How LED strip power is calculated

Every figure the Lightstrip Calculator produces comes from the rules on this page. They are published so the output can be checked rather than taken on trust, and so you can work the same numbers out by hand if you would rather.

The wattage figures behind them are measured rather than taken from packaging, which for LED strips is routinely optimistic.

Usable length and LED count

A strip can only be cut at its zone boundaries, so the requested length is rounded down to the nearest cut point. Cut spacing comes from the LED density and the zone size. Analog strips are the exception: they cut in fixed 10 cm zones of 12 LEDs regardless of what the reel specifies.

cut spacing (cm) = 100 ÷ (LEDs per metre ÷ LEDs per zone)
cut points = floor(length in cm ÷ cut spacing)
total LEDs = cut points × LEDs per zone

Power draw

Consumption is derived from the measured draw of a full reel, divided across that reel's LEDs and multiplied by the LEDs in your run. Reel figures are measured at full white rather than taken from the packaging, which is typically optimistic.

watts per LED = reel watts ÷ (LEDs per metre × reel length)
total watts = total LEDs × watts per LED
amps = total watts ÷ voltage

Source: Quindor's Real World Powersheet

Power injection points

A single feed carries about 4.26 A before the strip's copper traces limit it. Below that, one feed at the front is enough. Up to twice that, feed both ends. Beyond that, additional feeds are spaced along the middle.

1 feed            if amps ≤ 4.26
2 feeds           if amps ≤ 8.52
2 + ceil((amps − 8.52) ÷ 4.26)  otherwise

Source: quinled.info on strip wiring

Power supply size

The supply is sized for the calculated current plus 20% headroom, then rounded up to the smallest catalogued supply that covers it. A supply run continuously at its limit is a common cause of flicker and early failure.

required amps = calculated amps × 1.2

Wire gauge

The thinnest gauge whose round-trip voltage drop stays within 10%. Front and middle feeds carry current in both directions from the controller, so they are sized for double the current of an end feed.

resistance = ρ × (2 × cable length) ÷ conductor area,  ρ = 1.72e-8 Ω·m
voltage drop % = (amps × resistance ÷ voltage) × 100