Period Calculator — Predict Your Next Period & Track Your Cycle

Knowing when your next period is coming helps you plan everything from vacations to medical appointments. A period calculator uses your cycle history to predict future dates — no app required, just your last period date and average cycle length.

The CalcHub Period Calculator predicts your next period, fertile window, and ovulation date.

How to Calculate Your Next Period

Simple formula: Next period = First day of last period + Average cycle length

A "normal" menstrual cycle is 21–35 days (average: 28 days). Your cycle length is counted from the first day of one period to the first day of the next.

Period Prediction Table

Last Period Started	Cycle Length	Next Period Expected	Ovulation (Approx.)
March 1	28 days	March 29	March 15
March 1	30 days	March 31	March 17
March 1	26 days	March 27	March 13
March 1	32 days	April 2	March 19
March 1	35 days	April 5	March 22

Ovulation typically occurs 14 days before the next period (not 14 days after the last one).

Understanding Your Cycle Phases

Phase	Days (28-day cycle)	What Happens
Menstruation	Days 1–5	Uterine lining sheds; bleeding occurs
Follicular phase	Days 1–13	Egg develops in ovary; estrogen rises
Ovulation	Day 14 (approx.)	Egg is released; most fertile day
Luteal phase	Days 15–28	Progesterone rises; uterus prepares for implantation
PMS window	Days 22–28	Mood changes, bloating, breast tenderness (if no pregnancy)

Tracking Your Cycle Length

To find your average cycle length, track for at least 3 cycles:

Cycle	Period Start Date	Period Start (Next)	Cycle Length
1	Jan 5	Feb 2	28 days
2	Feb 2	Mar 3	29 days
3	Mar 3	Mar 31	28 days
Average	28.3 days ≈ 28 days

What's "Normal" and What's Not

Aspect	Normal Range	When to See a Doctor
Cycle length	21–35 days	Consistently shorter than 21 or longer than 35
Period duration	2–7 days	Longer than 7 days regularly
Flow	Light to moderate-heavy	Soaking through a pad/tampon every hour
Pain	Mild-moderate cramps	Pain so severe it prevents normal activity
Cycle variation	±1–5 days month to month	Variation of 8+ days between cycles
Missed periods	Occasional (stress, illness)	3+ consecutive missed periods (if not pregnant)

Irregular Cycles — What Causes Them

Cause	How It Affects Your Cycle
Stress	Can delay ovulation, making the cycle longer
Weight changes	Rapid gain or loss can disrupt hormones
PCOS	Often causes irregular, infrequent, or heavy periods
Thyroid disorders	Both hypo and hyper can alter cycle length
Birth control	Hormonal methods can shorten, lengthen, or stop periods
Perimenopause	Cycles become irregular before menopause (typically 45–55)
Excessive exercise	Can delay or stop periods in athletes
Breastfeeding	Periods may not return for months

If your cycles are consistently irregular, a period calculator becomes less accurate. Consider tracking additional signs (cervical mucus, basal body temperature) or using ovulation test strips.

PMS and Symptom Timing

PMS typically occurs 1–2 weeks before your period (during the luteal phase):

Symptom	When It Usually Starts	When It Ends
Mood changes / irritability	7–10 days before period	Day 1–2 of period
Bloating	5–7 days before	Day 1–2 of period
Breast tenderness	7–14 days before	Day 1–2 of period
Acne	7–10 days before	During period
Cramps	Day 1–2 of period	Day 2–3
Food cravings	7–14 days before	Day 1–2 of period

How to Use the Calculator

1. Open the CalcHub Period Calculator
2. Enter the first day of your last period
3. Enter your average cycle length (default: 28 days)
4. See: next period date, fertile window, ovulation date, and PMS window

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Why is my period late?

The most common reason (besides pregnancy) is stress — cortisol can delay ovulation, which delays your period. Other causes: illness, travel, weight changes, medication changes, and hormonal fluctuations. A period that's 1–5 days late is usually normal.

Can I predict my period 6 months ahead?

For regular cycles (variation ≤ 3 days), predictions 3–4 months ahead are fairly reliable. Beyond that, accuracy decreases. For irregular cycles, even next month's prediction may be off. Track consistently for best results.

Does the period calculator work if I'm on birth control?

Hormonal birth control (pills, patch, ring) creates an artificial cycle. If you take a 7-day break/placebo, your withdrawal bleed is predictable (usually starting day 2–3 of the break). The calculator works for tracking this pattern, but it's not predicting natural ovulation.

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Related Calculators

• Ovulation Calculator — find your fertile window
• Pregnancy Due Date Calculator — if you're expecting
• Age Calculator — exact age in years, months, days
• BMI Calculator — weight can affect cycle regularity