Also known as tenant's pro rata share. The portion of a building occupied by the tenant expressed as a percentage. When a tenant is responsible for paying its proportionate share of the landlord's costs for the building, such as operating expenses and real estate taxes, the tenant pays this amount over a base year. In general, the tenant's proportionate share is determined by taking the building's rentable square footage and dividing it by the tenant's rentable square footage. Local industry customs usually provide the landlord with the guiding principles for: