You need a total resistance of 100 ohms, and you already have a 300-ohm resistor in the parallel set. What value completes the circuit? This is the reverse of the usual parallel-resistance question — and it has a clean, exact answer.

The calculator above solves for the missing resistor. Enter your target total and the resistors you already have, and it returns the exact value the missing branch needs, plus a verification that the full set hits your target.

This guide explains the formula, the feasibility rules that catch impossible requests, and how to translate the exact answer into a real resistor you can buy.

What Does the Resistance Parallel Calculator Do?

You enter the target total resistance in ohms and up to three known resistors that will sit in parallel with the missing one. The first known resistor is required; the second and third are optional.

The headline gives the missing resistor's exact value. Below it, three rows restate your target total, list the known resistors, and verify the answer by recombining every resistor — known plus solved — into the parallel total.

The info line under the calculator reminds you of the two practical facts: every branch must exceed the target total, and real-world builds use standard E12 values near the computed answer.

How to Use the Resistance Parallel Calculator

Type your target total resistance into the first box — this is the equivalent resistance you want the whole parallel set to have. Then enter each resistor you already own; leave the optional boxes blank if you have only one or two.

Press Calculate. The headline shows the missing value, and the verification row proves the complete set lands on your target.

Press Reset to solve a different set. Trying the same target with different known resistors shows how the missing value moves — a fast education in parallel behavior.

The Missing-Resistor Formula

Parallel conductances add, so the missing branch's conductance is whatever the target needs minus what the known branches already provide.

The formula is:

1 ÷ R(missing) = 1 ÷ R(target) − 1 ÷ R1 − 1 ÷ R2 − 1 ÷ R3

For a 100-ohm target with a known 300-ohm resistor: 1 ÷ 100 − 1 ÷ 300 = 0.01 − 0.00333 = 0.00667, so R(missing) = 1 ÷ 0.00667 = 150 ohms. The calculator does this arithmetic, including the final reciprocal, for you.

Why Every Known Resistor Must Exceed the Target

The total resistance of a parallel set is always smaller than the smallest branch. That physical fact becomes a validation rule: if any known resistor is at or below the target total, no positive missing resistor can fix the set.

The calculator enforces this before doing any math. Enter a 90-ohm known resistor against a 100-ohm target and it explains the impossibility instead of returning a nonsense negative value.

This check catches the most common wiring-plan error: assuming you can parallel your way up to a larger total. You cannot — parallel combinations only go down.

The "Already There" Impossibility

A subtler failure: the known resistors' combined conductance already meets or exceeds the target's. Then the missing branch would need zero or negative conductance — physically meaningless.

The calculator detects this too, with a message explaining that the known set already reaches or passes the target. The fix is to remove a known resistor or raise the target, not to keep hunting for a missing value.

Together, the two checks mean every answer the calculator gives is a real, positive, buildable resistor value — or a clear explanation of why none exists.

How the Verification Row Works

After solving, the calculator recombines everything — the solved missing resistor plus all known ones — through the standard parallel formula and displays the result.

The formula is:

1 ÷ R(total) = 1 ÷ R(missing) + 1 ÷ R1 + 1 ÷ R2 + 1 ÷ R3

For the 100-ohm example: 1 ÷ (1 ÷ 150 + 1 ÷ 300) = 1 ÷ 0.01 = 100.00 ohms. The verification row should match your target to the decimal — if it does not, something was mis-entered.

Worked Example: Target 100, Known 300

You want 100 ohms total and own one 300-ohm resistor.

First: enter 100 as the target and 300 as Resistor 1, leaving the optional boxes blank, then press Calculate.

The missing conductance is 1 ÷ 100 − 1 ÷ 300 = 0.00667, so the missing resistor is 150.00 ohms.

Then: the verification row recombines 150 and 300 in parallel to confirm 100.00 ohms.

Answer: a 150.00-ohm resistor completes the set exactly.

Worked Example: Target 50, Known 100 and 200

You want 50 ohms total and own 100-ohm and 200-ohm resistors.

First: enter 50 as the target, 100 as Resistor 1, and 200 as Resistor 2, then press Calculate.

The missing conductance is 1 ÷ 50 − 1 ÷ 100 − 1 ÷ 200 = 0.02 − 0.01 − 0.005 = 0.005, so the missing resistor is 200.00 ohms.

Then: the verification row recombines 200, 100, and 200 in parallel to confirm 50.00 ohms.

Answer: a 200.00-ohm resistor — matching the value you already own — finishes the set.

Worked Example: Impossible Request

You want 100 ohms total and your known resistor is 90 ohms.

First: enter 100 as the target and 90 as Resistor 1, then press Calculate.

The validation rule fires: a 90-ohm branch is already below the 100-ohm target, so no parallel addition can ever raise the total to 100.

Then: the calculator explains the problem instead of calculating — the honest response to an impossible circuit.

Answer: no solution exists; pick a known resistor above 100 ohms or lower your target.

From Exact Value to Standard Resistor

The calculator returns exact values like 150.00 ohms, but stores sell standard E12 values: 120, 150, 180, 220, and so on. When the answer is 150.00, you are in luck — 150 is a standard value.

