Estimate the fertile window, ovulation date and next period from the last period date and usual cycle length, using the luteal phase method.
Ovulation Calculator runs in the page as you use it. There is no input to submit and no result to fetch, because the whole thing happens on this side of the wire.
Paste a customer record into it if you need to. The record never leaves the machine you are sitting at.
You will find it under Health, alongside 18 other tools for the same sort of job.
Runs in this tab, so nothing you type is uploaded.
lastPeriod: 2026-03-01 cycleLengthDays: 28
Ovulation date: 2026-03-15 Fertile window start: 2026-03-10 Fertile window end: 2026-03-16 Next period date: 2026-03-29
Yes. It is JavaScript running in this tab, so once the page has loaded it needs nothing further from the network. Put the machine in flight mode and it carries on. Nothing you do here is sent anywhere, because there is nowhere for it to go.
No. Reload the page and it starts from nothing. There is no account, no saved state and no cookie holding your last session. That is a deliberate trade: it cannot lose your data because it never keeps any.
Timing comes from performance.now(), the high resolution clock the browser provides, rather than from setInterval, which drifts by a few milliseconds every tick and compounds over minutes. What can still shift things is the browser throttling a tab in the background, so leave it visible if the timing matters.
Where a sound makes sense, it is generated with the Web Audio API rather than loaded as a file, so there is nothing to download and nothing to block. Browsers refuse to start audio until you have interacted with the page, which is why the first press is what switches it on.
Yes. The controls are sized for touch and the layout collapses to one column on a narrow screen. Bear in mind that a phone locking its screen will suspend the page, and timing stops with it.