Weeks Between Two Dates

Lay Down the Week Ruler

Quick Spans

Set the end date relative to the start with one click.

Days → Weeks Conversion Table

Need to add weeks to a date instead? Try the add days / weeks stepper.

The Weeks-Between Formula

Week Reference

No saved spans yet. Tap "Save to History" to remember up to six week measurements.

How to Measure Weeks Between Dates

1. Set the start date — for example, a last-menstrual-period date or a project kickoff.
2. Set the end date — a due date, deadline, or term end. Use the quick-span chips for common lengths.
3. Read the ruler: each major tick is one week; the green fill stops where the end date lands on the final partial tick.
4. Use the whole-weeks-plus-days figure for human-readable schedules (like '24+3' in pregnancy) and the fractional decimal for billing and accrual.
5. Tap "Save to History" to keep the measurement and compare spans side by side.

The Week: A Seven-Day Ruler for Time

In 2026, an expectant parent tracking a 40-week pregnancy, a scrum master counting sprints, and a payroll clerk reconciling bi-weekly cycles all measure time in weeks rather than days or months. The week is the only common time unit with no calendar irregularity — every week is exactly seven days, always — which makes it the cleanest ruler for spans between two dates. This calculator lays that ruler down between any start and end date and reads off both the whole weeks and the leftover fractional weeks.

The seven-day week is ancient and astronomical-cultural rather than astronomical-precise. It traces to Babylonian astronomy, which named the days after the seven classical 'planets' visible to the naked eye — the Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn. The Romans adopted it, and it spread through the Roman Empire and later Christianity and Islam. Unlike months and years, the week has no astronomical anchor; it simply repeats every seven days without interruption, which is exactly why it is such a reliable measuring unit.

To compute weeks between two dates, the engine first finds the total number of days between them — a simple subtraction of timestamps divided by 86,400,000 milliseconds — then divides by seven. The whole-number part is the count of complete weeks; the remainder (0 to 6 days) is the partial week. Expressed as a decimal, 16 days is 2.29 weeks (2 weeks and 2 days). The ruler above shows whole weeks as major ticks and the partial week as a shaded final segment.

Pregnancy is the most famous application of week-counting. Obstetricians measure gestational age in completed weeks plus days, written as '24+3' for 24 weeks and 3 days, dated from the first day of the last menstrual period (LMP) rather than conception. A full-term pregnancy is 40 weeks (280 days) from LMP. The 'weeks plus days' notation is exactly the whole-weeks-plus-remainder output this tool produces, which is why midwives and parents-to-be lean on week calculators.

ISO 8601 adds a formal week-numbering scheme used in business, manufacturing, and logistics. ISO weeks run Monday to Sunday, and week 1 of a year is the week containing the first Thursday. This gives 52 or 53 weeks per year. Many European payroll and planning systems schedule entirely by ISO week number ('we ship in week 38'), which is a different question from 'how many weeks between two dates' but rests on the same seven-day foundation. The sibling week-of-year tool covers ISO numbering.

Fractional weeks matter more than people expect. A project quoted as '6 weeks' that actually spans 44 days is 6.29 weeks — the extra two days can blow a deadline if treated as a clean 6. Payroll accrual, prorated subscriptions, and interest that compounds weekly all need the fractional part. This calculator always shows both the rounded whole-week figure and the precise decimal so you can choose the right one for billing versus planning.

Practically, decide up front whether your span is inclusive or exclusive of the end date. A Monday-to-Sunday span is 6 days exclusive but 7 days inclusive — and therefore either 0.86 or 1.0 weeks. This tool measures the exclusive elapsed span (end minus start) by default, which matches how most software and pregnancy dating works. For counting both endpoints, add one day. The week ruler makes the choice visible by showing exactly where the end date lands on the final tick.