Hire a Professional
Your results may vary - but they shouldn't have to…
If you're new at this then rather than simply estimating how much wallpaper you need (and especially rather than going by an online "wallpaper estimator") you should enlist the help of a knowledgeable professional. Most showrooms have experts on staff to help. If not then meet with a professional paper hanger to review the location for your installation. You'll need to have the wallpaper roll dimensions plus the pattern repeat and match information for the pattern you will be using (note - purely textural wallcoverings such as grasscloth have neither repeats nor matches) . This will allow them to estimate the number of rolls required for your job.