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Redmoon Calculators
Maker & workshop

LED Strip Power Supply Calculator

Free LED strip power supply calculator. Enter strip length, watts per foot, and voltage to get total wattage, a recommended PSU size with safety headroom, current draw, and a voltage-drop warning for long runs.

Built and maintained by Paul Clark, Redmoon Software · Last verified · Sources · Changes and corrections

When to use this

Use this before wiring up an LED strip lighting project — under-cabinet, accent, or architectural lighting — to size a power supply correctly and catch long runs that will suffer visible voltage drop before you've mounted anything.

How it compares

Compared to just reading a strip's wattage-per-meter spec sheet, this calculator bakes in the safety headroom convention, snaps the result to real off-the-shelf PSU sizes, and flags voltage-drop risk based on run length and voltage — three things a raw wattage number alone won't tell you.

Enter your values below. Calculations run locally as you type.

Strip setup

Power supply

Total load
78.7 W
1 run × 16.4 ft × 4.8 W/ft
Required PSU rating
94.5 W
incl. 20% headroom
Recommended PSU
100W
load 79% of rating
Current draw
6.56 A
at 12V

Run length is within the usual single-feed limit for 12V strip (5 m, 16.4 ft).

How it works

Total wattage is strip length × watts per foot × number of parallel runs sharing one power supply.

The required PSU rating adds your safety headroom percentage on top of that load, then the calculator picks the smallest standard supply rating that still covers it, from the sizes Mean Well's LED driver and enclosed supply ranges come in (20, 25, 35, 40, 50, 60, 75, 80, 100, 120, 150, 185, 200, 240, 300, 320, 350, 450, 480 and 600W).

Current draw (amps) divides total watts by voltage, and a voltage-drop warning triggers when a single run is longer than one standard 5 m (16.4 ft) reel at 12V or 10 m (32.8 ft) at 24V, the usual single-feed limits, beyond which power should be fed at both ends or injected along the run.

FAQs

Why shouldn't I run a power supply at 100% of its rated wattage?

Continuous full-load operation shortens a PSU's lifespan and increases heat, which is why this calculator adds a safety headroom (default 20%) on top of the strip's actual power draw before recommending a size.

When do I need power injection on a long run?

Strip makers rate a single feed at one 5 m (16.4 ft) reel on 12V and about twice that, 10 m, on 24V. Beyond that, voltage drop shows up as dimming or color shift toward the far end. Feeding power in at both ends of the run, or injecting it every reel length, evens out the voltage along the strip.

Should I choose 12V or 24V strips for longer runs?

24V strips carry the same power at half the current of 12V, which cuts voltage drop roughly in half and lets you run longer single runs before needing power injection. For runs longer than one 5 m reel, 24V is usually the better choice.

How is the recommended PSU size chosen?

The calculator takes your total wattage, adds the safety headroom percentage, and picks the smallest standard rating that still covers the padded load, from the sizes in Mean Well's LED driver and enclosed supply ranges: 20, 25, 35, 40, 50, 60, 75, 80, 100, 120, 150, 185, 200, 240, 300, 320, 350, 450, 480 and 600W. A 190W strip, padded to 228W, gets a 240W supply rather than jumping to 350W. Above 600W, split the load across supplies.

Worked example

Input

16.4 ft strip (one 5m reel), 4.8 W/ft, 12V, 20% headroom, 1 run.

Output

Total load 78.7W; recommended 100W PSU (79% loaded); 6.56A draw; run within the 5 m single-feed limit.

A 16.4 ft, 4.8 W/ft strip draws 78.72W; padding by 20% headroom requires 94.46W of PSU capacity, so the calculator recommends the next standard size up, a 100W supply, drawing 6.56A at 12V. One 5 m reel is the length 12V strip is made to be fed from one end, so no power injection is needed; a second reel joined on would trigger the warning.

Common pitfalls

  • Sizing a PSU to exactly match the strip's wattage with no headroom — continuous near-100% load shortens PSU life and increases fire risk; always pad the rating.
  • Running a single 12V feed past one 5 m (16.4 ft) reel (10 m at 24V) without power injection, which causes visible dimming and color/temperature shift toward the far end.
  • "Number of parallel runs" multiplies total load on a shared PSU — several runs from one power brick add up fast even if each run individually looks modest.

Watts per foot times length, plus headroom

Draw is the strip's rated watts per foot times its length, and the default 20% headroom on the supply is not padding — running a switch-mode supply continuously at its rated maximum shortens its life considerably and runs it hot. Keeping the load at or below about 80% of rated output is common practice; that takes 25% headroom, while the 20% default loads the supply to about 83%.

Rated watts per foot assumes full white at full brightness. Coloured or dimmed output draws less, so the figure is a worst case, which is the right basis for sizing a supply.

Voltage drop is why long runs fail

The most common LED strip problem is not power capacity but voltage drop along the strip: the far end is dimmer and often colour-shifted, because the thin copper traces have resistance and the current has to travel the whole length.

This is far worse at 12 V than 24 V. For the same power, 24 V draws half the current, and voltage drop scales with current — which is why long runs should be 24 V where there is a choice.

The fix for a long run is not a bigger supply. It is injecting power at both ends or at intervals, or splitting into multiple shorter runs, which is what the runs input models.

Read more about this

· 8 min read

Wiring several LED strip runs to one power supply

How the load of parallel strip runs stacks onto a single supply, why each run still keeps its own voltage-drop budget, and when the calculator is really telling you to split the job across two supplies.
· 8 min read

Sizing an LED strip power supply (and avoiding voltage drop)

Why you should never size a PSU to exactly your calculated load, the standard headroom convention, and when a long strip run needs power injection at both ends — with a worked PSU-size lookup.

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