The LED Series and Parallel Calculator plans a multi-LED array with safe series strings and independently limited parallel branches. Calculate the resistor for every branch, actual current, source demand, power and efficiency.
LED array calculator
How to use this calculator
- Enter the supply and typical LED forward voltage.
- Set the desired current for one branch.
- Choose how many LEDs are in series and how many identical strings are in parallel.
- Calculate, then use one displayed resistor in every parallel branch.
Important: Do not use one shared resistor for multiple parallel strings. Small forward-voltage differences can make one branch draw too much current.
LED Series and Parallel Calculator formulas
The LED Series and Parallel Calculator separates the array into identical branches so each string can be checked clearly.
For N LEDs in a series string, the combined forward voltage is N × VF. The resistor must drop the remaining voltage, so the ideal resistance for each branch is:
R = (VS − N × VF) ÷ I
Current must be expressed in amperes. Total source current equals the actual current of one branch multiplied by the number of parallel branches. Source power is supply voltage times total current. Approximate LED power is the combined LED forward voltage times branch current times the number of branches.
Real LEDs vary with temperature and manufacturing tolerance. This tool adds a five-percent resistance margin and, when selected, rounds upward to an E24 standard value. That conservative choice normally produces current slightly below the target. Confirm the component datasheet and measure a prototype.
Worked example: 12 V LED array
Suppose a 12 V source powers four parallel strings. Each string contains three 2 V LEDs and targets 20 mA. The LEDs use 6 V, leaving 6 V across the resistor. The ideal resistance is 6 ÷ 0.020 = 300 Ω. With the design margin and E24 rounding, the recommendation is 330 Ω.
| Result | Value |
|---|---|
| Total LEDs | 12 |
| Actual current per branch | 18.18 mA |
| Total source current | 72.73 mA |
| Source power | 0.873 W |
| Resistor dissipation per branch | 0.109 W; use at least 0.25 W |
Design notes for reliable LED arrays
Use the LED Series and Parallel Calculator as a planning check before selecting components. The LED Series and Parallel Calculator results should then be compared with LED datasheets.
Leave voltage headroom for the resistor. If the combined LED forward voltage is equal to or higher than the supply, a simple resistor circuit cannot regulate current reliably. Reduce the LEDs per string, raise the supply voltage within safe limits, or use a constant-current driver.
Resistor power ratings should not be operated at their limit. This calculator recommends a rating of at least twice the estimated branch dissipation and selects a common wattage. High-power LEDs often require a purpose-built driver and thermal management rather than a resistor.
Forward voltage depends on LED color, current and temperature. Use the typical value for planning, then check minimum and maximum values from the manufacturer. For more component background, see the SparkFun LED guide.
Choosing strings, branches and components
Start by selecting a series-string length that leaves practical voltage across the resistor. A resistor needs some voltage headroom to stabilize current when the supply or LED forward voltage changes. Very small headroom can make brightness inconsistent, while excessive headroom wastes more energy as heat. For ordinary indicator LEDs, a few volts across the resistor is often easier to manage than only a few tenths of a volt.
Use additional parallel branches when one string does not provide enough light. Every branch should contain the same number and type of LEDs and should receive its own resistor. If different LED colors or models are required, calculate them as separate branch groups because their forward voltages may differ substantially.
Check the power supply
The supply must provide at least the calculated total current, plus reasonable reserve. Continuous operation near a power supply maximum can cause voltage drop, heat or shortened life. A design margin of roughly 20 percent is a practical starting point for many low-power projects, but manufacturer guidance takes priority.
Prototype and measure
Build and test one branch before assembling the complete array. Measure the resistor voltage and branch current after the LEDs warm up. Confirm that the resistor remains within its temperature and wattage limits. Then duplicate the verified branch and measure the total supply current. For battery-powered designs, also test at the highest fresh-battery voltage and near the lowest usable voltage.
Brightness matching is best when LEDs come from the same production batch and operate at similar temperatures. Precision applications may benefit from active constant-current drivers, which control current more consistently than resistors when voltage and temperature vary.
Frequently asked questions
Can parallel LED strings share one resistor?
No. Give every string its own series resistor so current differences are limited independently.
Should I round resistance up or down?
Round upward for a safer current below the target. The E24 option does this automatically after applying a small design margin.
What if the calculator reports insufficient voltage?
The supply cannot overcome the selected LEDs’ combined forward voltage with useful resistor headroom. Use fewer LEDs per string or a suitable driver and supply.
Does this replace an LED driver?
No. It is best for low-power resistor-limited arrays. Use a constant-current driver for high-power LEDs, wide supply variation or precision brightness.
Related calculators
Educational estimate only. Verify voltage, current, component tolerance, thermal limits and wiring before building a circuit.