Why this site exists
Search for "how many string lights do I need for my patio" and you'll find dozens of articles that circle the question without answering it. Most offer a single example — usually a patio that isn't the size of yours — and leave you to do the arithmetic.
The maths isn't hard, but it's fiddly enough that people get it wrong in a specific and expensive way: they measure the straight-line distance, forget that draped lights sag, and end up one strand short on the evening they planned to hang them.
So this site does the calculation instead of describing it. Enter your measurements, get feet, strands, bulb count, and leftover length. No sign-up, no email capture, no forty paragraphs of preamble before the tool.
Try the calculator →Patio and deck perimeters, yard runs, tree wraps, and rooflines.How the estimates are calculated
Every mode uses a transparent formula. Nothing is hidden, so you can check our arithmetic against your own:
Patio / deck perimeter
feet = perimeter × (1 + drape allowance)
Perimeter is 2 × (length + width) for all four sides, 2L + W for three sides, 2L for two opposite sides, and L for a single side. Drape allowance is 0% for taut, 15% for a gentle café curve, and 30% for a deep swag.
Straight runs across a yard
feet = runs × (run length × (1 + drape) + 2 × end drop)
The end drop accounts for the vertical distance from your attachment point down to the post or wall bracket at each end of every run — a length that's genuinely easy to overlook.
Wrapping a tree
feet = (height in inches ÷ spacing) × trunk circumference in feet
The first term is the number of times the lights circle the trunk; the second is the distance covered per circuit. Branches are added at a rough allowance of 8 feet each. This is why trunk thickness matters far more than tree height.
Roofline / house
feet = roofline length × (1 + peak allowance)
Peak allowance is 0% for straight eaves, 15% for a typical home with one or two gables, and 30% for steep or multi-gable roofs, where tracing up and over each peak adds meaningful diagonal length.
Strands and bulbs
Strand count is ceiling(total feet ÷ your strand length) — always rounded up, because string lights can't be cut to fit. Bulb count is total feet ÷ (bulb spacing ÷ 12), which is why changing spacing changes the number of bulbs but never the footage you need.
Where the reference figures come from
The drape percentages, typical roofline lengths, and connection limits used across this site reflect the ranges commonly published by string light manufacturers and the values that show up consistently in product specifications. They're planning figures, not guarantees.
Two things vary too much to be worth generalising, so we always point you back to your own product:
- Maximum strands per chain. This depends on the wattage of your specific bulbs and the fuse rating in your specific plug. The number printed on your box is the one that counts.
- Weather ratings. "Outdoor" is a marketing word; wet-location and damp-location are specifications. Always read the label rather than the front of the packaging.
Our editorial approach
- Tool first. The calculator sits at the top of the homepage, not below a thousand words of introduction.
- Show the formula. Every number this site produces can be reproduced by hand. If you disagree with an estimate, you can see exactly where it came from.
- Safety isn't optional. Electrical guidance on this site is deliberately conservative. Where a manufacturer's limit exists, we defer to it.
- No pretending to be an electrician. This site helps you estimate quantities. It does not replace a qualified electrician for wiring, new circuits, or anything you're unsure about.
How this site is funded
String Light Calculator is free to use. The site is supported by display advertising, which is what pays for the domain and hosting. Ads never influence the calculations, and there's nothing to buy here.
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Get in touch
Found an error in a formula, or want a lighting scenario the calculator doesn't cover yet? We'd genuinely like to hear about it — corrections get fixed quickly. Visit the contact page.