periodcalculator.ai

Cycle prediction, engineered · private by architecture

Period Calculator.

Built like an instrument.

Predict your next period, ovulation day, fertile window, and menstrual cycle phase — in seconds. Three inputs. One calibrated forecast.

28 days
28
5 days
5
Fine-tune the luteal phase
14 days
14

The luteal phase — ovulation to your next period — is steadier than the cycle itself. If you've confirmed yours with ovulation tests, set it here for a sharper estimate.

Tracking mode. The dial centers on your next period; the forecast below maps three cycles ahead.

01Next period
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02Ovulation
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—
03Fertile window
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—
04Today
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Three cycles, one glance.

Three calibrated cycles on one instrument. Berry marks bleeding days; the bracketed brass zone is the fertile window; the needle is estimated ovulation.

01 · Cycle science

Four phases, one continuous signal

A cycle isn't a date on a calendar — it's a hormonal sequence. Knowing which phase you're in explains energy, mood, skin, and symptoms better than any countdown.

Days 1–5 · typical

Menstrual

The cycle starts on the first day of full bleeding. Estrogen and progesterone are at their lowest — fatigue and cramping are most common here.

Ends at ovulation

Follicular

Rising estrogen rebuilds the uterine lining while follicles mature. Many people notice energy and mood climbing through this stretch.

~24 hours

Ovulation

A surge of luteinizing hormone releases the egg. The egg lives 12–24 hours, but the fertile window opens five days earlier — sperm wait longer than eggs do.

~14 days · steadiest

Luteal

Progesterone dominates as the body prepares for a possible pregnancy. Its length barely varies — which is exactly why this calculator anchors on it.

Hormones across a 28-day cycle
Estrogen LH surge Progesterone
ovulation Day 1 Day 8 Day 14 Day 21 Day 28

Idealized curves for a 28-day cycle with ovulation on day 14. Your dial above adjusts these landmarks to your actual cycle length.

02 · How it works

Honest math, stated plainly

Most period calculators hide their assumptions. Ours are simple enough to print — and we'd rather tell you the limits than pretend there aren't any.

A note on accuracy. In an analysis of more than 600,000 real cycles, only a small minority were exactly 28 days, and cycle length commonly varied month to month. That's exactly why the dial calibrates to your averages — predictions sharpen as you enter real tracking data.

Next period = last period + cycle length

A cycle is counted from the first day of bleeding to the day before the next bleed begins. We project forward three cycles, not one.

Ovulation = next period − luteal phase

We count back from the projected period, because the luteal phase is far steadier than the cycle. Default is 14 days; you can override it.

Fertile window = 5 days before ovulation + ovulation day

Sperm can survive up to five days; the egg lives about one. Conception is possible across that six-day span — likeliest in the final two days.

Everything runs on this page

No account, no upload, no tracking pixel on your inputs. Close the tab and your numbers are gone.

Reference notes

REF · 01

What is a period calculator?

A period calculator predicts the start date of your next menstrual period by adding your average cycle length to the first day of your last period. This one also maps ovulation, the fertile window, and your current cycle phase on a calibrated dial.

REF · 02

How is ovulation calculated?

Ovulation is estimated by counting back the luteal phase — typically 14 days — from the projected next period. The luteal phase is the steadiest part of the cycle, which makes counting backward more reliable than counting forward.

REF · 03

What is the fertile window?

The six days ending on ovulation day: the five days sperm can survive, plus the roughly 24 hours the egg remains viable. Conception is most likely in the final two days of the window.

REF · 04

How accurate is a period calculator?

With regular cycles, predictions typically land within a day or two. In research covering more than 600,000 real cycles, only a minority ran exactly 28 days — which is why the calculator calibrates to your own averages. With irregular cycles, read the dates as a window.

REF · 05

Why is my period late?

The most common reasons are a cycle where ovulation simply happened later — shifted by stress, illness, travel, or weight change — rather than pregnancy. If your period is more than a week late, take a pregnancy test; if cycles go missing for 90 days, see a clinician.

REF · 06

What counts as an irregular cycle?

Cycles consistently shorter than 21 days or longer than 35, or that swing more than about 7–9 days month to month. Occasional variation is normal; a persistent pattern outside that range is worth raising with a clinician.

Why trust this

Built to clinical-review standards, not content-farm standards

Every claim on this site traces to peer-reviewed research or guidance from bodies like ACOG and the NIH. Reproductive health is YMYL territory — we treat it that way.

Medical review in progressReviewer byline publishes with our named board-certified OB-GYN at launch.
Private by architectureEvery reading is computed on this page, in your browser. There is nothing to leak because nothing is sent.
Cited, not assertedMethodology references ACOG patient guidance and large-scale cycle datasets published in peer-reviewed journals.
Honest about limitsWe show ranges and say "estimate" where it's an estimate — including that this is not contraception.

03 · The instrument family

One question per tool, answered properly

Six instruments, one engine — the same math, the same design system, the same review standard.

All instruments
Nº 01FertilityOvulation CalculatorPinpoint your estimated ovulation day across the next six cycles.Open Nº 02FertilityFertile Window CalculatorYour six best days, ranked by conception probability.Open Nº 03PregnancyDue Date CalculatorEstimate your due date from LMP, conception, or IVF transfer.Open Nº 04TrackingPeriod TrackerLog real cycles and let your averages sharpen every prediction.Open Nº 05PregnancyImplantation CalculatorThe likely implantation window after ovulation or transfer.Open Nº 06TrackingMenstrual Cycle CalculatorMap every phase of your cycle, not just the bleeding days.Open

04 · Questions, answered straight

Notes from the clinic

Private by architecture. Every calculation runs in your browser — nothing is stored or transmitted.

