How to calculate your next period date

Q: Does cycle length change with age?

Yes. In the first years after a first period, cycles of 21–45 days are normal for teens (ACOG); adults settle into 24–38 days (FIGO) and are most regular in the late 20s and 30s; perimenopause makes cycles irregular again. Recalculate your average when life stages change. Still waiting for a first period? Our first period quiz estimates when it might arrive from your development signs.

Q: My cycle length changes every month — how do I calculate?

Use your last 3+ cycles and predict a range , not a day: earliest = last period + shortest recent cycle, latest = last period + longest. A swing under about 7–9 days across a year still counts as regular (FIGO). Swings beyond that mean calendar math will keep disappointing you — track for a few months and consider evaluation; irregular cycles affect an estimated 14–25% of women (NICHD), and PCOS alone affects 6–13%, most undiagnosed (WHO).

Q: How do I calculate after a miscarriage, abortion, or stopping birth control?

The calendar method restarts from your first real period. Bleeding on hormonal birth control is withdrawal bleeding, so the formula doesn't apply while on it; after stopping, cycles can take weeks to months to re-establish. After an early pregnancy loss the first period commonly returns within about 4–6 weeks , sometimes up to 8 (Tommy's; NHS trust patient guidance) — use that first real period as your new day 1 (more in our guide to the first period after a miscarriage ). No period 90 days after = secondary amenorrhea, see a clinician.

Predicting your next period is simple arithmetic once you know two numbers. Here is the exact method, step by step, with examples and the caveats that matter.

Written by Mira Chen, founder and editorPublished 2026-08-19Updated 2026-10-06

Key takeaways

Next period ≈ the first day of your last period + your average cycle length. If your recent cycles vary, give yourself a range instead of a day: last start + your shortest recent cycle, to last start + your longest. Day 1 is the first day of full flow, not spotting (NICHD). A typical adult cycle runs 24–38 days (FIGO); 28 is only an average.

One datelast day 1 + average cycle length

A rangelast day 1 + shortest cycle → last day 1 + longest cycle

Ovulation (estimate)predicted next period − 14 days

Step 1 — Find day 1 of your last period

Everything hangs on this date. Day 1 is the first day of real menstrual flow; light spotting a day or two earlier does not count (NICHD).

Aug 29spotting · not day 1Aug 30full flow · day 1Aug 31day 2Sep 1day 3

Demo. The date you write down is the first day of real flow. A day or two of spotting before it belongs to the previous cycle.

No records yet? Start fresh: write down the first day of your next period. The same care applies at the other end — what counts as the last day decides your period length. Our guide to how to know when your period is ending defines it.

Step 2 — Work out your average cycle length

Count from day 1 of one period to day 1 of the next, for two or three cycles, and average the gaps — one cycle alone can mislead. No history at all? Start with 28 and replace it once real dates arrive.

Demo: four recorded start dates. Subtract each start date from the next one — that difference is the cycle length; don’t add 1.
From	To	Cycle length
Jun 1	Jun 29	28 days
Jun 29	Jul 29	30 days
Jul 29	Aug 30	32 days
Average (28 + 30 + 32) ÷ 3		30 days

The cycle length calculator does these subtractions from your past start dates, and the period calculator learns your average from the periods you log.

Step 3 — Add cycle length to day 1

Add the average to the last day 1 for a single date. Add your shortest and longest recent cycles for a range. With the demo dates above:

Demo — last period started Aug 30; recent cycles 28, 30 and 32 days.

~~Sep 26~~2729~~Oct 1Oct 2~~

range: Sep 27 – Oct 1 (Aug 30 + 28 … + 32)single date: Sep 29 (Aug 30 + 30)

OvulationCount back 14 days from the predicted period

Demo: Sep 29 − 14 = Sep 15. The fertile window is the 5 days before plus that day: Sep 10–15. It inherits every day of uncertainty in the period date.

Cycles that varyPlan with the range, not the middle

Have supplies ready from the earliest date; only call the period “late” after the latest one.

Every new periodRecalculate from the new day 1

Drop the oldest gap and add the newest, so the average follows how your cycles run now.

How accurate should you expect this to be?

With steady cycles the single date is usually close, but it stays an estimate: real cycles move by a few days, and ovulation moves with them.

