Work out the DC wattage and current for your LED strip project, then size a constant-voltage power supply with a clearly defined load allowance. Add different strip groups and check a candidate supply below.
LED strip power supply calculator
Free planning toolWhat size power supply do I need for LED strip lights?
Add the maximum power of every strip section that can operate at the same time. Include any other DC loads on that supply. Then divide the total by your chosen usable output fraction. The supply must also provide the correct constant DC voltage.
For example, a 96 W load with an 80% loading limit and no separate derating needs at least 96 ÷ 0.8 = 120 W of rated DC output. At 24 V, the load draws an estimated 4 A, while a 120 W supply is rated for 5 A. These two currents describe different things: what the load uses and what the supply can deliver.
The calculator gives a sizing target and an optional comparison with a product you have found. It does not choose a market model or assume that any listed wattage is a standard size.
Get the right numbers from the product label
Check voltage first. A strip marked 24 V DC needs a compatible 24 V constant-voltage supply. Extra wattage does not make a 12 V or 48 V supply compatible with it. The candidate check keeps voltage separate from power.
Next, find the power per length. Use W/m with metres or W/ft with feet, or let the calculator convert between them. Do not substitute LED count, a diode package name or a brightness claim for the product's wattage specification. If the manufacturer only gives a reel's total maximum power, divide it by that reel's stated length in the matching unit.
For RGB, RGBW or tunable-white tape, check which channels the stated figure includes and which can operate together. Use the maximum simultaneous state allowed by the actual strip and controller. A usual dimmed setting is not a reliable basis for sizing a system that can later run at full output.
Measure the powered strip after allowing for the product's permitted cut points. Keep cable length and unpowered offcuts out of the strip-length field. If two runs differ in length or watts per metre, enter them as separate groups.
LED strip power supply formula
Power for one strip group = length of one run × watts per matching length unit × number of identical runs.
Total DC load = sum of all strip-group watts + other simultaneous DC load.
Minimum nameplate DC output = total DC load ÷ (chosen loading percentage ÷ 100 × manufacturer-available output percentage ÷ 100).
Estimated DC load current = total DC load ÷ system DC voltage.
The two percentages have separate meanings. If 90% of a supply's nameplate output is available under the specified conditions and you choose to use at most 80% of that available output, the combined usable fraction is 0.9 × 0.8 = 0.72. Do not apply both if one input already includes the other reduction.
Why 80% loading is different from adding 20%
An 80% loading limit leaves 20% of the selected output capacity unused. Dividing the load by 0.8 is the same as multiplying it by 1.25. Multiplying by 1.2 instead would make the load 83.33% of the resulting supply rating.
Flexfire LEDs' power-supply guide uses the load divided by 0.8 method for its 80% guidance. This supports the calculator's initial loading value, but the setting remains editable because the chosen product's instructions govern the project.
MEAN WELL's technical guidance discusses temperature, load and product-specific derating. The calculator does not infer a derating value from room temperature. Read the relevant curve and conditions for the exact supply. An entered 100% availability does not verify that the product can deliver full output in a cupboard or other installation location.
Worked example: cabinet lighting and a shelf
Suppose two separate cabinet runs are each 2.5 m long and rated at 9.6 W/m. A shelf run is 3 m long at 14.4 W/m. All use 24 V DC. Add an illustrative 4.8 W of other simultaneous DC load:
- Cabinet strips: 2.5 × 9.6 × 2 = 48 W.
- Shelf strip: 3 × 14.4 = 43.2 W.
- Total: 48 + 43.2 + 4.8 = 96 W.
- Estimated load current: 96 ÷ 24 = 4 A.
- At 80% loading and 100% availability: 96 ÷ 0.8 = 120 W minimum.
A candidate rated 150 W at 24 V has a chosen 80% load budget of 120 W. The 96 W load leaves 24 W within that budget and uses 64% of nameplate output. A 100 W candidate at the same voltage would have only an 80 W budget, so it would not meet this chosen allowance.
If the manufacturer makes only 90% of nameplate output available under the installation conditions, the minimum becomes 96 ÷ (0.8 × 0.9) = 133.333... W. The calculator displays 133.4 W as an upward-rounded target. For the 150 W candidate, the corresponding budget is 150 × 0.9 × 0.8 = 108 W.
The example button also enters a hypothetical 5 m maximum for each run. Both 2.5 m cabinet runs and the 3 m shelf run are within that entered limit. The total of 8 m does not mean one continuous 8 m run is allowed. These are example specifications, not a product recommendation.
Power supply capacity does not decide the maximum run length
Waveform Lighting explains strip-length limitations as a separate issue from the supply's total capacity. A long run can have voltage drop or other limits even when the power supply has enough watts.
Enter the maximum run length for the exact strip and the intended feed arrangement. The calculator compares it with each physical run. It does not calculate cable resistance, voltage drop, feed locations or connector capacity. A warning needs a revised layout checked against the product instructions; increasing supply wattage alone does not resolve it.
Common questions
Can I enter the power supply rating in amps?
Yes. Select amps in the candidate section and enter the continuous DC output current. At the selected candidate voltage, watts = volts × amps. For example, 24 V × 5 A = 120 W. Use the rating for that output voltage and rail, not an AC input-current figure.
Should I multiply by the power supply efficiency?
The sizing calculation compares DC load with DC output capacity. It does not reduce that output rating again by an AC-to-DC efficiency percentage. Efficiency matters when estimating wall-socket energy use, but this calculator does not estimate electricity bills or mains input current. Manufacturer output derating is a separate input with a different meaning.
Does the current result size my controller or cable?
No. Total load current is useful for planning, but controllers can have both total and per-channel limits. Cable length, wire size, connectors, voltage drop and protection also matter. A matching supply rating does not confirm these parts. Check them separately for the actual circuit.
Can several supplies be treated as one larger supply?
Calculate each independently powered section with its own supply and allowance. Adding nameplate ratings does not establish that supplies can share an output. Follow a suitable connection plan and the equipment instructions.
Does this cover constant-current drivers or mains-voltage tape?
No. The model assumes a constant-voltage DC strip system at 5, 12, 24 or 48 V. It does not design constant-current LED circuits, direct-mains strips or AC connections. Have mains connections and protective measures handled by a qualified electrician.
Which calculators help with the rest of this update?
Use the furniture wrap calculator for cabinet film, the paint calculator for wall finishes or the tile calculator for a backsplash. Each calculates a different part of the project.
