π ICH Volume Calculator (ABC/2)
The radiologist's ABC/2 shortcut for bleed volume.
For informational purposes only, not medical advice. Consult a healthcare professional.
The ABC/2 method estimates bleed volume from three CT measurements: A, the largest bleed diameter on any slice; B, the diameter perpendicular to A on the same slice; and C, the number of slices showing blood multiplied by slice thickness. Multiply the three and divide by two. The result is the volume in cubic centimeters, and it matches planimetry (pixel-by-pixel measurement) closely enough for clinical use.
Volume drives decisions. The ICH Score assigns a point at 30 cm3 or more, surgeons weigh volume when considering evacuation, and expansion of even a few milliliters in the first hours predicts worse outcomes. A fast bedside number beats a precise number that arrives tomorrow.
How to Use This Calculator
Pull the three numbers from the CT. Measure A on the slice where the bleed looks biggest, in centimeters. Measure B at right angles to A on that same slice. Count every slice where blood is visible and multiply by the slice thickness from the scan protocol (usually 5 mm, which is 0.5 cm). Enter all four values and read the volume.
Find the slice with the largest bleed and measure its longest diameter (A) in cm.
Measure the widest line perpendicular to A on the same slice (B) in cm.
Count the slices that show any blood and note the slice thickness in mm.
Read the ABC/2 volume and whether it crosses the 30 cm3 ICH Score cutoff.
Why Dividing by Two Works
C = slice count x slice thickness
A bleed is roughly an ellipsoid, and the volume of an ellipsoid is (4/3) x pi x (A/2) x (B/2) x (C/2), which simplifies to about 0.52 x A x B x C. Rounding 0.52 down to 0.5 gives ABC/2, a formula anyone can do at the bedside. Validation studies against computer-measured volumes found ABC/2 runs within about 10 to 15 percent for typical rounded bleeds. It overestimates irregular or multilobed bleeds, which is worth knowing when the shape looks odd on the scan.
Worked Example
A bleed measuring 4.2 cm at its widest and 3.1 cm perpendicular, visible on 9 slices at 5 mm thickness, gives C = 4.5 cm and a volume of 29.3 cm3. That crosses the 30 cm3 line (30 cm3 cutoff: Not reached), which is exactly the volume that earns a point on the ICH Score. Shrink A to 2.5 cm and the volume drops to about 17.4 cm3, a different risk category from the same slice count.
How to Read the Volume
Under about 15 cm3 counts as a small bleed with generally better odds. Between 15 and 30 is moderate and deserves close watching for expansion in the first day. At or above 30 cm3 the ICH Score adds its volume point, and published mortality for the overall score climbs steeply. Location matters as much as size: 30 cm3 in the frontal lobe behaves differently from 30 cm3 in the brainstem, which is why the ICH Score also scores infratentorial origin separately.
A Measurement, Not a Verdict
ABC/2 is the standard bedside estimate radiologists and stroke teams use, but the official volume on the radiology report is the number that guides care. If you are estimating from images yourself, confirm against the report before drawing conclusions. Bleeds can expand early, so a volume measured at hour one may not match hour six. For informational purposes only, not medical advice. Consult a healthcare professional.
Related tools: ICH Score calculator, NIH Stroke Scale and mean arterial pressure.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is the ABC/2 method?
A bedside formula for bleed volume: multiply the largest diameter (A) by the perpendicular diameter (B) by the bleed height (C, from slice count times thickness), then divide by two. It approximates the ellipsoid volume formula.
How accurate is ABC/2?
Within roughly 10 to 15 percent of computer-measured volumes for rounded bleeds. It tends to overestimate irregular, finger-like, or multilobed hemorrhages.
What is a normal slice thickness for C?
Most non-contrast head CT protocols use 5 mm slices through the brain, so 9 slices means C = 4.5 cm. Thinner slices (2.5 mm) are common on newer scanners, so always check the protocol.
Why is 30 cm3 the important cutoff?
The ICH Score gives one point for volume at or above 30 cubic centimeters because that threshold separated survivors from non-survivors in the original study. It is the single most quoted volume number in hemorrhagic stroke.
Can I use ABC/2 for subdural hematomas?
No. ABC/2 was validated for roundish intraparenchymal bleeds. Subdurals are crescent-shaped and need different measurement methods, usually maximal thickness.
Does bleed volume change after the first scan?
Yes, and early. Hematoma expansion happens most often in the first 3 to 6 hours, which is why blood pressure control and repeat imaging matter in that window.
What does cm3 mean on a brain scan?
One cubic centimeter equals one milliliter. A 30 cm3 bleed is about two tablespoons of blood inside the brain, which is a lot in a closed box.
Who should measure bleed volume?
The radiologist's official measurement on the CT report is the clinical standard. This calculator reproduces the same ABC/2 math so you can follow along or estimate at the bedside.