Use this LED resistor calculator to connect one LED or several LEDs to a low-voltage DC supply. Choose an LED color as a starting point, enter the forward voltage and target current from the datasheet, and obtain a standard resistor value, actual current, resistor power rating, and wiring diagram. Multiple strings are calculated separately, so a shorter final string does not accidentally use the wrong resistor.
Why an LED Needs Current Limiting
An LED is a diode, not a resistor and not a fixed-voltage load. Its current rises steeply with forward voltage. A small change in supply voltage, device characteristics, or temperature can therefore cause a large current change. Connecting a bare LED directly across a voltage source is not a controlled way to drive it, even when the supply label appears to match the LED's forward voltage.
A series resistor provides negative feedback: when current increases, its voltage drop increases, leaving less voltage across the LED. This is a simple, inexpensive solution for indicator LEDs at modest currents. A regulated constant-current driver is usually a better choice for power LEDs, wide supply-voltage ranges, or tightly controlled brightness. A battery's internal resistance is not a reliable substitute for designed current limiting.
Identify Anode and Cathode
Connect the anode towards the positive supply and the cathode towards the negative return. On many through-hole LEDs, the longer lead is the anode and a flat edge on the package marks the cathode. These clues are not universal, and cut leads can no longer be compared: confirm the package drawing in the datasheet. The bar in the LED circuit symbol marks the cathode; the two outward arrows indicate emitted light.
Calculate the Resistor for One LED
Let \(V_S\) be the supply voltage, \(V_F\) the LED's forward voltage at the intended current, and \(I_F\) the target current in amperes. Kirchhoff's voltage law gives \(V_S=V_R+V_F\). Combining it with Ohm's law gives
\[R_{\mathrm{ideal}}=\frac{V_S-V_F}{I_F}.\]For a red LED at approximately 2 V, a 5 V supply and a target of 10 mA:
\[R=\frac{5-2}{0.010}=300\,\Omega.\]This is an E24 value. With nominal values, the current is 10 mA and the resistor dissipates
\[P_R=I_F^2R\] \[P_R=\frac{(V_S-V_F)^2}{R}.\]Here the loss is 0.030 W, or 30 mW. A 0.125 W resistor offers more than twice that nominal dissipation. The calculator selects a standard power rating at least twice the calculated resistor loss, using the worst-case loss when variation limits are enabled. That is a useful starting margin, not a universal thermal guarantee: follow the resistor's ambient-temperature derating, enclosure, mounting, working-voltage and pulse specifications.
Select a Standard Value by Rounding Up
If the ideal resistance is not available, select the next higher value in the chosen standard series when the goal is not to exceed the target current at the design point. Do not automatically choose the nearest value: it can be lower and allow too much current. The actual nominal current after choosing a resistor is
\[I_{\mathrm{actual}}=\frac{V_S-V_F}{R_{\mathrm{selected}}}.\]For example, 9 V, a 1.6 V LED and 15 mA give approximately 493 Ω. Rounding up in E12 gives 560 Ω, producing about 13.2 mA. A 470 Ω resistor would instead give about 15.7 mA. Neither the target nor a typical forward voltage is an absolute device rating: use the datasheet, account for tolerances, and leave operating margin.
LED Color Is Only a Voltage Estimate
Color helps identify a plausible starting voltage, but it does not uniquely determine the electrical characteristics. In particular, older yellow-green LEDs and modern blue-green LEDs can have very different forward voltages. The calculator's 2.2 V green preset is not appropriate for every green LED. White and blue presets also vary between devices, operating currents and temperatures.
| LED family | Illustrative forward-voltage range |
|---|---|
| Infrared | About 1.1–1.6 V |
| Red | About 1.8–2.2 V |
| Amber / yellow | About 1.8–2.4 V |
| Green | About 1.8–3.5 V, depending on technology |
| Blue / white | About 2.7–3.6 V |
These are examples, not guaranteed limits. Use forward-voltage limits at the relevant current and temperature from the actual part's datasheet. Many efficient indicator LEDs are bright enough at a few milliamperes. Do not assume that every LED tolerates 20 mA; some low-current LEDs require substantially less.
Several LEDs in Series
In a series string, the same current flows through every LED and the forward voltages add. One resistor can therefore limit the entire string. For \(n\) similar LEDs,
\[R=\frac{V_S-nV_F}{I_F}.\]For different LEDs, replace \(nV_F\) with the sum of their individual forward voltages. All LEDs in the string must tolerate the common operating current. The tool assumes identical LEDs within and across strings.
