Series Resistor Calculator

Add the individual values: Rt = R1 + R2 + R3 + ... . For example, 100, 200, and 300 ohms in series give 600 ohms total. Unlike parallel networks, there is no reciprocal math — series resistance is always larger than any single resistor in the string. Enter 2 to 8 resistors in the calculator above and the total is computed instantly, with per-resistor voltage drops and power.

Use the voltage divider rule: Vi = Vsupply x Ri / Rt, where Rt is the total series resistance. With 12 V across 100, 200, and 300 ohms (Rt = 600), the drops are 12x100/600 = 2 V, 12x200/600 = 4 V, and 12x300/600 = 6 V, and they always add up to the supply voltage. The calculator above shows every drop, each resistor's share as a percentage, and the dissipated power.

There is only one path for charge to flow, so whatever current leaves the supply must pass through each resistor in turn — this is Kirchhoff's current law applied to a single loop. That is why I = V / Rt describes the whole string with one number: 12 V across 600 ohms is 0.02 A everywhere. If you need different branch currents, you are describing a parallel network, which this tool does not cover.

A voltage divider is two resistors in series used to produce a lower voltage from a higher one: Vout = Vin x R2 / (R1 + R2), measured across R2. For example, 12 V with R1 = 4k and R2 = 8k gives 12x8000/12000 = 8 V. It works well for reference voltages and signal scaling, but not for powering loads — any load current drawn from the tap changes the division. The divider tab above computes Vout, current, and power in each resistor, plus a reverse solver.

Pick the tap current first (keep it at least 10x larger than any load the tap will feed, so the load barely disturbs the ratio), then R2 = Vout / I and R1 = (Vin - Vout) / I. To get 5 V from 9 V at 1 mA: R2 = 5/0.001 = 5000 ohms and R1 = 4/0.001 = 4000 ohms. The reverse solver in the divider tab does this math, then snaps both values to the nearest E12 or E24 standard resistors and shows the actual Vout you will get.

Pi = I^2 x Ri, using the single series current. With 0.02 A through 100, 200, and 300 ohms, the resistors dissipate 0.04 W, 0.08 W, and 0.12 W. Always choose a resistor rated at least twice the expected dissipation, then round up to the next standard rating (0.125, 0.25, 0.5, 1, 2, 5, 10 W). The calculator shows Pi per resistor and suggests the next standard rating automatically.

They are the standard preferred-value series manufacturers actually make: E12 has 12 values per decade (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82) typically at 5% or 10% tolerance, and E24 doubles that to 24 values per decade (adding 11, 13, 16, 20, 24, 30, 36, 43, 51, 62, 75, 82, 91) typically at 1% to 5%. You cannot buy a 350 ohm resistor off the shelf — the calculator's picker snaps any value to the nearest E12 or E24 part so your design uses real components.

R = (Vs - Vf) / If, where Vs is the supply voltage, Vf is the LED forward voltage, and If is the desired current. Example: a 9 V supply, a 2 V red LED, 20 mA gives (9-2)/0.02 = 350 ohms; the nearest E12 part is 330 ohms and the resistor dissipates 0.14 W, so use a 0.5 W rating (twice the dissipation, rounded up). The LED tab above does this with one click, including typical Vf presets for red, green, blue, and white LEDs, and warns you if the supply is too low for the LED.

Zero or negative resistance is not physically meaningful for a passive resistor, so the calculator rejects it with an error instead of silently computing nonsense. A zero-ohm entry is usually a wire link or a forgotten value — check the row and enter the real resistance. The tool requires at least 2 and at most 8 positive resistor values before it computes anything.

Yes, and it is the most common reason a divider misses its target: with 5% E12 resistors, each value can be 5% off in either direction, so a nominal 8.00 V tap can land anywhere in roughly a +/-10% band in the worst case. For reference voltages that must be accurate, use 1% E24 (or better) parts, measure the actual resistors, or buffer the tap with an op-amp. The calculator shows the ideal result — treat it as the center of a tolerance band, not a guarantee.

Use this series tool when resistors share one current path end to end — total Rt = R1 + R2 + ..., voltage divides across them. Use the parallel resistor calculator when resistors connect across the same two nodes and share the voltage — total Rt = 1/(1/R1 + 1/R2 + ...), current divides between branches. If your circuit has both, simplify each series or parallel group step by step; start with whichever group is obvious, then feed its equivalent resistance into the other tool.

The physics is the same idea seen from two sides: a voltage divider is the deliberate design (choosing R1 and R2 to create a target Vout), while voltage drop is the analysis of what is lost across resistance that is already there (Vd = I x R, including wire resistance). This tool covers series networks and divider design; for drop in a single resistor or in wiring runs by AWG gauge, use the resistor voltage drop calculator, which also handles the reverse problem of finding maximum current for a drop budget.

Scope boundary: this tool handles series resistor networks (total resistance, per-resistor voltage drop, power) and voltage divider design, including the LED helper. The parallel resistor calculator handles parallel networks with current division. The resistor calculator covers color codes, Ohm's law, and SMD codes. The resistor noise calculator covers Johnson thermal noise. The resistor voltage drop calculator covers Vd = I x R in single resistors and wire runs by AWG. Pick the one matching your topology; they cross-link each other and do not overlap.

The formulas assume ideal DC resistance. For AC, resistors still follow the same arithmetic for the resistive part, but real circuits add reactance from inductance and capacitance, skin effect, and phase shifts that this tool does not model. It is accurate for DC electronics, LED circuits, sensor dividers, and low-frequency references. For mains AC or RF work, treat the numbers as a first approximation and verify with measurement.