Counted back, not forward. Ovulation is estimated from your cycle length and luteal phase.

Calibrated to you. The dial tunes to your real cycle averages — not the textbook 28.

01When do I ovulate?
Definition

Typically about 14 days before your next period starts — not 14 days after your last one began.

How it works

Subtract your luteal phase (default 14 days, adjustable on the dial) from your projected next period. On a 21-day cycle that's around day 7; on a 28-day cycle, day 14; on a 35-day cycle, day 21.

Why it matters

Signs like fertile cervical mucus or a positive LH test can confirm the calendar estimate; a shifted month moves ovulation with it.

Key takeaway

Your ovulation day moves with your cycle length — the dial recalibrates it automatically.

02How many days after my period can I get pregnant?
Definition

Pregnancy is possible during the fertile window: the five days before ovulation plus ovulation day itself.

How it works

Sperm can survive up to five days inside the body; the egg lives roughly one day. On shorter cycles, ovulation arrives early — the fertile window can open within a few days of bleeding ending.

Why it matters

The idea that the days right after a period are automatically “safe” fails exactly when cycles run short or irregular.

Key takeaway

With short cycles, fertility can begin almost as soon as the period ends.

03Why do you count back from my next period to find ovulation?
Definition

Ovulation is estimated as the projected next period minus the luteal phase — counting backward, not forward.

How it works

The luteal phase — ovulation to the next period — holds steady near 14 days for most people, while the first half of the cycle absorbs nearly all the variation. Counting back from the projected period anchors the estimate to the stable half.

Why it matters

The popular “day 14” rule silently assumes a 28-day cycle. On a 33-day cycle, ovulation lands near day 19 — counting forward to day 14 misses the entire fertile window.

Key takeaway

Ovulation tracks your next period, not your last one.

04How accurate is a period calculator?
Definition

With regular cycles, a period calculator typically predicts within a day or two — accuracy scales with how regular your cycles are.

How it works

The math assumes your next cycle behaves like your average cycle. In research covering more than 600,000 real cycles, only a minority ran exactly 28 days, and length commonly varied month to month.

Why it matters

With irregular cycles, read each date as the center of a window — a window that narrows as you enter your real averages instead of the defaults.

Key takeaway

Predictions sharpen when you enter your own averages from real tracking — not the textbook 28.

05My cycles are irregular. Can I still use this?

How it works

Yes — but read ranges, not dates. Enter your best average and treat every projection as the center of a window that widens with your variation. Tracking three to six real cycles tightens the inputs considerably.

Why it matters

Cycles consistently shorter than 21 days, longer than 35, or swinging more than about 7–9 days month to month are worth raising with a clinician — calendar math alone can't see the cause.

Key takeaway

Irregular cycles need a window, a tracking habit, and sometimes a clinician — in that order.

06When is the best time to take a pregnancy test?
Definition

From the first day of your missed period — earlier tests trade accuracy for speed.

How it works

Home tests detect hCG, which rises after implantation. Testing from the day your period was due gives high reliability; testing days earlier can return false negatives even when pregnant.

Key takeaway

If the dial says your period is overdue and the test is negative, retest in two to three days.

07How long does a period last?
Definition

Two to seven days is the typical range; around five is average.

Why it matters

Bleeding that regularly runs past seven to eight days, or heavy enough to soak through protection hourly, is a pattern worth bringing to a clinician rather than tracking quietly.

Key takeaway

The dial's period-length input accepts 2–10 days — set it to your real average, not the default.

08What is a normal menstrual cycle length?
Definition

For adults, 21 to 35 days counts as typical; for adolescents, up to 45. Exactly 28 is the textbook case, not the norm.

Why it matters

Large-scale cycle data shows most people don't run 28 days, and many vary month to month. The calculator's range is 21–40 because that's where calendar math stays meaningful.

Key takeaway

Normal is a range — calibrate the instrument to yours.

09Can I rely on this to avoid pregnancy?
Definition

No. A calendar calculator is an awareness tool, not contraception.

Why it matters

Calendar-based methods have markedly higher real-world failure rates than modern contraception, because sperm survival and ovulation timing both vary. The avoid-pregnancy mode on the dial shows estimated risk only.

Key takeaway

For preventing pregnancy, talk to a clinician about reliable contraception — use this instrument for awareness.

10Is anything I enter saved?
Definition

No. Every calculation runs in your browser; nothing is transmitted, stored, or tracked.

Why it matters

Reproductive health data is sensitive, and several popular cycle apps have faced regulatory action over sharing it. This page is private by architecture: close the tab and your numbers are gone.

Key takeaway

Privacy here isn't a policy — it's the absence of a server.

05 · Clinical thresholds

When a calculator isn't the right instrument

Prediction is for planning — not diagnosis. These six readings are outside calendar math. Any of them is a reason to talk to a clinician:

T·01

No period for 90+ days and you're not pregnant

T·02

Cycles consistently shorter than 21 or longer than 35 days

T·03

Bleeding that soaks through protection hourly

T·04

Periods lasting longer than 7 days

T·05

Severe pain that disrupts daily life

T·06

Bleeding between periods or after intercourse

06 · The instrument

Your cycle, on the dial.

You've read the science. The instrument is calibrated and waiting — three inputs, ten seconds, nothing stored.

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