Sleep Calculator

The alarm is set for tomorrow and now you're doing the sum in your head: the hours you want, plus the time it takes you to drift off, counted backwards from the wake-up time. Sleep Calculator does that subtraction for whatever numbers you choose. Its own worked example is a 7 AM alarm, 8 hours of sleep and 15 minutes to fall asleep, which means planning for bed at 10:45 PM the previous evening.

You start from a wake-up time, a bedtime or Sleep now. The sleep target can be 7, 7.5, 8, 8.5 or 9 hours, or you can plan by cycle estimate. Time to fall asleep runs from 0 to 30 minutes, or a custom figure. The result shows bedtime, estimated sleep onset and wake-up time.

Dates stay attached to results, so a bedtime that falls the evening before is labelled that way. The calculator reads your device's time zone, and there's a setting for the clock time that repeats when daylight saving ends, which is a nice touch. It can ask you, or take the earlier or later occurrence.

The cycle mode uses an assumed cycle length of 80 to 100 minutes, 90 by default. The about page is clear that cycles are estimates and don't predict REM, deep sleep or the best instant to wake.

It's free and there's no account. The site states that it isn't a healthcare provider, a sleep laboratory or a medical device, and it claims no medical review. Its content was prepared with AI assistance and checked against cited sources, by its own account.

According to the privacy policy, effective November 11, 2026, your times, dates and other inputs are used locally and aren't sent to a calculation API. Results aren't saved, and reloading resets the form. Visits are measured with PVUV.ai and Ahrefs Web Analytics, and Cloudflare hosts the site.

No person or company is named, and by the site's own admission its support channel isn't active yet. An iPhone app is described as in development and isn't on the App Store. If all you want is a sensible bedtime for tomorrow's alarm, the web page is a handy one to keep open.