You have to take into account several different aspects when making this calculation. The first thing you need to decide is how many people, maximum, will be at your event at a time. If you have 50 guests, you will need one toilet. If you have 100 guests, you will need two, and if you have 1000 people attending, you will need twenty. The longer the event, the more toilets should be provided. You can assume that your guests or attendees will need to use the facilities once every three or four hours. With smaller party sizes, one or two additional toilets will suffice, but with larger numbers, you will need considerably more.
Please consider the following: take into consideration the number of females who will be attending the event. You should be ready to add one or two toilets to your total needs to accommodate your female guests if they are at least half of your party.
The use of alcohol will increase your number of toilets. Alcohol has the tendency to increase the amount of times a person has to use the toilet, estimated at 30 to 40 percent more than usual. Therefore the amount of restrooms will increase as well. The number you add, will vary, based on what type of event it is. To be on the safe side, plan for 10% more.
If the portable toilets will be used to supplement the restrooms available, you can count those permanent facilities as part of your total. For example, if your home has two restrooms, you can decrease your calculated need by two. And please, hand sanitation is very important. People like to wash their hands after using the facilities, but this is too often ignored when arranging for outdoor facilities. Our event toilets have hand basins.
Consider people with disabilities when making your portable toilet plans.