Three LEDs at 2.2 V each on a 9 V supply leave 2.4 V across the resistor. At 15 mA, the ideal value is 160 Ω. The next higher E12 value is 180 Ω, giving about 13.3 mA, not 150 Ω. A series string does not multiply the supply current by its LED count: all three LEDs carry the same branch current.
Parallel Strings Need Separate Resistors
Do not place several bare LEDs in parallel behind one shared resistor. A shared resistor limits only their combined current. Small differences in forward voltage let one LED take more current than another; heating can aggravate the imbalance. If a branch fails open, the remaining LEDs can receive a larger share of the current.
For twelve LEDs at 2.2 V on 12 V, one practical layout is three parallel strings of four LEDs. Each string has its own resistor. At a 15 mA target,
\[R=\frac{12-4(2.2)}{0.015}\approx213.3\,\Omega.\]Select 220 Ω in E12. Each string draws approximately 14.5 mA, so the supply provides about 43.6 mA in total. Each resistor dissipates about 46.5 mW. A 0.125 W resistor exceeds the calculator's two-times nominal minimum; a common 0.25 W part provides additional thermal margin. Enclosure temperature and component specifications still matter.
What if the Final String Is Shorter?
A shorter string drops less voltage across its LEDs, leaving more voltage across its resistor. It must not reuse the full-string resistor without recalculation. For ten 2 V LEDs on 12 V arranged as 4 + 4 + 2 LEDs at a 10 mA target, the two four-LED strings need 430 Ω each in E24, while the two-LED string needs 820 Ω. The tool lists both values and draws a representative string for each. Actual currents may differ slightly after standard-value rounding.
Supply Headroom, Tolerances and Temperature
The supply voltage must exceed the total LED voltage by enough for the resistor to control current under actual operating conditions. If \(V_S=\sum V_F\), calculating zero ohms does not make direct connection safe. With very little voltage across the resistor, a small voltage variation causes a large relative current change:
\[\Delta I\approx\frac{\Delta V_S-\sum\Delta V_F}{R}.\]There is no universal rule that LEDs must use exactly 70%, 80%, or any other fixed fraction of the supply. More resistor headroom usually makes current less sensitive to voltage changes, but wastes more power. Select the arrangement using supply limits, LED variation and temperature, acceptable brightness variation, and thermal loss.
Enable the variation option to design against stated limits. If resistor tolerance is \(t_R\), the smallest actual resistance is \(R(1-t_R)\). A conservative minimum nominal resistance for a specified maximum current is
\[R_{\min}=\frac{V_{S,\max}-nV_{F,\min}}{I_{\mathrm{target}}(1-t_R)}.\]The calculator rounds this up and reports the resulting current interval:
\[I_{\max}=\frac{V_{S,\max}-nV_{F,\min}}{R(1-t_R)}\] \[I_{\min}=\frac{\max(0,V_{S,\min}-nV_{F,\max})}{R(1+t_R)}.\]These are constant-forward-voltage engineering estimates. They are only as trustworthy as the limits entered: LED voltage also depends on current and junction temperature, and the model does not replace a nonlinear diode model or thermal testing. Enter datasheet limits over the relevant operating range, not just typical values. A lower voltage at elevated temperature can increase current in a resistor-driven LED. Zero calculated minimum current means reliable illumination is not assured at that corner, not that the LED has a perfectly sharp threshold.
Power, Supply Capacity and Safe Wiring
For several parallel strings, currents add. Sum each branch's actual current to size the supply:
\[I_S=\sum_j I_j,\qquad P_{\mathrm{in}}=V_S I_S.\]The resistor and LEDs divide the input electrical power. The fraction delivered electrically to the LEDs is not their optical efficiency: most LED input power still becomes heat. Do not confuse resistor wattage with the supply's current rating or the LED's maximum permitted current.
- Use an isolated low-voltage DC source. This tool is not for direct connection to mains electricity.
- Switch power off before wiring; check LED polarity and resistor value before powering up.
- Start below the datasheet's operating-current limit and measure the actual branch current.
- Do not assume reverse connection is harmless. Many LEDs tolerate little reverse voltage.
- For RGB LEDs, calculate each color channel separately and follow the common-anode or common-cathode pinout.
- Use a suitable transistor or driver for loads beyond a microcontroller pin's ratings. Its voltage drop must also be included in the design.
- LED strips and indicators with built-in resistors or drivers have their own rated supply requirements; do not treat them as bare LEDs.
The resistor color-code calculator helps identify physical resistor values, while the voltage-divider calculator covers a different circuit function. A voltage divider is not a substitute for a series LED current limiter.