LED string / array calculator
Design an LED array: put LEDs in series or parallel, choose the supply voltage, and read the current-limiting resistor, total current and power.
Circuit values
LED string results
Total forward drop
Vf × LEDs in series
Series resistor (per string)
(V − series drop) ÷ If
Resistor power (per string)
If²·R — pick a part rated at ~2×
Total supply current
If × parallel strings
Total power
Vsupply × total current
About the model
Each LED in series adds its forward voltage, so the string drop is Vf × N and the series resistor sees the supply voltage minus that drop. Parallel strings share the supply current — the supply must deliver If × (number of strings). A separate resistor per string is safer than one shared resistor, because a shared resistor cannot balance small Vf differences between strings.
This assumes a constant Vf. Real LEDs shift forward voltage with temperature and batch, so use the datasheet curve for a production design and rate the resistor at roughly 2× its dissipation.
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Frequently asked questions
How do you size a resistor for a series LED string?
Each LED in series adds its forward voltage, so the string drop is Vf × N and the series resistor is R = (V − N·Vf) ÷ If. With a 5 V supply, 3.0 V white LEDs and 20 mA, a single LED needs (5 − 3.0) ÷ 0.02 = 100 Ω, while two in series would need the supply to stay above 6 V.
How much current does a parallel LED array draw from the supply?
Each parallel string carries the full LED current If, so the supply must deliver If × (number of strings). The tool reports total supply current as If × parallel count and total power as Vsupply × that current. A separate resistor per string is safer than one shared resistor, because a shared resistor cannot balance small Vf differences between strings.
What happens if the series forward drop exceeds the supply voltage?
The driveable condition requires the series drop to stay below the supply, otherwise the resistor has no headroom and current is not controlled. The tool flags this case and tells you to raise the supply voltage — presets of 3.3, 5, 9, 12 and 24 V — or reduce the number of LEDs in series.
Assumes constant forward voltage. Real LEDs shift Vf with temperature and batch — verify against the datasheet. ICBOMS provides this tool for reference only.