2.6 days

average spread (one standard deviation) of a person’s own cycle lengths

13%

of cycles were exactly 28 days long

10–15 days

typical luteal phase — why ovulation is an estimate

Bull et al., npj Digital Medicine 2019: 612,613 ovulatory cycles from 124,646 app users (people with PCOS excluded; funded by the app’s maker). These describe groups of cycles. They explain why a single date is often off by a couple of days — they are not a forecast for you.

Stress, illness, travel, weight change, new medication and perimenopause can all shift a cycle after you have calculated it. As the date gets close, your body offers its own cross-check — see the signs your period is coming tomorrow.

When the formula needs a new day 1

Some bleeds aren’t a natural cycle, and some events restart the count. Here is where day 1 moves:

When the usual formula needs a new day 1. Each link goes to the guide for that situation.
Situation	Does the formula apply?	Your next day 1
On the combined pill, patch or ring	No — bleeds in the break are withdrawal bleeds set by the pack, not a cycle.	Not used while you’re on it.
After stopping hormonal contraception	Yes, once natural cycles return — that can take weeks to months.	The first natural period after stopping.
After a miscarriage	Yes, from the first real period.	The first period after the loss — it can take up to 8 weeks (NHS). First period after a miscarriage
After giving birth	Yes, from the first real period — not lochia.	Timing depends mostly on feeding. First period after birth
After emergency contraception	Roughly — the next period can come earlier or up to about a week later (Mayo Clinic).	The next period you get. No period within 3 weeks of taking it: take a pregnancy test. Why is my period late?

When to involve a clinician

Worth a clinician’s visit:

Cycles repeatedly shorter than 24 or longer than 38 days (FIGO)
Your shortest and longest cycle in a year differ by more than 7–9 days (FIGO)
You’ve missed your period 3 times in a row (NHS), or your periods have become irregular
Bleeding longer than 8 days, soaking a pad or tampon every hour for several hours (ACOG), or severe pain

Bring your recorded start dates — they turn “my periods are odd” into something a clinician can read.

Frequently asked questions

Can I calculate my next period without an app?

Yes — the calendar method needs only your last period's first day and your average cycle length. A paper calendar works; our calculator just automates it and projects months ahead.

Does cycle length change with age?

Yes. In the first years after a first period, cycles of 21–45 days are normal for teens (ACOG); adults settle into 24–38 days (FIGO) and are most regular in the late 20s and 30s; perimenopause makes cycles irregular again. Recalculate your average when life stages change. Still waiting for a first period? Our first period quiz estimates when it might arrive from your development signs.

What if I only remember roughly when my period started?

Use your best estimate — a day or two of uncertainty simply widens real-world accuracy. Then log the next two or three periods precisely to lock in your pattern.

I got two periods in one month — is something wrong?

Usually not. A calendar month is 30–31 days, so any cycle shorter than that will occasionally put two period starts in the same month. What matters is cycle length, not the calendar: 24–38 days between day 1s is the normal adult range (FIGO). If your cycles are consistently shorter than that, mention it to a clinician.

My cycle length changes every month — how do I calculate?

Use your last 3+ cycles and predict a range, not a day: earliest = last period + shortest recent cycle, latest = last period + longest. A swing under about 7–9 days across a year still counts as regular (FIGO). Swings beyond that mean calendar math will keep disappointing you — track for a few months and consider evaluation; irregular cycles affect an estimated 14–25% of women (NICHD), and PCOS alone affects 6–13%, most undiagnosed (WHO).

Does day 1 count as the first day, or do I start counting the day after?

Day 1 counts. The first day of full flow is cycle day 1, and you count inclusively up to the day before the next period starts. Next expected period = day 1 date + your cycle length.

How do I calculate after a miscarriage, abortion, or stopping birth control?

The calendar method restarts from your first real period. Bleeding on hormonal birth control is withdrawal bleeding, so the formula doesn't apply while on it; after stopping, cycles can take weeks to months to re-establish. After an early pregnancy loss the first period commonly returns within about 4–6 weeks, sometimes up to 8 (Tommy's; NHS trust patient guidance) — use that first real period as your new day 1 (more in our guide to the first period after a miscarriage). No period 90 days after = secondary amenorrhea, see a clinician.

References

This tool provides estimates for personal planning only and is not a substitute for professional medical advice, diagnosis or treatment. Predictions must not be used as contraception. If you have concerns about your cycle, heavy bleeding, severe pain or missed periods, please consult a licensed clinician.