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Menstrual Cycle Calculator

Predict next period start date. Free online Menstrual Cycle Calculator. Calculate menstrual cycle online — fast, accurate, mobile-friendly, no signup needed.

Days until next
18

Derivation

  1. ├── 01Givenlast = 10, cycle = 28
  2. ├── 02Formulae.cycle-t
  3. ├── 03Substitutee.28-t
  4. └── 04Compute Days until next18
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§01What is

Understanding the Menstrual Cycle Calculator

The Menstrual Cycle Calculator computes Days until next from 2 inputs: days since last period, average cycle (days). Predict next period start date.

Health metrics give us objective checkpoints against a body that can feel unreliable. Used alongside professional guidance — not in place of it — these numbers help you track progress and spot trends that matter. The Menstrual Cycle Calculator sits in that toolkit — it predict next period start date. Enter your numbers above and the result updates instantly; every step of the math is shown in the Derivation panel so you can see exactly how the answer was reached.

§02The Formula

How it’s calculated

e.cycle-t

Where

last
Days since last period
cycle
Average cycle (days)
§03Practical Example

Step-by-step walkthrough

Scenario

Apply the formula to a realistic set of inputs: Days since last period = 10, Average cycle (days) = 28.

  1. 01Start by noting the input — Days since last period: 10.
  2. 02Start by noting the input — Average cycle (days): 28.
  3. 03Substitute these values into the formula: e.cycle-t
  4. 04Compute Days until next: the calculator returns 18.
  5. 05Cross-check the answer by opening the Derivation panel above — every line of math is shown so you can follow the computation end-to-end.
§04Variants

Common Menstrual Cycle Problems

The formula gets rearranged depending on which variable you need. Here are the patterns you’ll run into in the real world — find the one that matches your problem and follow the worked steps.

01 · PATTERN

Days since last period halved

last = 5 (from 10)

Keep every other input at its default and halve the days since last period. See how days until next responds.

  1. 01New Days since last period: 5
  2. 02Baseline Days until next: 18
  3. 03New Days until next: 23
  4. 04Days until next increases by 27.8% → use this sensitivity to plan for real-world variation.
02 · PATTERN

Days since last period doubled

last = 20 (from 10)

Keep every other input at its default and double the days since last period. See how days until next responds.

  1. 01New Days since last period: 20
  2. 02Baseline Days until next: 18
  3. 03New Days until next: 8
  4. 04Days until next decreases by 55.6% → use this sensitivity to plan for real-world variation.
03 · PATTERN

Average cycle (days) halved

cycle = 14 (from 28)

Keep every other input at its default and halve the average cycle (days). See how days until next responds.

  1. 01New Average cycle (days): 14
  2. 02Baseline Days until next: 18
  3. 03New Days until next: 4
  4. 04Days until next decreases by 77.8% → use this sensitivity to plan for real-world variation.
04 · PATTERN

Average cycle (days) doubled

cycle = 56 (from 28)

Keep every other input at its default and double the average cycle (days). See how days until next responds.

  1. 01New Average cycle (days): 56
  2. 02Baseline Days until next: 18
  3. 03New Days until next: 46
  4. 04Days until next increases by 155.6% → use this sensitivity to plan for real-world variation.
§05FAQ

Frequently asked questions

Yes. The calculator implements the standard formula as documented and returns exact floating-point results. No approximations are used unless noted in the formula.
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