Lights
Type feet and inches like 12'6" or just 12.5. Add one place for each part of the house, and use How many for windows or bushes of the same size. Everything is saved on this device.
About the Christmas lights calculator
This free Christmas lights calculator tells you how many strands of lights to buy for the outside of your house. Add the places you want to light: the roofline, windows, doors, bushes, trees and wrapped tree trunks. The calculator measures each one, picks the number of strands, and adds them up.
It also counts the bulbs, estimates the watts and amps for LED or incandescent lights, and the electricity cost for the season.
Features
- Six kinds of places — roofline or fence, window and door outlines, bushes, cone-shaped trees and wrapped trunks
- Strand presets — mini lights (50 or 100 bulbs), C7 and C9 bulbs, or your own strand
- Bushes and trees by area — light, standard or full look, from the "100 lights per 1.5 × 1.5 ft" rule
- Strands per place — each place is rounded up to whole strands on its own
- Watts and amps — LED or incandescent, at 120 V (230 V in metric)
- Power cost — kWh and cost for the hours and nights you choose
- Feet or metres — strand lengths in feet or metres
- Saved on this device — the list is still there in the store
How the lights are counted
There are two kinds of places:
Lines and outlines need a length of lights. The calculator divides the length by the lit length of one strand and rounds up.
- Roofline, gutter or fence: the length you measure along it
- Window: all four sides, 2 × (width + height)
- Door: both sides and the top, 2 × height + width
- Tree trunk or post: the lights go round in a spiral. A trunk 6 ft tall and 24 in around, with 4 in between turns, takes 18 turns of 24.3 in — about 36.5 ft of lights
Bushes and trees need a number of lights, from the area you see from the street:
- Bush or hedge: width × height
- Tree: a cone looks like a triangle, so width × height ÷ 2
The usual rule is 100 lights for every 1.5 × 1.5 ft of bush. That is about 44 lights per square foot — the Standard look. Light is 100 per 2 × 2 ft, Full is 100 per 1 × 1 ft.
Worked examples
These numbers come from the calculator with LED mini lights, 100 bulbs on a 33 ft strand, and the standard look:
| What you light | Lights needed | Strands |
|---|---|---|
| 40 ft roofline | 40 ft | 2 |
| 4 windows, 3 × 5 ft | 64 ft | 2 |
| Front door, 3 × 7 ft | 17 ft | 1 |
| 3 bushes, 3 × 3 ft | 1,200 lights | 12 |
| Tree, 7 ft tall and 4 ft wide | 622 lights | 7 |
| Trunk, 6 ft tall, 24 in around | 36.5 ft | 2 |
A whole house: a 60 ft roofline (2 strands), the 4 windows (2), the door (1) and the 3 bushes (12) need 17 strands with 1,700 bulbs. As LED lights that is about 102 W (0.85 A at 120 V). The same strands as incandescent lights draw about 680 W (5.7 A).
With C9 bulbs: an 80 ft roofline needs 4 strands of 25 ft (100 bulbs). LED: about 80 W. Incandescent: about 700 W.
In metres: a 12 m roofline needs 2 strands of 10 m with 100 bulbs.
Lights for bushes and trees
How many lights a bush or tree needs, from the calculator (front area × the look):
| Bush (width × height) | Light | Standard | Full |
|---|---|---|---|
| 2 × 2 ft | 100 | 178 | 400 |
| 3 × 3 ft | 225 | 400 | 900 |
| 4 × 4 ft | 400 | 711 | 1,600 |
| 5 × 5 ft | 625 | 1,111 | 2,500 |
| Tree (height × width) | Standard look | Strands of 100 |
|---|---|---|
| 4 × 2.5 ft | 222 | 3 |
| 6 × 4 ft | 533 | 6 |
| 9 × 5 ft | 1,000 | 10 |
| 12 × 6 ft | 1,600 | 16 |
For a Christmas tree inside the house, the Christmas Tree Calculator uses the indoor rule (lights per foot of height) and adds ornaments and garland.
Watts, amps and the power cost
LED lights use far less power than incandescent ones. The calculator uses these typical watts (type your own under Watts and power cost if the box says something else):
| Strand | LED | Incandescent |
|---|---|---|
| Mini, 100 bulbs | about 6 W | about 40 W |
| Mini, 50 bulbs | about 3 W | about 20 W |
| C7, 25 bulbs | about 12.5 W | about 125 W |
| C9, 25 bulbs | about 20 W | about 175 W |
The whole house above, on for 6 hours a night for 40 nights, uses about 24.5 kWh with LED lights and 163 kWh with incandescent ones. At $0.17 per kWh that is about $4.16 against $27.74.
Safety: every strand box says how many strands may be connected end to end. Never connect more, even if the plugs fit. A household circuit of 15 A at 120 V should carry at most about 12 A (1,440 W) for a long time; the calculator warns you above that. Use lights and extension cords made for outdoor use outside.
Frequently asked questions
How many Christmas lights do I need for my roofline?
Measure the roofline in feet and divide by the lit length of one strand. A 40 ft roofline needs 2 strands of 33 ft mini lights, or 2 strands of 25 ft C9 lights.
How many lights do I need for a bush?
Use about 100 lights for every 1.5 × 1.5 ft of bush. A 3 × 3 ft bush needs about 400 lights (4 strands of 100). For a softer look, 225 lights are enough.
How do I measure a window for lights?
Measure the width and the height of the window frame. The outline is 2 × (width + height): a 3 × 5 ft window needs 16 ft of lights.
How many strands can I connect together?
It depends on the strand. Read the box or the tag on the cord: it gives the most strands (or watts) you may connect end to end. LED strands allow many more than incandescent ones.
How much does it cost to run Christmas lights?
Watts × hours × nights ÷ 1,000 gives kWh; multiply by your price per kWh. 17 strands of LED mini lights (102 W), 6 hours a night for 40 nights, use 24.5 kWh — about $4 at $0.17 per kWh.
Are the watts exact?
No. They are typical figures for each type of strand. The real number is on the box or the tag. Type it as Watts per bulb (watts of the strand ÷ bulbs) for an exact total.
Related tools
- Christmas Tree Calculator — lights, ornaments and garland for the tree inside
- Christmas Countdown — days until Christmas
- Electricity Cost Calculator — the running cost of any device
- Wrapping Paper Calculator — paper and ribbon for the gifts
- Feet to Meters — strand lengths in metres
How to use it
- Choose Feet & inches or Metres & cm
- Pick what the first place is — roofline, window, door, bush, tree or trunk — and enter its size
- Use Add another place to light for each part of the house, and How many for the same window or bush
- Pick the light strand you will buy and LED or Incandescent
- For bushes and trees, pick the look: light, standard or full
- Read the strands and bulbs, check the watts, then press Copy the list
