Period Calculator

Predict your upcoming period dates from your last period, cycle length and period length. Free, instant, no signup.

First day of last period

Cycle length (days)

Period length (days)

How to use the Period Calculator

Enter your values. Fill in the fields with your numbers.
Calculate. Press Calculate to run the period calculator.
Use the result. Copy the result or try a related tool next.

Why use our Period Calculator

Instant results. Enter your figures and the period calculator returns an answer in seconds.

Free & private. Runs in your browser — no signup, and nothing is sent to a server.

Accurate. Uses standard formulas so you can rely on the numbers.

Free to use — premium coming soon

FREE

✓ Unlimited calculations
✓ Instant results
✓ No signup

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★ Save & compare scenarios
★ Export results

About the Period Calculator

The Period Calculator estimates when your next menstrual period is likely to start, along with your approximate ovulation date and fertile window, based on two simple inputs: the first day of your last period and your usual cycle length. A cycle is counted from the first day of one period to the first day of the next, so the math is straightforward: it adds your cycle length to your last start date to project the next one, then repeats that to forecast several months ahead. It is a quick planning aid, not a medical diagnosis.

People reach for a period calculator for many practical reasons. Some want a heads-up so they are never caught without supplies before a trip, event, or exam. Others are trying to conceive and want to know their fertile days, or are timing things to avoid an overlap with a wedding, holiday, or competition. It is also a low-effort way to start understanding your own pattern before committing to a full tracking app, and a handy reference when a clinician asks for your last period date or cycle length.

Under the hood, the tool uses the calendar (rhythm) method. The next period is calculated as last period start plus cycle length. Ovulation is estimated by working back from the projected next period: because the luteal phase (ovulation to the next period) is fairly consistent at around 14 days, ovulation is placed roughly 14 days before your next expected period. The fertile window is then the six days ending on ovulation day, since sperm can survive several days and the egg lives about a day. A 28-day cycle puts ovulation near day 14; a 35-day cycle pushes it closer to day 21.

Accuracy depends entirely on how regular your cycles are. For cycles in the typical 21 to 35 day range that stay consistent, calendar predictions are reasonably close. If your cycles vary widely, or you have a condition such as PCOS, are postpartum, perimenopausal, or recently stopped hormonal birth control, the estimates can be well off and should not be relied on for contraception or conception decisions. On privacy: this calculator runs entirely in your browser, so the dates you enter are never sent to a server, stored, or shared, and nothing is saved when you close the page.

Frequently asked questions

How does the period calculator predict my next period?

It adds your usual cycle length to the first day of your last period. For example, a last period starting June 1 with a 28-day cycle gives a predicted next start of June 29. It then repeats this to project the following months.

How does it estimate ovulation and my fertile window?

Ovulation is placed about 14 days before your next expected period, because the luteal phase is fairly consistent at roughly 14 days. The fertile window is the six days ending on ovulation day, reflecting that sperm can survive a few days and the egg about one.

What if my cycles are irregular?

Calendar-based prediction works best for regular cycles in the 21 to 35 day range. If your cycle length varies a lot, the dates are only rough estimates, so use a midrange or average cycle length and treat the result as a guide rather than a fixed date.

Is this calculator accurate enough to use as birth control?

No. The calendar method alone is not a reliable form of contraception, and the fertile window shown is an estimate. If you are avoiding pregnancy, use a proven contraceptive method and talk to a healthcare provider.

Are the dates I enter saved or shared?

No. The calculation happens locally in your browser, so your dates are not uploaded, stored, or shared, and nothing is retained after you leave the page. Re-enter your details each time you visit.

From our blog

How to Estimate Your Website's Ad Revenue Before You Build It

By the Super Simple Digital Tools Team · Updated June 2026

Most people discover ad revenue math the hard way, after months of writing, when the first payout lands far below what they imagined. A few minutes with an ad revenue calculator up front can prevent that disappointment by translating a traffic dream into a realistic dollar figure. The only honest starting question is: how many pageviews can you reasonably reach, and what is a typical earning rate for your topic? Get those two numbers roughly right and the rest is arithmetic.

The cleanest mental model is RPM, or revenue per 1,000 pageviews. RPM already bundles together how many ad slots you run, how often they fill, and the commission your network keeps, which is why it is the number publishers quote to each other. The formula is just earnings equals pageviews times RPM divided by 1,000. A site doing 100,000 monthly pageviews at a $5 RPM earns about $500 a month. Double the RPM by moving into a higher-value niche and you double the income without adding a single visitor.

CPM works from the advertiser's side and is useful when you are quoted a rate per 1,000 impressions rather than per pageview. Here you must account for how many ad units sit on each page, because impressions equal pageviews times ads-per-page. Three ad units at a $5 CPM behave like a $15 page RPM. That multiplier is tempting, but stacking ad units hurts page speed and viewability, and low-viewability impressions often pay little, so more ads rarely means proportionally more money.

Niche choice is the single biggest lever the calculator will reveal. Lifestyle, recipes, and entertainment commonly land around $1-$3 RPM, mid-tier tech and how-to content often reaches $5-$15, and finance, health, real estate, and insurance can climb well beyond that, especially with audiences in the United States, United Kingdom, Canada, or Australia. Plug in two different RPMs for the same traffic and the gap is usually larger than any traffic increase you could realistically achieve in a year.

Finally, remember what the number cannot capture. Ad rates rise in Q4 and dip in January, fill rates vary, ad blockers shave off a slice of impressions, and your network takes its cut before you see a cent. Use the calculator to compare scenarios, set a target, and decide whether a project is worth your time, then update your inputs with real data from your dashboard once traffic starts flowing. The estimate is a compass, not a contract.

Start with RPM rather than CPM when you can, because it already includes ad count and the network's cut, leaving fewer ways to overestimate.
Run the numbers twice, once with a conservative RPM and once optimistic, to see your realistic earnings range instead of a single hopeful figure.
When modeling CPM, keep ads-per-page modest; piling on units inflates the estimate but real viewability and page-speed penalties claw most of it back.
Adjust your RPM upward for Tier-1 (US, UK, Canada, Australia) traffic and downward for broad global audiences, since visitor location heavily drives ad rates.

Read the full guide →

Tool by the Super Simple Digital Tools Team. Reviewed by our editorial team. Free to use, no signup required.