Bladder volume
Calculate bladder volume or post-void residual on ultrasound from three diameters, with a choice of coefficient and the thresholds used for residual urine.
Dimensions
Residual
77mL
Residual 77 mL
A residual of 77 mL is borderline. It is common in older patients without symptoms; tie it to the voiding complaints before drawing conclusions.
This result uses a coefficient of 0.52. The geometric value is 0.52; many bladder scanners use 0.7 and therefore read systematically higher. State which you used, or a follow-up measurement will not be comparable.
At low filling the bladder is rarely a neat ellipsoid, and that is exactly where the relative error is largest. This method cannot reliably distinguish a residual of 30 from one of 60 mL.
How to read the result
For a filled bladder the number mostly describes measuring conditions. For a residual it describes emptying, and that is where the thresholds matter.
The formula
Volume (mL) = transverse × anteroposterior × craniocaudal × coefficient
The geometric ellipsoid coefficient is 0.52 (π/6). Many bladder scanners use 0.7 and therefore report higher volumes for the same bladder.
Measure the transverse and anteroposterior diameters on the transverse view and the craniocaudal on the longitudinal view, in centimetres.
For a post-void residual, measure as soon as possible after voiding: the bladder refills faster than most protocols assume.
When to use it
- Measuring a post-void residual as part of a urological ultrasound.
- Checking whether the bladder is adequately filled before assessing the wall or the prostate.
- Quantifying retention in a patient with voiding complaints.
- Comparing with a bladder scanner reading, which usually uses a different coefficient.
⚠️ Pitfalls
- State the coefficient. A volume calculated with 0.7 is roughly a third higher than the same bladder at 0.52, and comparisons across methods are meaningless without it.
- At low filling the bladder is not an ellipsoid, and the relative error is largest exactly where small residuals are being judged.
- Measure the residual immediately after voiding. Every minute of delay adds urine and turns a normal result into a borderline one.
- A single raised residual is weak evidence. Repeat it before it ends up in a conclusion.
- A large residual is a finding, not a diagnosis. Look for the cause, and check the upper tracts for hydronephrosis.
Frequently asked questions
How is bladder volume calculated on ultrasound?
By multiplying the three diameters and a coefficient. The geometric ellipsoid coefficient is 0.52, while many bladder scanners use 0.7.
What is a normal post-void residual?
Below 50 mL is normal. Between 50 and 100 mL is borderline and common in older patients, and above 100 mL warrants a repeat measurement before conclusions are drawn.
Why does my result differ from the bladder scanner?
Most scanners use a coefficient of 0.7 rather than the geometric 0.52, so they read systematically higher for the same bladder.
How full should the bladder be to assess the wall?
Roughly 150 to 400 mL. Below that the wall looks falsely thickened, and above it the bladder is uncomfortable and the wall falsely thin.
Does a raised residual mean obstruction?
Not by itself. It can reflect detrusor underactivity, medication or an incomplete void on the day. It is a finding to explain, not a diagnosis.
References
📄Dicuio M, et al. Measurements of urinary bladder volume: comparison of five ultrasound calculation methods in volunteers. Arch Ital Urol Androl. 2005;77(1):60–62.
📄EAU Guidelines on Non-neurogenic Male Lower Urinary Tract Symptoms.↗This calculator supports clinical judgement and does not replace it. Always check the result against your own measurements and the guideline in force.
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