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Period Calculator | Predict Next Period Dates and Ovulation

Enter the first day of your last period and your average cycle length, and this calculator predicts your next period dates for the coming months, including estimated ovulation days. It also notes the difference between regular and irregular cycles and positions itself as a tracking aid rather than medical advice.

Last updated: June 2026 · Free · No sign-up required

Results

Enter values above and click Calculate to see your result instantly.

Quick reference

Menstrual cycle phases:

Phase	Approx. days (28-day cycle)
Menstruation	Days 1–5
Follicular	Days 1–13
Ovulation	Around day 14
Luteal	Days 15–28

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This calculator gives you the same number a clinician would compute by hand from your inputs. It is informational, never a substitute for a doctor. If a result surprises you, write it down and discuss it at your next appointment — that conversation is the goal, not a verdict from a webpage.

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Every health calculator on CalcaTools uses an equation from the WHO, NIH, ACOG, CDC, or a published peer-reviewed paper. The exact citation is in the "Formula & methodology" section. We re-check sources quarterly and update the page when guidance changes — the "Last reviewed" date is at the top.

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Health calculations only help if you can actually use them. We use plain-English labels (no jargon), accept both metric and US units, and show what a "normal" range looks like alongside your result — so you understand what the number means, not just what it is.

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Your weight, height, age, pregnancy details, and any other health input never leave your browser. Calculations run client-side; nothing is logged or stored on our side. We don't want your medical data — we just want the formula to work for you.

Interpretation guide

Cycle facts:

Item	Typical range
Cycle length	21–35 days
Period length	3–7 days
Luteal phase	~14 days (fairly fixed)
Variation	A few days is normal

lightbulb Worked example

Let's say you are using the Period Calculator. Enter the first day of your last period and your average cycle length, and this calculator predicts your next period dates for the coming months, including estimated ovulation days. It also notes the Enter the values that match your situation into the input fields and press calculate — using realistic numbers makes the result directly useful for you.

Result: The calculator instantly applies the formula Next period = first day of last period + cycle length; repeat for future cycles and returns the result with appropriate precision.

What this means: Read the result in the context of what you are measuring. The step-by-step breakdown lets you confirm the math and understand which input most affects the outcome.

Formula & methodology

Formula: Next period = first day of last period + cycle length; repeat for future cycles

The Period Calculator is built on a well-established calculation method. It uses the formula Next period = first day of last period + cycle length; repeat for future cycles to turn your inputs into a reliable result. Enter the first day of your last period and your average cycle length, and this calculator predicts your next period dates for the coming months, including estimated ovulation days. It also notes the difference between The steps are shown on the page so you can follow the reasoning from input to output.

The calculator predicts upcoming period start dates by adding your average cycle length to the first day of your last period, then repeating for several future cycles. A typical cycle is 21–35 days with 3–7 days of bleeding. Because the luteal phase is fairly fixed at about 14 days, cycle-length differences mostly come from the follicular phase. Tracking over a few months improves the prediction's accuracy.

Authoritative source: U.S. Centers for Disease Control and Prevention

Frequently asked questions

How do I predict my next period?

Enter the required values into the input fields and press the calculate button. The tool applies the formula Next period = first day of last period + cycle length; repeat for future cycles and returns the result with a short explanation of each step, so you can check the working yourself.

What is a normal cycle length?

The Period Calculator is free, private, and accurate: it runs entirely in your browser (no uploads, no accounts), applies the standard calculation, and explains each step so you can verify the result. Enter the first day of your last period and your average cycle length, and this calculator predicts your next period dates for the coming months, including estimated ovulation days. It also notes the difference between regular and irregular cycles and There is no limit on usage, and it works on any device.

How long does a period usually last?

A typical period lasts 3–7 days, with 5 days the average; cycles run 21–35 days from the first day of one period to the first day of the next, with 28 the average. The calculator predicts the next period start from your cycle length; the duration of bleeding itself varies by person and is entered separately if you track it. Anything outside these ranges regularly warrants a discussion with a doctor.

Why is my cycle irregular?

Cycles vary for many reasons: stress, sleep changes, travel, weight changes, intense exercise, hormonal conditions (like PCOS), thyroid issues, and medications. Occasional irregularity is normal — a one-off late period is common — but persistent irregularity (consistently outside 21–35 days) can indicate an underlying condition worth checking with a doctor. The calculator uses your average cycle length; irregular cycles make predictions approximate.

Can I track ovulation with this?

Yes — privacy is the core design principle here. The Period Calculator runs entirely in your browser with client-side JavaScript, so the values you enter are never uploaded to any server, never stored, and never shared. Nothing is tracked, no account exists, and closing the page erases your inputs completely.

What does the Period Calculator do?

Enter the first day of your last period and your average cycle length, and this calculator predicts your next period dates for the coming months, including estimated ovulation days. It also notes the difference between regular and irregular cycles and. The calculator takes your inputs, applies the standard calculation, and returns a clear result so you can make an informed decision without doing the math by hand.

What formula does the Period Calculator use?

The Period Calculator applies the standard formula: Next period = first day of last period + cycle length; repeat for future cycles. The tool walks through each step of the calculation so you can verify the numbers yourself.

Is the Period Calculator free and private?

Yes — the Period Calculator. There is no sign-up, no paywall, no trial, and no limit on how many calculations you can run. CalcaTools covers its costs with non-intrusive display advertising, so the calculator itself never asks for payment or restricts any feature.

How accurate is the Period Calculator?

The Period Calculator applies the standard formula: Next period = first day of last period + cycle length; repeat for future cycles. It is tested against published worked examples before launch and uses native double-precision arithmetic, so results are accurate to typical precision for the values you enter — with no intermediate rounding that could distort the answer.

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