Calculators
Free, accurate cycle and period predictor
Ahrefs Domain Rating set in type 25 Oct 2026, reset 18 Oct 2026.
| Metric | Value |
|---|---|
| Domain Rating | 15 no change since last check |
| Trust Flow | 20 no change since last check |
| Citation Flow | 9 no change since last check |
| Referring domains | 77 no change since last check |
| External backlinks | 191 no change since last check |
Ahrefs Domain Rating set in type 25 Oct 2026, reset 18 Oct 2026. Lead topic: Health/Alternative (20). Majestic set 18 Oct 2026, reset 18 Oct 2026. Link data typeset by Majestic. Trust Flow, Citation Flow and Topical Trust Flow are trademarks of Majestic-12 Ltd.
| Domain age | 4 months |
|---|---|
| Registered | |
| Expires | |
| Registrar | Name.com, Inc. |
| DNSSEC | Not signed |
Free mobile-first calculators that predict your next 6 periods, ovulation day, and fertile window with no signup or data storage.
Period Tools is a suite of free, mobile-first calculators designed for personal cycle and fertility planning. The Period Calculator predicts your next six periods, ovulation day, and fertile window by using the standard luteal-phase method — adding your cycle length to your last period start date and estimating ovulation as 14 days before your predicted next period.
All calculators run entirely in your browser with no signup required and no data storage on servers. The site also offers a free period tracker for logging your cycle privately on your device, and an optional paid subscription (Period Tools Pro) that adds cloud sync and device backup for your tracker history.
The tools are designed around accuracy, mobile-first usability, and honest framing. For regular cycles, predictions typically land within one to two days about 75% of the time. The site emphasizes that all results are estimates for planning purposes only and are not substitutes for medical advice.
Pricing model: Freemium
All calculators and guides are free. Period Tools Pro (optional): $4.99/month or $19.99/year.
Enter the first day of your last period and your average cycle length. The calculator uses the standard luteal-phase method — it adds your cycle length to your last period start date to predict your next period, and estimates ovulation as 14 days before your predicted next period. It then projects six future cycles for planning.
For someone with regular cycles, predictions land within one to two days about 75% of the time. Accuracy depends on cycle regularity — if your cycle varies by more than 7–9 days month to month it is considered irregular and predictions become rougher estimates.
A typical cycle is 21 to 35 days, measured from the first day of one period to the first day of the next. The often-quoted 28-day cycle is an average, not a rule. Cycles shorter than 21 days or longer than 35 days are still within range for some people but are worth tracking.
The calculator uses the luteal-phase formula. The luteal phase — the time between ovulation and your next period — averages 14 days for most people, so predicted ovulation equals your next period start date minus 14 days. The fertile window is the five days leading up to ovulation plus ovulation day itself, because sperm can survive up to five days inside the body.
No. Everything runs in your browser. Your dates are never sent to a server, saved in a database, or shared with any third party. Refreshing the page clears your inputs. The only exception is if you use Period Tools Pro's cloud sync feature, which stores your optional tracker log on secure servers so your history can follow you across devices.
No review of Period Tools set in type yet. Set the first.
Reviews are free to set. None are paid for. Read the proofs
Readers' notes · Reprints · Share the page · Your company in print? Claim it · Report a misprint
Period Tools has a free listing. The owner can unlock the full profile with a paid launch for $5:
AstroCalcoloAscendant calculator for Italy and the world with IANA timezone conversion and historical rules
TDEE CalculatorFree online calculator to estimate your Total Daily Energy Expenditure and daily calorie needs
TidyUseFree calculators and printable tools for home, garden and everyday money questions
Instant answers. Zero reloads.
Can we set one analytics cookie to count which pages readers flip to? Nothing is set unless you say yes. Privacy