When the answer falls between standards, the note advises picking the nearest standard value at or above the computed answer. The total then lands within a few percent of the target, which most circuits tolerate.

Choosing at or above rather than below is deliberate: it keeps the missing branch's conductance at or under the computed value, so the total stays on the safe side of the target instead of overshooting into unknown territory.

For precision work, combine standards: two resistors in series or parallel can hit almost any value, and this calculator tells you exactly what to aim for.

Edge Case: The Missing Resistor Equals a Known One

The second worked example produced a 200-ohm answer when a 200-ohm resistor was already known. This is not a coincidence pattern — it falls out of the conductance arithmetic whenever the numbers align.

Symmetric answers are worth double-checking against your parts bin before ordering. If you already own the value, the build is free.

The verification row is especially reassuring here: seeing 50.00 ohms confirmed from 200, 100, and 200 builds confidence in an answer that looked too neat.

Symmetric sets also have a practical bonus: one spare of the common value covers two positions in the circuit. If a resistor fails, the identical twin in your parts bin is already the right replacement.

Finally, keep a record of the exact computed value alongside the standard part you installed. If the circuit ever needs retuning, the exact figure — not the rounded standard — is the correct starting point for the next calculation.

Common Missing-Resistor Mistakes

The most common mistake is entering the known resistors as if they were in series — adding them instead of combining conductances. Parallel math runs on reciprocals, and the calculator's formula box shows the reciprocal form explicitly.

The second mistake is ignoring the feasibility checks' message. A "no solution" result is not a calculator bug; it is the physics telling you the target cannot be built from those parts.

The third mistake is over-precision. Ordering a custom 149.73-ohm resistor when a standard 150-ohm part gives 100.00 ohms in the verification row is engineering theater — the standard part wins.

Where Missing-Resistor Calculations Are Useful

Hobbyists use the calculator to design around parts on hand. Knowing the exact missing value before ordering avoids the drawer full of almost-right resistors every builder accumulates.

Students use it to check homework in reverse: given a target and all-but-one resistor values, solve for the last one and verify — the calculator's verification row mirrors the textbook method.

Repair technicians use it when a parallel network must be rebuilt with substitute values. The original schematic's total is the target; the available substitutes are the knowns.

How to Interpret Your Result Correctly

Read the headline as the theoretical ideal, then translate it to the nearest standard value for the real build. The verification row confirms the theory; your multimeter confirms the practice.

If the calculator refuses with an impossibility message, believe it and redesign. No algebraic trickery creates conductance from nothing.

Remember tolerance. Real resistors vary a few percent from their marked value, so a verification of 100.00 ohms means 95 to 105 in the physical world — plan accordingly.

Frequently Asked Questions

1. What is this calculator solving for?

The unknown branch in a parallel set: given the target total resistance and the other resistors, it finds the missing value via 1/R(missing) = 1/R(target) − sum of 1/R(known).

2. Why must known resistors exceed the target?

Because a parallel total is always smaller than its smallest branch. A known resistor at or below the target makes the target unreachable — the calculator rejects such inputs.

3. What does the verification row prove?

That the solved resistor plus your known ones recombine to exactly the target total. It is an independent recheck of the same arithmetic, shown to two decimals.

4. What if no solution exists?

You get a plain-language explanation, not a number. Either a known resistor is too small or the known set already reaches the target — adjust the inputs and recalculate.

5. How many known resistors can I enter?

Up to three, with the first required and the other two optional. Most real problems need only one or two knowns.

6. Why is the answer given to two decimals?

Two decimals are plenty for resistor work — 150.00 ohms tells you the value precisely while standard parts come in much coarser steps anyway.

7. Should I buy the exact computed value?

Buy the nearest standard E12 value at or above it. The calculator's note recommends this, and the resulting total stays within a few percent of target.

8. Can the missing resistor be larger than the known ones?

Easily. In the 100-ohm example the answer is 150 ohms against a 300-ohm known; with small targets and large knowns, the missing value routinely exceeds every known resistor.

9. Does this work for capacitors or inductors?

The reciprocal math is identical for parallel capacitors, but units and practical concerns differ. The calculator is labeled and validated for resistors in ohms.

10. What units does the calculator use?

Ohms throughout — enter kilohms as thousands of ohms (4,700 for 4.7k) so the reciprocal arithmetic stays consistent.

11. Why not just use series resistors instead?

Series addition is simpler, but sometimes the circuit topology demands parallel branches — for current sharing or redundancy. This calculator serves the parallel case specifically.

12. How is this different from a total-resistance calculator?

A total-resistance calculator takes all resistors and finds the equivalent. This one takes the equivalent and all-but-one resistors and finds the missing branch — the inverse problem.

13. What is conductance?

The reciprocal of resistance, 1/R. Parallel branches add conductances, which is why the formula works in reciprocals before the final flip back to ohms.

14. Can I solve for two missing resistors?

No — one equation cannot fix two unknowns. Specify all but one branch, or pick a value for one missing resistor and solve for the other.

15. Is my circuit data saved anywhere?

No. The calculation runs entirely in your browser and nothing is transmitted or stored. Reset or close the page and the values are gone.