LED Strip & Power Supply Sizer
Turn strip length into watts, amps and PSU size
LED strip power, headroom and wire FAQ
Why do I need headroom on the power supply?
LED strips draw power continuously when on. Running a power supply right at its maximum rating can make it run hotter and shorter-lived. Adding 10–30% headroom means the supply coasts instead of straining, which usually improves reliability and reduces fan noise.
Is it bad if my power supply is “too big”?
As long as the voltage matches the strip (for example 12 V strip with 12 V supply), a higher-wattage supply is normally fine. The strip only draws what it needs; the extra capacity just sits in reserve. Extremely oversized supplies can be less efficient at very light loads but are usually safe if wired correctly.
Can I mix 12 V and 24 V strips on one power supply?
No. A constant-voltage supply must match the LED strip voltage. Mixing 12 V and 24 V strips on one output will either under-drive one strip or over-drive the other. Use separate supplies or a multi-voltage system with the correct output for each type.
What if my datasheet only lists watts per reel, not per metre?
Divide the total reel watts by the reel length to get a watts per unit figure, then use that here. For example, a 5 m reel that uses 48 W is about 9.6 W/m. Do the same with feet if the label shows watts per 16.4 ft or per 32 ft.
Why does cable length matter for LED strips?
LED strips run at low voltage, so even a small voltage drop along long or thin cable can cause visible dimming and colour shift. Short, thick cables keep resistance low so that the strip actually sees close to its rated voltage at the far end.
Can I just use one long strip powered from one end?
For short runs this is fine, but long high-power runs often benefit from feeding power at both ends or from the middle. That reduces voltage drop and keeps brightness more even along the strip. Some manufacturers give maximum run lengths before you should add a new feed.
Should I use a dimmable power supply?
If you dim via a low-voltage LED controller (PWM on the DC side), you usually use a non-dimmable constant-voltage PSU. If you dim from the mains side (Triac/phase-cut wall dimmer), you need a power supply designed for that. Always follow the wiring diagrams for your controller and power supply.
How to use this LED strip and power supply sizer
This tool turns the basic strip specs you see on a carton or datasheet into a clear power and wiring snapshot: how many watts and amps your run needs, what size power supply to buy and a rough wire gauge suggestion.
1. Set units, voltage and strip rating
Start by choosing US / Imperial (ft) or metric (m). Then add your strip’s supply voltage (usually 5 V, 12 V or 24 V) and the wattage per length from the datasheet. In US mode the tool treats the rating as W/ft; in metric mode it treats it as W/m.
2. Add total lit length on this power supply
Add up all the segments this power supply will run and enter the total length. For example, three 2 m runs is 6 m total. If you have multiple supplies, size each one separately so cable lengths and currents stay realistic.
3. Pick a safety headroom
Headroom is the percentage you layer on top of the calculated strip watts. Many installers choose around 20% so the supply runs cooler. Lower values make the supply smaller but harder worked; higher values make it larger and more relaxed. The flag in the result helps you see whether your choice is lean, typical or very generous.
4. Estimate your longest cable run
In the cable box, enter the longest one-way run from the power supply to the start of a strip. This gives enough information for a simple wire size hint. For complex layouts or long distances you may still want a dedicated voltage-drop calculator.
5. Read watts, amps, PSU size and wire hint
After you tap Size power supply & wire, the summary shows:
- Strip watts with and without headroom.
- Estimated current draw from the supply at your voltage.
- A recommended PSU rating and the next common size up.
- A simple wire gauge suggestion based on current and run length.
Use this as a planning baseline, then cross-check with manufacturer wiring diagrams and local electrical rules before installing.
6. Copy the summary into your project notes
Tap Copy summary to drop the numbers into a text file, email or project document. It becomes a quick reference when you buy parts or talk with your electrician, and it keeps your layout and wiring plan on the same page.
How the LED strip watt, amp and wire maths works
The maths is kept deliberately simple and transparent so you can check it by hand or adapt it for more complex projects.
1. Strip watts from watts-per-length and total length
Let Punit be the strip watts per length unit (W/ft or W/m) and L the total strip length (ft or m) on this supply. The base strip load is:
Base strip watts = Punit × L
This is the “raw” power draw with no safety margin added.
2. Adding safety headroom to size the power supply
Let H be your headroom percentage (for example 20). The headroom factor is 1 + H/100. The tool multiplies:
Recommended PSU watts = Base strip watts × (1 + H/100)
It then rounds this up to a nearby “common” PSU size so you see both the exact requirement and a realistic catalogue size you might buy.
3. Current draw from watts and voltage
Let V be the supply voltage in volts. The approximate current at the recommended wattage is:
Current (A) ≈ Recommended PSU watts ÷ V
This gives a convenient single number to compare with wire ampacity charts and controller ratings.
4. A simple wire gauge suggestion
The calculator uses a small set of heuristics that blend current and one-way cable length to suggest a minimum copper wire gauge and an approximate mm² size for metric users. Short, low-current runs get thinner wire; longer or higher-current runs nudge the suggestion to thicker wire.
For demanding or code-critical installs, you should also check an ampacity / voltage-drop chart or a dedicated wire calculator and follow any minimum sizes required by your local electrical rules.
5. Understanding the headroom flag
The green/amber/red headroom flag is based solely on your headroom percentage:
- GREEN for a mid-range headroom (roughly 15–35%).
- AMBER for lean (around 10–15%) or generous (around 35–60%) margins.
- RED if there is almost no headroom or extremely high headroom.
It’s simply there to nudge you towards a comfortable middle ground where the supply is neither overloaded nor excessively oversized for what the strip actually needs.
References and further reading on LED strip power and wiring
Use this sizer alongside these LED strip power and wire resources:
- Simple Lighting — How to calculate power requirements for LED strip lights — step-by-step examples of strip wattage, headroom and PSU sizing.
- LEDs.co.za — LED strip power supply sizing — practical guide on matching strip wattage and adding sensible headroom.
- LEDSupply — How to choose an LED power supply — background on constant-voltage supplies and common sizing mistakes.
- Waveform Lighting — LED strip light wire gauge & voltage drop calculator — deeper dive into voltage drop and conductor sizing for long runs.
- Wired4Signs USA — Why wire size matters in low-voltage LED systems — explains the 3% voltage drop idea and why cable thickness affects brightness.
Always confirm final wire sizes, overcurrent protection and installation methods with local electrical codes, manufacturer recommendations and a qualified electrician where required.