Last updated: April 2026
29 Jan 2026 - 2 Feb 2026
12 Feb 2026
7 Feb 2026 - 13 Feb 2026
| Cycle | Period dates | Ovulation | Fertile window |
|---|---|---|---|
| 1 | 29 Jan 2026 - 2 Feb 2026 | 12 Feb 2026 | 7 Feb 2026 - 13 Feb 2026 |
| 2 | 26 Feb 2026 - 2 Mar 2026 | 12 Mar 2026 | 7 Mar 2026 - 13 Mar 2026 |
| 3 | 26 Mar 2026 - 30 Mar 2026 | 9 Apr 2026 | 4 Apr 2026 - 10 Apr 2026 |
This calculator provides an estimate only. Period dates and ovulation timing can vary and should not replace medical advice.
The next period is estimated by adding your average cycle length to the first day of your last period.
Longer cycles push period and ovulation estimates later. Shorter cycles bring those estimates earlier.
Irregular cycles, stress, illness, travel, medication, and hormonal factors can all change period timing.
The Period Calculator estimates upcoming period dates, period end dates, ovulation dates, and a fertile window from the last period date, average cycle length, period length, and number of cycles. It is useful for planning and tracking, not for confirming pregnancy or diagnosing cycle problems. Use this expanded guide when you need more than a quick result. It explains the assumptions behind the Period Calculator, the records to gather, and the decisions the estimate can support. It is especially useful for people tracking menstrual cycles, estimating next period dates, and understanding likely fertile-window timing. The strongest use of the page is scenario comparison: change one input at a time, compare the output, and keep a note of which assumption changed.
The calculator treats the first day of the last period as cycle day 1, adds the average cycle length for future periods, estimates ovulation as about 14 days before the next period, and estimates a fertile window around ovulation. The calculator result depends on the quality of the inputs and on the rule set or formula selected in the calculator above. For practical use, treat the output as a structured estimate: start with the core inputs, review the main outputs, then test the decision points that matter most to your situation. Key decisions include when the next period may start, when ovulation may occur, whether cycle variation makes predictions less reliable.
Input: Last period 1 January 2026, 28-day cycle, 5-day period
Calculation: Next period starts 28 days later and ends 4 days after that.
Result: The next period is estimated as 29 January to 2 February 2026.
Input: Last period 1 January and cycle length 35 days.
Calculation: The calculator adds 35 days for the next period and estimates ovulation 14 days before it.
Result: The predicted dates shift later than a standard 28-day cycle.
Input: A user records 27, 29, and 31 day cycles.
Calculation: The average may be useful, but variation should be noted.
Result: The calculator can plan likely dates but should not be treated as exact.
Menstrual cycles naturally vary. Illness, stress, contraception, breastfeeding, travel, weight changes, perimenopause, and some health conditions can shift timing. Calendar estimates are most useful when combined with real tracking over several cycles.
A useful Period Calculator result starts with the same evidence you would use if you were checking the answer manually. The calculator can organise the arithmetic, but it cannot know whether a payslip is final, a bill is estimated, a quote excludes fees, or a personal circumstance has changed since the last statement.
Before making a decision, compare the calculator result with the source document that controls the real outcome. For this topic, that usually means checking NHS periods and fertility guidance. If there is a difference between the calculator and an official statement, contract, assessment, or professional advice, treat the official document as the stronger source.
The most important input is not always the largest number on the form. Sometimes a date, threshold, percentage, eligibility flag, or timing assumption changes the result more than the headline amount. This is why scenario testing is more useful than a single calculation.
| Input | Why it matters | What to double-check |
|---|---|---|
| last period date | It feeds directly into the estimate or changes which rule is applied. | Check the period, units, eligibility, and whether the figure is final or estimated. |
| average cycle length | It feeds directly into the estimate or changes which rule is applied. | Check the period, units, eligibility, and whether the figure is final or estimated. |
| period length | It feeds directly into the estimate or changes which rule is applied. | Check the period, units, eligibility, and whether the figure is final or estimated. |
| number of cycles to forecast | It feeds directly into the estimate or changes which rule is applied. | Check the period, units, eligibility, and whether the figure is final or estimated. |
The output should be read as a decision aid, not just a number. For Period Calculator, the useful question is often what the result means for timing, affordability, eligibility, comparison, or next steps.
A single estimate is a snapshot. A better approach is to save a base case, then adjust one assumption at a time. This shows whether the result is stable or whether a small change in timing, rate, usage, income, or cost creates a very different answer.
| Scenario | Change one assumption | What the comparison shows |
|---|---|---|
| Base case | Use the best current evidence. | Shows the result you would expect if nothing important changes. |
| Conservative case | Use lower income, higher cost, slower growth, or less favourable timing. | Shows whether the decision still works with less optimistic assumptions. |
| Improved case | Use the realistic upside, such as lower cost, better rate, higher usage, or stronger evidence. | Shows the potential benefit without treating it as guaranteed. |
Most errors come from using the right formula with the wrong assumption. Dates can be counted differently, rates can change, official thresholds can move, and real bills or contracts often include conditions that a simple calculator cannot infer automatically.
Once you have a result, write down the key assumptions and compare them with NHS periods and fertility guidance. If the number affects a deadline, tax return, benefit claim, employment issue, medical question, finance agreement, or major purchase, use the calculator as preparation for a more formal check.
For lower-stakes use, the next step may simply be comparing two or three scenarios. For higher-stakes use, the next step should be checking the official guidance, speaking to the relevant organisation, or getting qualified advice before acting.
This calculator is general information only and is not medical advice. It cannot diagnose pregnancy, infertility, hormonal conditions, or menstrual disorders. This is general information only and is not medical advice. The calculator is designed to support understanding and planning, but it cannot verify documents, predict future rule changes, or account for every exception. Use it as an estimate and check the official source before acting where the result matters.
No. It is a common estimate, but real ovulation timing varies.
No. Calendar estimates alone are not reliable contraception.
Cycle length can vary because of stress, illness, travel, medication, contraception, and normal biological variation.
Cycle day 1 is usually the first day of period bleeding.
Yes. Stress, illness, travel, contraception, and other factors can change timing.
Seek advice for severe pain, very heavy bleeding, missed periods, unexpected bleeding, or concerns about fertility or pregnancy.
This calculator is designed to help you understand the likely number before you make a decision or start an application.
Your result should be checked against official UK guidance, especially if your circumstances include dependants, exemptions, prior leave, or a complex immigration history.
Treat the figure as a planning tool rather than legal advice. Where the answer affects an application deadline or major payment, speak to an authorised adviser.
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