Reference formulas and worked examples below keep their stated units. Monetary examples use USD; dimensional examples use US customary units unless labeled otherwise. Metric calculator inputs are converted to these reference units before calculation. Choosing another currency does not convert example amounts or exchange money.
How to use this christmas lights calculator
Estimate light strands for a measured decorating run, plus electrical energy use and cost for the display period.
How the calculation works
Required lit length = measured run × (1 + allowance ÷ 100). Strands = required length ÷ lit length per strand, rounded up.
Total watts = whole strands × watts per strand. Energy = watts ÷ 1,000 × daily hours × display days.
Energy cost = kWh × entered price per kWh.
Worked example
A 120 ft run plus 10% allowance needs 132 ft. With 25 ft strands, use 6 strands. At 5 W each, six hours daily for 30 days uses 5.4 kWh, or about $0.86 at $0.16 per kWh.
Limits and assumptions
A length and energy estimate, not a circuit safety assessment. Follow manufacturer limits for connected strands and indoor/outdoor ratings. It does not calculate tree wrapping geometry.
Frequently asked questions
Which strand length should I enter?
The lit section length from the product specifications.
Can I connect all the calculated strands together?
Not necessarily. Follow the product’s maximum end-to-end connection limit and electrical requirements.
Do LED and incandescent lights cost the same to run?
No. Enter the wattage of the actual product to compare usage.