Qmax urine flow rate explained

Qmax: peak urine flow rate

Qmax is the fastest rate your urine flows during a flow test (uroflowmetry), measured in milliliters per second (mL/s). A low Qmax can mean the outflow from the bladder is blocked, for example by an enlarged prostate. In a study of 1,271 men, those with a blockage had a lower average Qmax (9.7 vs 12.6 mL/s)¹.

A plain-language definition from the ProcedurePath glossary.

Written and reviewed by Varun Danda, MD, interventional radiology resident physician · Last reviewed October 25, 2026

How the test works

You urinate into a special funnel or toilet that measures the flow. The machine draws a curve of your stream and reports the peak, which is the Qmax, along with how much urine you passed.

What it can and cannot tell you

In the same study, a cutoff of 10 mL/s helped point to a blockage, but flow tests could not replace pressure-flow tests, which also measure the pressure inside the bladder¹. A low Qmax says the stream is weak. It does not say why on its own.

Read more: Prostate size and anatomy terms

Common question

Does a low Qmax always mean a blocked prostate?

No. Flow tests help, but they cannot replace pressure-flow tests for finding a blockage¹. Ask your doctor what your result means.

Talk with your doctor before you decide. This information does not replace personal medical advice. Read the medical disclaimer.

Sources

Reynard et al., the ICS-BPH Study: uroflowmetry, urinary symptoms and bladder outlet obstruction, British Journal of Urology 1998 (PMID 9839573) ↩︎