How Many ORs? It’s Complicated

Surgery SuitePlanning a surgery suite used to be fairly simple. General operating rooms were used for a wide range of procedures with dedicated operating rooms limited to cardiac surgery and orthopedics. Today, the question “How many operating rooms are needed?” is complicated by more stringent distinctions between operating rooms and procedure rooms, changing technology and increasing specialization, and the convergence of diagnostic imaging and surgical procedures. From a facility planning perspective, the number, size, and specialization of the operating rooms (ORs) is the single biggest factor contributing to the overall footprint of the surgery suite (and project cost). Moreover, the numbers and sizes of related patient care and support spaces are planned based on the number and types of ORs. And, from an operational standpoint, the number of ORs drives ongoing staffing and related operational costs.


In the surgery suite, a distinction is made between operating rooms and procedure rooms. ORs are designed to provide a safe environment for procedures that carry a high risk of infection. Any form of anesthetic may be administered in an OR as long as the appropriate anesthesia gas administration devices and exhaust systems are provided. On the other hand, procedure rooms are used for procedures that do not require an aseptic field although they may use sterile instruments and supplies. Local anesthesia, or minimal or moderate sedation, may be administered in a procedure room, but anesthetic agents used in procedure rooms should not require special ventilation. Procedure rooms are considered unrestricted areas and may be used for endoscopy, pain management, laser, or other procedures in which there is no cutting of the mucous membranes.


Mobile ultrasound, endoscopy units, or c-arms (their name derived from their shape) have been a standard part of surgical operating rooms for many years. However, today, hybrid operating rooms have permanently installed equipment, such as intraoperative CT, MRI, and fixed c-arms, which is typically used in conjunction with cardiovascular, thoracic, neurosurgery, spinal, and orthopedic procedures to enable diagnostic imaging before, during, and after surgery. This arrangement enables the surgeon to assess the effectiveness of the surgery and perform further resections or additional interventions in a single encounter. Many equipment vendors offer highly specialized, proprietary imaging systems that are permanently integrated with operating rooms while others offer designs that position the CT or MRI unit so it can be used independently for diagnostic procedures when surgery is not in progress.


As imaging procedures are becoming more interventional and no longer limited to diagnostic uses, surgery is becoming less invasive. With the melding of interventional radiology and minimally-invasive, image-guided, and traditional open surgical techniques, the co-location of the interventional suite with the surgical suite, when feasible, provides additional long-term flexibility.


Whether planning a traditional surgery suite or an integrated surgery and interventional radiology area, the process begins with determining the number of procedure rooms — surgical operating rooms (ORs), interventional rooms (IRs), minor procedure rooms (MPRs), and hybrid rooms. Some healthcare organizations may have already settled on the number and type of procedure rooms based on a detailed business plan (What can we afford?). Also, partnerships with equipment vendors, or the existence of an enthusiastic donor, may result in early agreement on one or more hybrid or specialty procedure rooms regardless of the current workload.

  • Current and historical workload. When a detailed analysis is required, a look at the existing workload is a good starting point. Historical data — such as the past three years — is also used to identify trends in conjunction with projecting future workload. The current annual number and minutes of surgery cases and interventional procedures (as applicable) should be tabulated by specialty and patient type (inpatient, same-day admit, and outpatient) and sorted by the type of procedure room used. For example, surgical specialties that may use a dedicated procedure room include cardiovascular surgery, neurosurgery, orthopedics, gastrointestinal and genitourinary (GI/GU), gynecology, and ophthalmology. Many other procedures can be accommodated in the same flexible OR or procedure room. Ideally, the number of dedicated rooms should be kept to a minimum. If an institution has a busy trauma center, the total annual minutes should be adjusted to reflect emergency procedures that occur outside the normal hours when elective procedures are scheduled. Procedures that occur outside the primary weekday shift should not be used to calculate procedure room demand. The average minutes per procedure, or room time, can be calculated for various specialties by dividing the annual minutes by the annual number of procedures. Hospitals also typically record the average room turnaround time which is the time between when one patient leaves the procedure room and the room is ready for the next patient. Throughput is the rate of patient flow through the OR or procedure room — such as 2.5 cases per OR per day or 900 annual cases per OR.
  • Projected workload. Once the projected workload is confirmed, the annual minutes of room time can be calculated based on a review of existing data with adjustments to reflect anticipated future changes in case length. A target room turnaround time also needs to be established (usually 15 to 20 minutes). This should be based on an analysis of the existing room turnaround time and adjusted to reflect potential operational improvement initiatives. The target room turnaround time is multiplied by the projected procedures and added to the annual minutes of room time. This results in the total annual minutes required which is used to determine the number of procedure rooms.
  • Procedure room availability. Most procedure rooms are available eight hours per day for a total of 480 minutes per day. However, many institutions are extending hours of operation into the early evening to optimize the utilization of these expensive facilities. The number of days per year the suite will be in operation must then be determined. If the suite operates five days per week, 50 weeks per year (assuming 10 holidays), 250 annual days of operation will be calculated. Multiplying the annual days of operation by the minutes per day will provide the annual minutes available per procedure room at 100 percent utilization (typically 120,000 annual minutes).
  • Scheduling efficiency. A scheduling efficiency factor is multiplied by the annual minutes available per procedure room to account for periods of time that a case cannot be scheduled. This factor may range from 70 percent for ORs or procedure rooms used for complex (and sometimes unscheduled) procedures — such as cardiothoracic, neurosurgery, and trauma/orthopedics — to 90 percent for an outpatient suite where all procedures are scheduled.
  • Estimating the number of procedure rooms needed. The total annual minutes required are then divided by the annual minutes available per room (adjusted for scheduling efficiency). It must be noted that if the hospital is designated as a trauma center, an OR may need to be available at all times for emergencies. This must be considered in calculating the number of ORs for the surgical suite.


In an integrated surgical and interventional radiology suite, optimal flexibility could be achieved if all the procedure rooms were interchangeable. However, the physical configuration and patient and staff flow of an OR and an IR are different. ORs are typically designed with dual access to the procedure room — one from a perimeter corridor for the patient and staff and the other from a central clean “core” area from which case carts and other sterile supplies and equipment are placed in the procedure room. An IR is configured with both a contiguous control room, housing staff workstations, and a contiguous equipment component room, rather than a clean core area. IRs also require radiation protection.

The difficulty in projecting future workload comes when procedures performed in an OR or IR today are expected to migrate to a hybrid room. Also, new, and often experimental technology, may involve longer procedure times thus reducing the number of procedures to be accommodated in a specific room initially.

Note: Detailed calculations for determining the number of ORs, IRs, and various other procedure rooms can be found in the SpaceMed Guide.

See related post: Factors Affecting the Size of the Surgery Suite.

This article is an update of a previous post.