{
    "name": "Period Calculator",
    "url": "https://calculators.geekmidst.com/calculators/period-calculator",
    "markdown": "https://calculators.geekmidst.com/calculators/period-calculator.md",
    "category": "Health & fitness",
    "description": "Predict the start and end dates of your next periods from the first day of your last period, your cycle length and how long your period usually lasts.",
    "summary": "If your cycle is regular, each period starts one cycle length after the previous one. Enter your last period to see the dates for the coming months.",
    "also_called": [
        "Menstrual cycle calculator",
        "Period predictor"
    ],
    "applies_to": "Worldwide",
    "last_updated": "2026-10-29",
    "computation": "Results are computed in the visitor’s browser on the HTML page. This API describes the calculator; it does not compute results.",
    "inputs": [
        {
            "name": "lmp",
            "label": "First day of your last period",
            "type": "date",
            "default": "2026-01-05"
        },
        {
            "name": "cycle",
            "label": "Usual cycle length",
            "type": "number",
            "default": "28",
            "unit": "days"
        },
        {
            "name": "period_days",
            "label": "Period length",
            "type": "number",
            "default": "5",
            "unit": "days"
        },
        {
            "name": "count",
            "label": "Periods to show",
            "type": "number",
            "default": "6",
            "optional": true
        }
    ],
    "outputs": [
        {
            "key": "next_period",
            "label": "Next period starts",
            "format": "text"
        },
        {
            "key": "next_period_end",
            "label": "Next period ends",
            "format": "text"
        },
        {
            "key": "following_period",
            "label": "The one after starts",
            "format": "text"
        }
    ],
    "formula": {
        "text": "Period n starts = LMP + n × cycle length;  ends = start + period length − 1;  ovulation ≈ start − 14 days",
        "variables": {
            "LMP": "first day of the last menstrual period",
            "n": "1 for the next period, 2 for the one after, and so on"
        }
    },
    "worked_example": {
        "inputs": {
            "lmp": "2026-01-05",
            "cycle": "28",
            "period_days": "5",
            "count": "6"
        },
        "steps": [
            "Next period: 5 Jan + 28 days = 2 Feb 2026.",
            "It lasts 5 days, so it ends on 6 Feb (2 Feb + 4 days).",
            "The one after: 5 Jan + 56 days = 2 Mar 2026."
        ],
        "result": "With a 28-day cycle that started on 5 January 2026, the next period is due on 2 February and the one after on 2 March."
    },
    "interpretation": "A period arriving a few days early or late is common. If periods stop for three months or more, become very irregular or are unusually heavy or painful, it is worth seeing a doctor or nurse.",
    "assumptions": [
        "Your cycle length stays the same from month to month.",
        "Ovulation is estimated 14 days before each period."
    ],
    "limitations": [
        "Predictions are less reliable for irregular cycles, in the first years after periods start, around menopause, after pregnancy or when changing contraception.",
        "Not suitable for avoiding pregnancy."
    ],
    "rules": null,
    "faqs": [
        {
            "question": "How do I work out my cycle length?",
            "answer": "Count from the first day of one period to the day before the next one starts; average several cycles for a better figure."
        },
        {
            "question": "Is a 28-day cycle normal?",
            "answer": "It is the average, but longer and shorter cycles are also normal. What matters most is what is typical for you."
        }
    ],
    "sources": [
        {
            "title": "Tommy’s: Ovulation and fertility",
            "url": "https://www.tommys.org/pregnancy-information/planning-pregnancy/how-get-pregnant/ovulation-and-fertility"
        }
    ],
    "related": [
        {
            "name": "Ovulation Calculator",
            "url": "https://calculators.geekmidst.com/calculators/ovulation-calculator"
        },
        {
            "name": "Conception Calculator",
            "url": "https://calculators.geekmidst.com/calculators/conception-calculator"
        },
        {
            "name": "Date Difference Calculator",
            "url": "https://calculators.geekmidst.com/calculators/date-difference-calculator"
        }
    ],
    "disclaimer": "This is a general screening estimate for adults, not medical advice or a diagnosis. Talk to a qualified health professional about your health.",
    "publisher": "GM¹X CalcVerse"
}