To work out a roughly how many hours and days will you actually be working as a contractor, ie invoiceable hours per year.
A non leap year consist of 365 days across 52 weeks. Take away weekends and you have a maximum of 260 days to invoice.
You need some holidays and you may be sick. You may want to take time off for training and conferences. You may have gaps between contracts. You will need time for administration and sales/interviews.