LED Strip Power Supply Calculator (Watts, Amps, Voltage Drop, Power Injection & Feed Wire)

How it works

watts = W/m × metres; amps = watts ÷ V; PSU = watts × (1 + headroom) → next standard size · far-end drop (fed one end, uniform load) = (A/m) × R_strip × L² ÷ 2, R_strip ≈ 0.12 Ω/m loop · max run for x% drop = √(2 × x × V ÷ (A/m × R)); fed both ends doubles it · feed wire: 2 × I × R_wire × length ≤ 3% of V

LED strip is a long, thin printed circuit carrying every downstream LED's current through a few square millimetres of copper, so the voltage falls along its length — with the square of the length, because both the current and the distance grow. That is why a 5 m roll of 12 V strip fed from one end is visibly dimmer and warmer at the far end, and why 24 V strip, feeding both ends and "power injection" wires back to the supply are standard practice. The supply is sized from watts per metre with headroom so it never runs at its limit, and the feed wire from the supply is chosen so it does not add its own drop.

Worked example

Five metres of 14.4 W/m 12 V strip is 72 W (6 A) — use a 96 W (or 100 W) supply for 20% headroom. Fed from one end it drops about 1.8 V (15%) by the far end; a 5% limit allows only about 2.9 m from one end, so feed both ends (about 5.8 m capacity). A 3 m feed at 6 A needs 14 AWG to stay under a 3% drop; a 6 A load is at the limit of most small RGB controllers, so use an amplifier or a higher-rated unit.

Frequently asked questions