Measuring from the ground is fine: pace the front of the house and count roof peaks twice for the slope.
How the calculations work
Tree lights use bulbs per foot of tree height: 50 for a subtle look, 100 for the classic look most stores recommend and 150 for a bright, dense tree. The strand count rounds up to whole strands. Garland is estimated as about three times the height of the tree, which suits a loose spiral.
Outdoor strands add up every run you measured, add ten percent for corners, peaks and the drop to the outlet, and divide by the strand length. Circuit load multiplies strands by watts per strand, converts to amps at 120 volts and compares it with 80 percent of the breaker rating, the usual limit for loads that run for hours. Running cost multiplies the total watts by the hours and days and your electricity price.
Safety reminders
Always follow the maximum number of sets that can be connected end-to-end, printed on the box or tag. Use outdoor-rated cords and GFCI-protected outlets outside, keep connections off the ground and out of puddles, and remember that other appliances on the same circuit count towards the load. When in doubt, ask a licensed electrician.
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Frequently asked questions
How do I measure my roofline?
Measure the width of the house for straight gutters; for each peak, measure one slope and double it.
Why add 10 percent?
Corners, gaps and the run down to an outlet always use more length than the straight measurement.
C9 bulbs or mini lights for rooflines?
C9 and C7 bulbs read better from the street on rooflines; minis suit bushes, railings and windows.
Is HowManyLights free?
Yes. All four calculators are free with no sign-up.
Are these numbers exact?
They are planning estimates. Check the wattage and end-to-end limits printed on your lights, and ask an electrician if unsure.