Convert weeks to days

One week is exactly 7 days. That is a fixed convention, not a measurement: 60 seconds to the minute, 60 minutes to the hour, 24 hours to the day and 7 days to the week, with the second as the SI base unit. Leap seconds adjust the clock against the Earth's rotation; they do not change the length of a minute or an hour as a unit, so this factor is exact.

Worked through: 6 week x 7 = 42 day.

Going the other way, one day is 0.1428571429 weeks.

For mental arithmetic: Multiply by 7: double it three times, then subtract the original.

Multiply by seven, which is exact and still manages to mislead. A project quoted at six weeks is 42 calendar days and 30 working days, and those two numbers get used interchangeably in planning with predictable results. The conversion here is calendar days; if what you want is business days, that depends on which days you work and which holidays you take, which is a different calculation entirely.

Precision is worth matching to the input. A measurement given as 1 week carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.