If you worked 40 hours per week, 52 weeks a year, you’d work 2,080 hours. But of course you need to allow for holidays and sick days, plus all the hours you’ll spend on things you can’t charge for like administration, meetings, travelling, breaks etc.
As the table shows, a more realistic number of hours you can invoice clients for might be 1,350 hours.
You can tailor this calculation to your own situation, but don’t be tempted to skip on holidays as that’s a sure-fire way to burn yourself out. Also don’t underestimate the number of non-chargeable hours you’ll need to invest in admin, marketing and all the other aspects of running your business.