How many Christmas lights do you need?

Two questions, two answers

Decorating with lights involves two separate questions. The first is how many strings to buy, which is a matter of taste and arithmetic. The second is whether the strings you plug in are safe on the circuit, which is a matter of watts, amps and the manufacturer’s limits. The calculator handles both: choose a tree or a roofline for the first, and “Check my circuit” for the second.

The methods

Tree. Lights = tree height × lights per foot. Strings = lights ÷ lights per string, rounded up, and the length is strings × the length of one string. The lights-per-foot figure is a decorator’s rule of thumb, with around 100 mini lights per foot a common start and more for a fuller look. No safety code or standard sets it. Roofline. Strings = length × (1 + extra%) ÷ length of one string, rounded up. Circuit. Total watts = strings × watts per string. Amps = watts ÷ 120 V. For a load that stays on 3 hours or more, the National Electrical Code (210.20(A)) requires the breaker to be rated at 125% of it, so the lights plus anything else on the circuit must stay at or below 80% of the breaker: 1,440 W on a 15 A circuit, 1,920 W on a 20 A circuit.

A worked example

A 7-foot tree at 100 lights per foot needs 700 lights, or 7 strings of 100 lights; if each string is 20 feet long, that is 140 feet of lights. A 48-foot roofline with 10 percent extra for corners is 52.8 feet, which takes 3 strings of 25 feet. For the circuit, a 100-bulb ENERGY STAR LED string can draw at most 100 × 0.2 = 20 W, so 12 of them draw at most 240 W, 2.0 A and 17% of the 80% limit of a 15 A circuit: no problem. Twelve strings labeled 150 W each, on the other hand, are 1,800 W and 15 A, which is 125% of the 1,440 W limit; the circuit can carry 9 of them, or 12 on a 20 A circuit.

Limits of this method

The label wins. A manufacturer’s limit on how many strings can be joined end to end reflects the wire in the set and its fuse. The calculator asks for it as an optional entry and shows how many separate plug-in runs you need. The UL 588 figure of 216 W end to end is only a reference: it comes from a UL representative’s comments in a magazine (Carolina Country, December 2017), not from the text of the standard, and the CPSC’s older advice was no more than three standard-size sets on one extension cord.

Other loads count. An outdoor outlet may share its circuit with a garage, a freezer or a bathroom. Enter what else is on it or, if you do not know, use a separate circuit for the lights.

Outdoor and indoor sets differ. The CPSC advises using only lights marked for outdoor use outdoors, plugging them into GFCI-protected outlets and throwing away damaged sets. Never put electric lights on a metal tree.

Costs. The Department of Energy’s 2015 article gave $0.27 against about $10 to run a 6-foot tree’s lights for 12 hours a day for 40 days, LEDs against incandescent. Use the electricity cost calculator for your own rate.

Common questions

How many Christmas lights do I need per foot of tree?

There is no standard. Decorators commonly start at about 100 mini lights per foot of tree height and go higher for a fuller, brighter look, which for a 7-foot tree is 700 lights, or 7 strings of 100. The calculator lets you change the number per foot, so use fewer for a subtle tree and more for a dense one.

How many strings of Christmas lights can I connect together?

As many as the label on the set allows, and no more. UL’s Robert La Rocca told Carolina Country in December 2017 that the old edition of the UL 588 standard let you connect only three strings end to end, and that the current one allows up to 216 watts end to end, which is why a low-power LED string can be joined many times over. The CPSC’s older holiday advice was no more than three standard-size sets per extension cord. When the label and these numbers disagree, follow the label.

How much power do LED lights use compared with incandescent?

The U.S. Department of Energy says LED strands use about 70 percent less energy than conventional incandescent strands and, in its 2015 example, cost $0.27 to light a 6-foot tree for 12 hours a day over 40 days, against about $10 for incandescent lights. ENERGY STAR certifies decorative LED strings at no more than 0.2 watts per bulb and says they use about 92 percent less energy than incandescent strands.

How many amps does a string of lights draw?

Divide the watts on the label by the voltage, 120 in a US home. A 150 W string draws 150 ÷ 120 = 1.25 A. Twelve of them draw 15 A, which is the entire rating of a 15 A circuit, so the circuit is already past the 80% limit for lights that stay on for hours.

Why is the limit 80% of the circuit?

The National Electrical Code treats a load that runs for 3 hours or more as continuous, and a circuit breaker for a continuous load must be rated at 125% of it, which is the same as loading the circuit to 80%. A 15 A circuit at 120 V is 1,800 W, so the lights should stay at or below 1,440 W; a 20 A circuit gives 1,920 W. Holiday lights are usually on all evening, so we use 80%. See the breaker size calculator.

Can I use an extension cord for outdoor lights?

Yes, if it is rated for outdoor use and for the load. The CPSC advises that outdoor lights and decorations be marked for outdoor use and plugged into outlets with ground-fault circuit interrupter (GFCI) protection, and that damaged sets, with cracked sockets, bare wire or loose connections, be thrown away.

Convert watts and amps with the watts to amps calculator, size a breaker with the breaker size calculator, or work out a long cord run with the voltage drop calculator.

For general information only. Follow the instructions and markings on every light set and extension cord, and local electrical code; this calculator is not a substitute for an electrician. Sources, read October 27, 2026: U.S. Department of Energy, “How Do Holiday Lights Work?” (December 16, 2015); ENERGY STAR, Decorative Light Strings (specification effective January 2011; up to 0.2 W per bulb); U.S. Consumer Product Safety Commission holiday-decorating safety releases (1998 to 2005; outdoor rating, GFCI, three sets per extension cord); Carolina Country, “The evolution of holiday lighting” (December 2017; UL’s Robert La Rocca on UL 588); NFPA 70, National Electrical Code, 210.20(A) (continuous loads, 125%), as explained on our breaker size calculator page. How we check our calculators.