Do you mean so you have a better view of your margin? If so, you might find these calculations helpful:
Subtract the total cost of the event from the total sales revenue and then divide by total cost of the event. The result is expressed as a percentage, which you multiply by 100. For example, if an event cost £1,000 to stage and generated £2,500 in revenue the sum would look like:
£2,500 – £1,000 = £1,500
£1,500 ÷ £1,000 = 1.5
1.5 x 100 = 150% event ROI
Another way to calculate event ROI is dividing the total revenue by the total event cost. The resulting number is expressed in pounds e.g. £2.50 was generated for every £1 invested. Using these easy calculations, you can set out at the start of planning your event what you hope to achieve in terms of ROI.