How to use the led strip power supply calculator
- Choose meters or feet and enter the total strip length you plan to power.
- Enter the power per meter or foot and the voltage from the label on the strip.
- Set the headroom you want, so the supply is not run at its limit. The page starts with an example value that you can change.
- In the advanced options, add the length of the wire from the supply to the strip and its resistance to estimate the voltage lost in it.
Formula
Strip load (W) = power per length x total length
Supply size (W) = strip load x (1 + headroom / 100)
Current (A) = watts / volts
Wire drop (V) = current x 2 x one-way wire length x resistance per length
The wire drop counts both the supply wire and the return wire, which is why it uses twice the one-way length. The drop inside the strip itself is not counted.
Five meters of 12 volt strip
A strip rated at 14.4 W per meter and 5 m long loads the supply with 72 W, which is 6 A at 12 V. With 20 percent headroom the supply should be at least 86.4 W, or 7.2 A. The next size in the example list is 100 W.
If the supply sits 2 m from the strip on 18 gauge wire at 0.021 ohm per meter, the wire loses 6 x 2 x 2 x 0.021 = 0.504 V, which is 4.2 percent of 12 V. A 16 foot run of 24 V strip at 4.5 W per foot loads the supply with 72 W, or 3 A.
Why leave headroom
A power supply that runs near its rating gets hot and may wear out sooner. Many people pick a supply with 20 percent or more spare capacity. The right amount depends on the supply, so the page lets you set it and does not claim a rule.
Use the wire voltage drop calculator for a longer cable run, and the AWG wire gauge converter to see wire sizes. For a single LED, the LED resistor calculator finds the series resistor.
Voltage drop and long strips
LED strips lose voltage along their length, so the far end can look dimmer than the start. Feeding power at both ends or in several places helps. At 12 volts a given load draws twice the current that it does at 24 volts, so thin wire loses more, and a 24 volt strip is often easier for long runs.
Assumptions and limits
- The strip is a constant voltage type that draws its rated watts at full brightness.
- The power per length and the voltage come from the label on your strip.
- The list of common supply sizes is an example for rounding up, not a rule or a product list.
- Dimmers, controllers and their own power use are not included, so add them if they are significant.
Frequently asked questions
How do I calculate the power supply for an LED strip?
Multiply the power per meter or foot by the length to get the load in watts, then add headroom. Five meters at 14.4 W per meter is 72 W, and with 20 percent headroom that is 86.4 W.
How many amps does an LED strip draw?
Divide the watts by the voltage. A 72 W strip on 12 V draws 6 A, and the same strip on 24 V draws 3 A.
Should the supply be bigger than the strip load?
Most guides suggest leaving some spare capacity, which this page calls headroom. The amount is your choice, and the page uses 20 percent as an example.
Why does the end of my strip look dim?
Voltage falls along the strip because of its resistance. Powering both ends, using a higher voltage strip or thicker feed wire reduces the effect.