I do not have children. I have Waffles, who naps on the radiator for most of the afternoon and is still wide awake at 05:40 the next morning. People kept picking "under 13" on the adult sleep calculator, and that calculator counts adult sleep cycles, which is the wrong arithmetic for a child. This page counts hours instead.
Pick the age, the wake-up time and the day's naps added up. You get a window to be asleep in: no earlier than the first time, no later than the second.
How the math works
The hours come from one place: the American Academy of Sleep Medicine consensus statement on sleep for children (Paruthi et al., Journal of Clinical Sleep Medicine, 2016), which the American Academy of Pediatrics endorsed. It gives a range per age, and for every age up to 5 that range is per 24 hours including naps.
So the calculator takes the naps off first. A 4 to 12 month old needs 12 to 16 hours. If today's naps add up to three hours, the night has to carry 9 to 13 hours. With a 6:30 a.m. wake-up, 13 hours back is 5:30 p.m. and 9 hours back is 9:30 p.m. That is the window.
For 6 to 12 year olds and teenagers the ranges are per 24 hours too, but most of them do not nap. If you leave the naps empty for those ages, the calculator counts none and says so in the result.
There is no fall-asleep time added and no cycle counting. The window is when to be asleep, so lights out goes a little before its start.
Why a window and not one time
Every range in the statement is two to four hours wide, and picking one time inside it would mean inventing a number the panel did not give. A school night with an early bus sits near the late end of the night, and a weekend after a busy day can sit near the early end. Both are inside.
For teenagers the window often lands later than a family would like. A 13 to 18 year old who has to be up at 6:30 a.m. needs to be asleep by 10:30 p.m. to get the bottom of 8 to 10 hours. That is arithmetic, not a judgment about anyone's teenager.
Under 2, naps first
Under 2 the calculator will not give a bedtime until you tell it about the naps. At that age the naps are a large share of the day, and a bedtime that ignores them could be hours out in either direction. Pick the total, or pick No nap. It is one extra field, and it is the field that makes the answer yours.
Under 4 months there is no range to give. The statement leaves newborns out because their sleep varies too widely, and I am not going to fill that gap with a number of my own.
When to trust it and when not to
Trust it for timing. If your child is fine during the day and you just want to know whether 8:00 p.m. or 9:00 p.m. makes more sense after a long nap, this is the arithmetic.
It is not a tool for sleep problems. If your child snores most nights, stops breathing or gasps in their sleep, cannot settle night after night, or is exhausted during the day despite enough hours, take that to your pediatrician. I am not a doctor, and a calculator cannot see any of that.
For the grown-ups in the house, the sleep calculator counts adult cycles, and the nap calculator times your own nap.
Here is the step for tonight. Add up today's naps, put in tomorrow's wake-up, and aim to have the lights off a little before the window opens.