About the Christmas Lights Calculator
This Christmas lights calculator plans a whole holiday display in one go: how many strands to wrap the tree, how many to run along the roofline and windows, and how many net lights cover your bushes. It then adds up the bulbs to estimate total wattage, current draw, how many household circuits you need and what the lights will cost to run for the season.
Use it before a trip to the store so you buy enough of the same strand (colours and bulb spacing vary between batches), or when deciding whether switching from old incandescent minis to LEDs is worth it. The electrical figures help you avoid tripping a breaker with too many strands on one outdoor circuit.
The tree rule of thumb is about 100 mini lights per foot of tree height for a standard look (50 for a light sprinkle, 150 for a dense, glowing tree). Net lights are assumed to be 4 × 6 ft with 150 bulbs. Wattage uses typical figures of about 7 W per 100 LED mini lights and 40 W per 100 incandescent minis — check the label on your strands.
With the default inputs, the light strands to buy is 9. Change any value above to recalculate instantly.
How to use the christmas lights calculator
- 1Enter your tree height and pick how densely you like it lit.
- 2Measure the roofline, windows and railings you want to outline in feet.
- 3Count your bushes and estimate their average width and height.
- 4Check the strand box for bulbs per strand and lighted length.
- 5Choose LED or incandescent and enter your usage and electricity rate.
Formula and method
The tree uses a lights-per-foot rule (50, 100 or 150 mini lights per foot of height), divided by the bulbs on each strand and rounded up. Rooflines are covered by strands laid end to end, so the outline length is divided by each strand’s lighted length. Bushes are covered with 4 × 6 ft (24 sq ft) net lights of 150 bulbs, using the bush’s width × height to decide how many nets each needs.
Power is estimated from the total bulb count at about 7 W per 100 LED minis or 40 W per 100 incandescent minis. Amps are watts ÷ 120 V, and a 15 A household circuit is limited to 80% of its rating (1,440 W) for continuous loads. Energy is watts × hours per day × days ÷ 1,000, multiplied by your electricity rate.
- W × H
- Bush width × height (sq ft)
- 1,440 W
- 15 A × 120 V × 80% continuous-load limit
Worked examples
7 ft tree, 40 ft roofline and four bushes with LEDs
A 7 ft tree at 100 lights per foot needs 700 bulbs, or 7 strands of 100. The 40 ft roofline takes 2 strands of 25 ft, and each 3 × 3 ft bush needs one net. The 1,500 LED bulbs draw about 105 W; running 6 hours a day for 35 days uses 22 kWh, about $3.75 at $0.17/kWh.
9 ft heavily lit tree with incandescent minis
At 150 per foot a 9 ft tree wants 1,350 lights — 14 strands. 120 ft of roofline needs 5 strands and six 4 × 5 ft bushes need one net each. With 2,800 incandescent bulbs at 40 W per 100 the load is 1,120 W (9.3 A), costing about $40 for the season.
Big display that needs two circuits
A 7.5 ft tree needs 750 lights (8 strands), 200 ft of roofline needs 7 strands of 33 ft, and each 5 × 5 ft bush needs two nets. The 3,900 incandescent bulbs draw 1,560 W — more than the 1,440 W continuous limit of one 15 A circuit — and cost about $62 to run.
Frequently asked questions
How many lights do I need for a Christmas tree?+
A common rule is about 100 mini lights per foot of tree height, so a 7 ft tree needs around 700. Use 50 per foot for a subtle look or 150–200 per foot for a densely lit tree.
How many Christmas light strands can I connect together?+
It depends on the strand. Incandescent mini strands often allow only three to five end to end, while many LED strands allow 20 or more. The maximum is printed on the tag or box — never exceed it.
How much does it cost to run Christmas lights?+
A 100-bulb LED mini strand uses about 7 W, so 6 hours a night for 35 days is about 1.5 kWh — roughly 25 cents. The same incandescent strand uses about 40 W and costs about six times as much.
How many Christmas lights can go on one outlet?+
A standard 15 A, 120 V circuit should carry no more than about 1,440 W continuously. That is roughly 3,600 incandescent minis or about 20,000 LED minis, but all outlets on the same circuit share that limit.
How do I measure for roofline lights?+
Measure along the gutters, peaks and any windows or railings you want to outline, and add a few feet to reach the nearest outdoor outlet. Divide by the lighted length printed on the strand box.