter than series: 100 + 220 || 330 equals 100 + (220 || 330).
• Check the power of each part, not just the total: in a series string the largest resistor takes the most power, in parallel the smallest.
• For E48 and denser series the first two parts of a triple are taken from one decade either side of the target to keep the search fast; the third part is always solved exactly.

FAQ

How does the search work?

For pairs, every series value is tried with the exact partner it would need, rounded to the two neighbouring series values. Triples try every pair and solve the third part the same way for each shape.

Can I enter values like 4k7?

Yes. 4k7, 4.7k and 4700 are the same. In expressions the R prefix is optional.

Why is a single resistor listed even when the error is large?

It shows the starting point, so you can see how much the combination improves on simply using the nearest standard value.

Can the tool combine inductors or capacitors?

No, it is resistor-only. Use the impedance calculator for networks with L and C.