How to use this string light calculator
Pick the tab that matches your project, type in a few measurements, and the results update as you type. Here's what each mode is for:
- Patio / Deck perimeter — for lights that run around the edge of a rectangular patio, deck, pergola, or gazebo. Enter the length and width and choose how many sides you're lighting.
- Straight runs across yard — for lights strung in parallel lines between posts, a house and a fence, or across a backyard. Enter how many runs and how long each one is.
- Wrap a tree — for spiraling lights up a trunk (and optionally out onto branches). Enter the height you'll cover and the trunk circumference.
- Roofline / house — for outlining your roof edges, eaves, and gables with lights (great for Christmas displays). Enter the length of roofline you're covering and how many peaks it has.
How many string lights do I need? (quick reference)
If you just want a fast estimate before measuring, these are typical amounts. Your exact number depends on hang style, which the calculator handles for you.
| Space | Typical size | Feet of lights (gentle drape) |
|---|---|---|
| Small balcony / patio | 10 × 8 ft | ~40 ft |
| Average patio or deck | 16 × 12 ft | ~65 ft |
| Large patio / pergola | 20 × 16 ft | ~80 ft |
| Backyard (3 straight runs) | 3 × 25 ft | ~85 ft |
| Wrap one tree trunk | 8 ft high, 30 in around | ~60 ft |
| Outline a house roofline | ~120 ft of eaves + peaks | ~140 ft |
Hang styles: taut vs. draped vs. swag
The single biggest factor people forget is sag. Lights strung with a droop use noticeably more length than the straight-line distance:
- Taut / straight — pulled tight along a wall or edge. No extra length.
- Gentle drape (+15%) — the classic café look with a light curve between supports. The best all-round choice.
- Deep swag (+30%) — dramatic U-shaped dips. Beautiful, but budget noticeably more feet.
How many strands can you safely connect?
Every string light set lists a maximum number you can link end-to-end before overloading the first plug. As a rule of thumb:
- Incandescent café lights — usually only 2–3 strands per outlet.
- LED plug-in strands — often 3–8, sometimes more (LEDs draw far less power).
- Low-voltage / solar sets — each set is independent; no linking limit, but each needs its own transformer or panel.
Always check the printed max on your box. If you need more length than one chain allows, run a second chain from another outlet or use a weather-rated outdoor extension cord.
How many Christmas lights do I need for my roofline?
For roofline and eave lighting, the length of lights you need is simply the total length of the roof edges you plan to outline — plus extra if you follow the peaks up and over each gable. Measure the horizontal run of each section of roof you want to line, add them up, then use the Roofline / house tab and choose how many peaks your roof has:
- Straight eaves only — lights run flat along the gutters. Length ≈ your measured roofline.
- A few peaks (+15%) — a typical single-gable home where you trace up and over one or two peaks.
- Steep / multi-gable roof (+30%) — complex rooflines with several tall peaks use noticeably more.
A typical single-story home has roughly 100–150 feet of roofline. For a clean look, use lights with bulbs spaced about every 12 inches and clip them to the gutter or shingle edge with plastic light clips (never staples or nails).
Frequently asked questions
How many feet of string lights do I need for a 20×12 patio?
For a 20 × 12 ft patio lit on all four sides with a gentle café drape, you need roughly 74 feet of lights — that's about two 48-ft strands. Lighting only two opposite sides drops it to around 46 feet. Use the calculator above for your exact measurements and hang style.
How many string lights to wrap a tree?
It depends on trunk thickness and how tightly you wrap. Wrapping the first 8 feet of a trunk that's 30 inches around, with medium 4-inch spacing, takes about 60 feet of lights. Thicker trunks and tighter spacing use more — the "Wrap a tree" tab calculates it from your measurements.
Should I add extra length just in case?
Yes. Buy the next strand up rather than cutting it close — you'll want slack to reach the outlet, to re-position after your first attempt, and to replace any dead section later. The calculator already rounds up to whole strands and shows your leftover feet.
What size string lights should I buy — bulb spacing?
Bulb spacing (the gap between bulbs) changes the look, not the length you need. Wider spacing (12–18 in) gives an airy café feel; tighter spacing (6 in) looks fuller and brighter. Set your spacing in the calculator to see the approximate bulb count.
How many Christmas lights for the roofline of a house?
Most single-story houses have about 100–150 feet of roofline. Outlining 120 feet of eaves with a couple of peaks takes roughly 140 feet of lights. Measure each roof edge you'll cover, add them together, and use the "Roofline / house" tab with your peak setting for an exact figure.
Can I use indoor string lights outside?
No. Use lights rated for outdoor or wet/damp locations, plug into a GFCI-protected outlet, and use outdoor-rated extension cords. Indoor-only sets can fail or become a hazard when exposed to rain and humidity.