About Period Tools
A small independent team building tools we want to use ourselves — clean, accurate, and respectful of your time.
Why we built it
We kept opening Google to answer the same simple questions about our cycles — when’s my next period, am I in my fertile window, how late am I — and getting served clunky brand calculators that wouldn’t work on a phone, blog posts that buried the answer under 2,000 words of stock-photo filler, or apps that wanted our data first.
We wanted a place that just worked. So we made one. Open the page, pick a date, see your answer. No signup, no login, no tracking pixel sending your cycle dates to ad networks. Everything runs in your browser. We don’t see your dates and we can’t — there’s nowhere on our server for them to go.
What we’re careful about
- Accuracy. The math behind every calculator on this site is the same standard method your healthcare provider uses — Naegele’s rule for due dates, the luteal-phase ovulation estimate, ABO/Rh genetics for blood type (the Chinese gender chart is clearly labelled fun-only). It’s tested.
- Mobile first. Most people search these terms on a phone. Every screen, every input, every output is designed for a small screen first.
- Honest framing. A calculator is an estimate. We say so. The folklore tools are clearly labelled as folklore. Anything that could mislead about a real health decision gets a disclaimer.
- No dark patterns. No fake countdown timers, no fake low-stock warnings, no aggressive pop-ups, no signup walls in front of free tools.
How we keep the lights on
Period Tools is funded mainly by Period Tools Pro, an optional subscription ($4.99/month or $19.99/year) that backs up and syncs your private cycle tracker across devices. Every calculator stays free whether or not you subscribe. Right now the site runs no ads at all; if we add them to the free tier later to help cover costs, Pro will always stay ad-free. In the future we may also add a small number of affiliate links to products we genuinely think are helpful — and if we do, we’ll always say so clearly and never let a sponsor or advertiser decide our content.
What we don’t do: sell your cycle data, share it with advertisers, or use it for anything but showing you your own results. The calculators are client-side — those inputs never leave your browser — and the free tracker stores its log on your device. The only cycle data that ever reaches our servers is the optional Pro cloud-sync log (so your history can follow you across devices), and you can delete it at any time. Details in our privacy policy.
Who’s behind Period Tools?
A small, independent team that builds health-adjacent calculators and clear, carefully-researched guides. Every page is a shared effort — researched, written, and checked by more than one of us, and kept current long after it first goes up. Questions or corrections reach a real person on the team; just email us.
If you have a question, a correction, or just want to tell us something we got wrong, we genuinely want to hear it. Reach out via our contact page.