Weeks Between Dates Calculator — Count Exact Weeks

How to Use the Weeks Between Dates Calculator

1. Enter the start date: Use the start date picker to select your first date. It defaults to approximately 3 months ago, providing an immediate result of about 13 weeks. Navigate the calendar or type the date in YYYY-MM-DD format. The calculator accepts any valid date.
2. Enter the end date: Select your second date. It defaults to today. The order of dates does not matter — the calculator always computes the absolute difference between the two dates, so the result is the same regardless of which date you enter first.
3. Read the primary result: The large number at the top shows the complete weeks between the two dates. If there are remaining days beyond complete weeks, they appear next to the week count (for example, "13 weeks + 4 days").
4. Check the supporting details: Below the main result, you will find the total day count and the full year-month-day duration. The total days equals weeks times 7 plus remaining days. The duration breakdown provides a natural-language representation of the same time span.

This calculator is ideal for sprint planning, pregnancy week tracking, academic semester scheduling, training program design, and any scenario where the weekly unit provides the most useful perspective on a time duration.

Weeks Calculation Formula

Variables Explained

• Total Days: The absolute number of calendar days between the two dates, computed from the millisecond difference.
• Complete Weeks: The integer quotient when dividing total days by 7. This represents how many full 7-day periods fit between the dates.
• Remaining Days: The remainder when dividing total days by 7. This is always between 0 and 6, representing the leftover days after extracting all complete weeks.
• 86,400,000: Milliseconds in one day (24 * 60 * 60 * 1,000), used to convert the millisecond time difference to days.

Step-by-Step Example

Calculate the weeks between January 6, 2026 (Monday) and April 20, 2026 (Monday):

1. January: 31 - 6 = 25 remaining days
2. February: 28 days (2026 is not a leap year)
3. March: 31 days
4. April 1-20: 20 days
5. Total: 25 + 28 + 31 + 20 = 104 days
6. Complete weeks: floor(104 / 7) = 14 weeks
7. Remaining days: 104 mod 7 = 6 days
8. Result: 14 weeks and 6 days (or equivalently, 3 months and 14 days)

Practical Examples

Example 1: Tom's Agile Sprint Planning

Tom is a scrum master planning a software project from March 2, 2026 to August 28, 2026. His team runs 2-week sprints and he needs to determine how many sprints fit in the timeline:

• Total days: 179
• Complete weeks: 25 weeks and 4 days
• Number of 2-week sprints: 12 full sprints (24 weeks)
• Remaining time: 1 week and 4 days for buffer and release

Tom plans 12 sprints with a final week-and-a-half buffer for integration testing, bug fixes, and release preparation. He numbers the sprints 1 through 12 and creates a sprint calendar starting every other Monday from March 2. The remaining 11 days serve as a vital safety net for the inevitable scope adjustments. For tracking his team's daily hours, he uses the time card calculator.

Example 2: Jennifer's Pregnancy Progress

Jennifer's last menstrual period was on November 15, 2025. As of February 20, 2026, she wants to know her gestational age in weeks to share with her obstetrician:

• Total days: 97
• Complete weeks: 13 weeks and 6 days
• Medical notation: 13+6 (13 weeks, 6 days)
• Trimester: Late first trimester / early second trimester

At 13 weeks and 6 days, Jennifer is about to enter her second trimester (which begins at week 14). Her doctor uses this precise week count to schedule appropriate screenings — the nuchal translucency scan is typically done between weeks 11 and 14. The remaining days matter clinically, as certain tests have narrow timing windows. For detailed pregnancy milestones, our pregnancy calculator provides trimester-by-trimester guidance.

Example 3: Alex's Training Program

Alex signed up for a marathon on October 11, 2026, and wants to start training on April 6, 2026. Most marathon training plans are structured in 12, 16, or 20-week blocks:

• Total days: 188
• Complete weeks: 26 weeks and 6 days
• Best fit: 20-week plan starting June 1 with 6 weeks of base building beforehand
• Taper week: The last 2-3 weeks before the race for reduced training

With nearly 27 weeks available, Alex has enough time for a comprehensive preparation plan. He chooses a 20-week marathon training plan, uses the first 4 weeks for base fitness building, and has about 3 weeks of buffer for any interruptions from weather, minor injuries, or schedule conflicts. The weekly structure of training plans makes this calculator particularly useful for athletes planning competition preparation.

Weeks Reference Table

Tips and Complete Guide

Understanding the ISO Week Calendar

The ISO 8601 standard defines an alternative week-based calendar where every year has either 52 or 53 weeks. Each ISO week starts on Monday and ends on Sunday. Week 1 of a year is defined as the week containing the first Thursday of January. This means that January 1 might belong to the last ISO week of the previous year, or December 31 might belong to the first ISO week of the following year. This system is widely used in Europe for business planning and is the standard in many industries for fiscal week tracking.

Weeks in Medical and Health Contexts

Weeks are the standard unit for pregnancy tracking (gestational age), neonatal development, and early childhood milestones. In pregnancy, the due date is calculated as 40 weeks from the last menstrual period. Critical milestones include viability at 24 weeks, term at 37 weeks, and post-term at 42 weeks. Pediatricians track infant development in weeks for the first several months, as weekly changes are significant in early development. Drug treatment programs, physical therapy plans, and rehabilitation protocols are also commonly structured in weekly cycles.

Week-Based Planning Strategies

Many productive planning systems are organized around weeks. The 12-Week Year methodology compresses annual goals into 12-week cycles for increased urgency. Agile software development uses 1-2 week sprints as the fundamental planning unit. Academic institutions structure coursework around 15-16 week semesters. Fitness and nutrition programs typically run in 4, 8, or 12-week phases. Understanding how many weeks you have available for a goal helps break it into manageable weekly targets. If you have 20 weeks and need to complete 100 tasks, that is 5 tasks per week — a more actionable framing than "100 tasks in about 4.5 months."

Common Mistakes to Avoid

• Assuming 4 weeks equals 1 month: Four weeks is 28 days, but most months have 30 or 31 days. Over the course of a year, the difference is significant: 12 months is approximately 52.18 weeks, not 48 (12 * 4). Using 4 weeks as a month will undercount the actual time available by about one full month per year.
• Ignoring the remaining days: The remaining days after complete weeks can be critical. In pregnancy tracking, being "32 weeks and 6 days" is clinically different from "32 weeks and 0 days." In sprint planning, 2 extra days might accommodate a deployment. Always check the remaining days, not just the week count.
• Confusing ISO weeks with calendar weeks: The ISO week system (Week 1 contains the first Thursday of January) may number weeks differently from your intuitive counting. December 29-31 in some years belong to Week 1 of the next year. For business reporting, clarify which week numbering system is being used.
• Not accounting for incomplete weeks at boundaries: If a project starts on Wednesday and you count "4 weeks," the 4th week ends on a Tuesday. The first and last weeks may be incomplete, giving fewer working days than expected. Factor in partial weeks when planning deliverables.

Frequently Asked Questions