Christmas Lights Calculator
Pick what you are lighting - a tree or a roofline - enter the size, and get the strand count, bulb count, total wattage for incandescent vs LED, and what the display costs to run this season.
How the math works
For trees, the classic rule is 100 mini bulbs per foot of height for a full decorated look - a 7 ft tree takes roughly 700 bulbs. Cozy and designer looks scale that to 50 and 150 per foot. Larger bulbs are spaced wider: C6 works around 60 per foot, C9 around 25 per foot on a tree. The calculator converts bulb count into strand counts using the strand sizes printed on the box, and always rounds strands up.
For rooflines and eaves, work from bulb spacing instead: standard residential C9 spacing is one bulb per linear foot (12-inch centers), the bright commercial look is one per 6 inches, and a minimal outline stretches to one per 18 inches. Measure every run you plan to light - both eaves, the rake edges, porch columns - and the calculator adds 10% slack for dips between clips and the drop down to the outlet.
LED vs incandescent: where the money actually is
Bulb type dominates the running cost. A 100-bulb incandescent mini strand draws about 40 W; the LED equivalent draws about 10 W. At 6 hours a day for 35 days, ten strands of minis cost roughly $12 in incandescent versus $3 in LED at $0.17/kWh - small money either way. The gap explodes with large C9 bulbs: incandescent C9s pull 7 W each, so a 100-bulb roofline costs about $21 a month, while the same run in LED (0.4 W each) costs about $1.40. If you display big C9s every year, LED pays for itself in one or two seasons.
Safety limits the calculator respects
The 210-watt rule printed on UL tags caps what you can daisy-chain per run: combined strand wattage must stay under 210 W. Incandescent minis (40 W) allow about 5 strands per run; a 25-bulb C9 incandescent strand (175 W) allows essentially one strand per run before you exceed it. LED strands draw so little that the limit becomes physical length and plug connections, not wattage. Split long runs across outlets, use a grounded timer, and keep connections off wet ground.
FAQ
- How many lights for a 6-foot vs 9-foot tree?
- At the classic 100-per-foot rule: about 600 bulbs (6 strands of 100) for 6 ft, and 900 bulbs (9 strands) for 9 ft. Slim or pencil trees take roughly 60% of those numbers; full Noble or Fraser firs take the full count or more.
- Do LED lights really last longer?
- Yes - LED sets are rated for 25,000-75,000 hours versus about 3,000 for incandescent, and they run cool, which reduces socket corrosion and fire risk. The one historical drawback (harsh blue-toned white) is solved by buying warm-white LED, so check the label rather than the shelf.
- What size bulb should I use where?
- Minis (and wide-angle LED minis) belong on trees, wreaths and garlands where density creates the effect. C6 works on small outdoor trees and bushes. C9 is the roofline and walkway bulb - visible from the street and easy to align one per foot. Mixing one bulb family per surface looks tidier than mixing everywhere.
- How do I measure my roofline without a ladder?
- Walk the house perimeter with a tape for the straight eave lengths, or pace it: an adult stride is roughly 2.5 ft. Count gutter sections separately, add porch and column wraps, then let the 10% slack cover measurement error. Most single-story homes need 100-200 ft of roofline lighting; most two-story homes 150-250 ft.
- Should I use a timer?
- Absolutely - the cost math scales linearly with hours, and nobody watches lights at 3 a.m. A $10 mechanical or smart timer set to 5-6 hours (dusk to bedtime) cuts the season cost nearly in half versus 12-hour manual operation, and it removes the daily on/off chore.
This tool provides general planning estimates, not electrical advice. Always follow the wattage limits printed on the UL tag of your specific strands, and never connect more strands than the manufacturer allows.