Physical Therapy Billing Units & the 8-Minute Rule
Physical therapy revenue lives or dies on unit counting. Bill too few units and you lose money you earned; bill too many and you invite an audit. Here is how physical therapy billing units work, how the 8-minute rule decides them, and where clinics get it wrong.
Timed vs untimed PT codes
Physical therapy CPT codes come in two kinds, and they are counted differently.
- Timed (direct, one-on-one) codes. Billed in 15-minute units based on how long you spent. Examples: 97110 (therapeutic exercise), 97112 (neuromuscular re-education), 97140 (manual therapy), 97530 (therapeutic activities), 97116 (gait training), 97535 (self-care training).
- Untimed (service-based) codes. Billed once per session no matter the time. Examples: 97010 (hot/cold packs), 97012 (mechanical traction), 97014 or G0283 (electrical stimulation, unattended), 97161 to 97164 (PT evaluations and re-evaluation).
The 8-minute rule only applies to the timed codes.
What is the 8-minute rule?
Under Medicare, you can bill one 15-minute unit of a timed service once you have provided at least 8 minutes of it. The number of units you can bill comes from the total timed minutes in the visit, using this table.
| Total timed minutes | Units you can bill |
|---|---|
| 8 to 22 minutes | 1 unit |
| 23 to 37 minutes | 2 units |
| 38 to 52 minutes | 3 units |
| 53 to 67 minutes | 4 units |
| 68 to 82 minutes | 5 units |
| 83 to 97 minutes | 6 units |
The pattern is simple: 15 minutes per unit, and every extra unit needs at least 8 more minutes into the next 15-minute block.
A worked example
Say a visit includes 20 minutes of therapeutic exercise (97110), 10 minutes of manual therapy (97140), and 8 minutes of gait training (97116). That is 38 total timed minutes, which the table allows 3 units. You then assign the units to the services by their minutes: 97110 gets 1 unit (its 20 minutes is the largest single block over 15), and the remaining two units go to the services with the most leftover minutes. Getting this split right is where a PT-trained biller protects revenue.
Medicare 8-minute rule vs the AMA rule
Medicare uses the total-minutes method above. Many commercial payers instead follow the AMA (Rule of Eights), which counts each timed code separately: each service needs its own 8 minutes to bill a unit. The two methods can produce different unit counts for the same visit, so you have to know which rule each payer follows. Mixing them up is a common and costly error.
Where PT clinics lose money on units
- Counting untimed services toward the 8-minute total (only timed minutes count).
- Applying the Medicare rule to a commercial payer that uses the AMA rule, or the reverse.
- Not documenting minutes per service, which cannot support the units billed in an audit.
- Missing the KX modifier once the therapy threshold is exceeded, or missing modifier 59 on distinct services.
We cover the therapy-specific coding in more depth on our physical therapy billing page. If unit counting is costing your clinic revenue, a free revenue analysis will show you where.
FAQs
What is the 8-minute rule in physical therapy?
The 8-minute rule is how Medicare decides how many 15-minute units of a timed service you can bill. You need at least 8 minutes to bill 1 unit, 23 minutes for 2 units, 38 for 3, and so on, based on the total timed minutes in the visit.
How many units can I bill for 40 minutes of therapy?
For 40 total timed minutes you can bill 3 units under Medicare, because 38 to 52 minutes equals 3 units on the 8-minute rule table.
Do untimed codes count toward the 8-minute rule?
No. Only timed, one-on-one service minutes count toward the total. Untimed service-based codes like hot/cold packs or unattended e-stim are billed once per session and are excluded from the minute total.
What is the difference between the Medicare 8-minute rule and the AMA rule?
Medicare counts the total timed minutes across all services to set the unit total. The AMA (Rule of Eights) counts each timed code separately, so each service needs its own 8 minutes. Different payers follow different methods, which can change the units you can bill.