Anesthesia: Base and Time Units - How to Calculate

Anesthesia services are reimbursed differently from other procedure codes. Part of the payment for anesthesia is based on "base units," which are assigned to anesthesia CPT codes by the Centers for Medicare & Medicaid Services (CMS). The remainder of the payment allowance is based on the time the patient was under anesthesia. Both the base and time units are then multiplied by an anesthesia conversion factor (CF), which CMS releases annually and is specific to the locality where the anesthesia service is rendered.

The formula to calculate the allowed amount for anesthesia is:

(Base Units + Time [in units]) x CF = Anesthesia Fee Amount

The base units assigned to anesthesia CPT codes and the annual anesthesia conversion factors are available at the CMS Anesthesiologists Center.


  • Payment for services that meet the definition of "personally performed" is based on the base units (as defined by CMS) and time, in increments of 15-minute units
  • Services that are "medically-directed" are reimbursed at 50 percent of the "personally performed" rate. Refer to the CMS Medicare Claims Processing Manual, chapter 12, sections 50.B-50.F for more information regarding the definitions of "personally performed" and "medically directed."
  • Payment for services that are "medically-supervised" is based on three base units per procedure with an additional unit of time if the physician documents that he or she was present at induction

Claim Submission

  • Report actual anesthesia time in minutes on the claim
  • Example: submit 17 minutes of anesthesia as "0017" in the units field (Item 24G of the CMS-1500 claim form). Contractors compute time units by dividing reported anesthesia time by 15 minutes (17 minutes = 1.13 units).

Note: This method is used to calculate anesthesia services that are "personally performed." Services that are "medically directed" are reimbursed at 50 percent of the amount received if the service was personally performed. To find the definitions of "personally performed," "medically directed," and to learn about other payment exceptions, please refer to Sections 50.B–50.F of CMS Pub.100-04, Chapter 12.

Separate payment is not allowed for the anesthesia service performed by the physician who also furnishes the medical or surgical service. In that case, payment for the anesthesia service is made through the payment for the medical or surgical service. For example, separate payment is not allowed for the surgeon’s performance of a local or surgical anesthesia if the surgeon also performs the surgical procedure.


  • CMS Medicare Claims Processing Manual (PDF, 1.05 MB) (Pub. 100-04), Chapter 12
  • Definitions of personally performed, medically directed and medically supervised: Section 50
  • Definition of concurrent procedures: Section 50.C
  • Monitored Anesthesia Care: Section 50.H
  • Anesthesia claims modifiers: Section 50.I
  • Billing Modifiers for qualified nonphysician anesthetists: Section 140.3.3 
  • Additional information regarding anesthesia modifiers is available in the Palmetto GBA Modifier Lookup Tool

Last Updated: 05/28/2020