Weeks Calculator
1 week = 7 days
How Week Calculations Work
A week is exactly seven days by international convention (ISO 8601 defines week 1 as containing the year's first Thursday). Converting weeks to days multiplies by 7: 4 weeks = 28 days, 40 weeks = 280 days. Converting days to weeks divides by 7, yielding whole weeks plus a remainder: 45 days = 6 weeks and 3 days.
Pregnancy gestation is measured in weeks — a full-term pregnancy is approximately 40 weeks (280 days) from the last menstrual period. If the first day of the last period was March 1, 2026, adding 12 weeks (84 days) reaches May 24, 2026 — the end of the first trimester. Adding 40 weeks (280 days) reaches December 6, 2026, matching Naegele's due-date rule.
Project sprints commonly run 2-week (14-day) cycles. A sprint starting Monday, June 1, 2026 ends Sunday, June 14 (inclusive) or Friday, June 12 (if counting 10 working days). Adding 6 weeks to a kickoff date forecasts milestone dates: June 1 + 6 weeks = July 13, 2026. School terms of 18 weeks equal 126 calendar days.
ISO week numbering labels weeks within a year (W01 through W52 or W53). Week 1 contains January 4. This differs from simple "weeks from a start date" but matters for manufacturing, payroll, and European business reporting. This calculator focuses on duration arithmetic — N weeks from a date — rather than ISO week labels.
Enter a number of weeks to convert to days, days to convert to weeks, or a start date plus weeks to find the target date. Essential for pregnancy week tracking, fitness programs, and any planning measured in weekly increments.
Examples
| Example | Result |
|---|---|
| 4 weeks → days | 28 days |
| 40 weeks → days | 280 days |
| 45 days → weeks | 6 weeks, 3 days |
| Mar 1, 2026 + 12 weeks | May 24, 2026 |
| Mar 1, 2026 + 40 weeks | Dec 6, 2026 |
| Jun 1, 2026 + 6 weeks | Jul 13, 2026 |
| 18 weeks → days | 126 days |
| 100 days → weeks | 14 weeks, 2 days |
Frequently asked questions
One week equals exactly 7 days. Four weeks is 28 days; 40 weeks is 280 days.
Multiply 12 × 7 = 84 days, then add 84 days to your start date. Or use this calculator directly with the start date and 12 weeks.
Gestational age uses completed weeks plus extra days — '12 weeks 3 days' means 12 full weeks and 3 additional days from the last menstrual period.