Week Number Calculator

ISO week number identifies which week of the year a date falls in. ISO 8601 defines week 1 as containing the first Thursday of the year, with weeks starting on Monday

The calculator uses the standard published formula for week number. Results are accurate to the precision of the inputs you provide. For financial, medical, or legal decisions, always verify with a qualified professional.

This calculator works with inches, watts. You can enter values in the units shown — the calculator handles all conversions internally.

The calculator applies the standard formula for this type of calculation. See the 'How It Works' steps above for the detailed formula breakdown.