ABC/2 formula: how to estimate intracerebral hemorrhage volume
The ABC/2 formula estimates the volume of an intracerebral hemorrhage (ICH) from three measurements on a CT scan: A × B × C ÷ 2. For a bleed measuring 6 cm by 4 cm across, spanning 3 cm of slices, the volume is (6 × 4 × 3) ÷ 2 = 36.0 mL.
Not a medical device. Not a diagnosis or dosing recommendation.
What is ABC/2?
ABC/2 is a quick way to turn three straight-line measurements of a brain bleed into a volume in milliliters. It treats the hemorrhage as an ellipsoid, a rounded egg shape, and uses a simplified version of the ellipsoid volume formula.
It was described by Kothari and colleagues in 1996, so it's also called the Kothari method, ABC over 2, or the ABC2 volume.
The measurements come from a person reading the scan. The calculator doesn't look at images and doesn't choose A, B or C; it does the multiplication.
This page explains the arithmetic. It doesn't give a cutoff, and it doesn't say what a particular volume means for a patient.
The ABC/2 formula
volume_mL = (A_cm * B_cm * C_cm) / 2Exact expression the calculator uses:
volume_ml = (a_cm * b_cm * c_cm) / 2In words: multiply the three measurements in centimeters together, then halve the result. Because 1 cubic centimeter is 1 milliliter, the answer is in mL. Magent's calculator reports it to 1 decimal place.
What each part means
| Term | Plain-English meaning | Unit | Accepted range |
|---|---|---|---|
a_cm (A) | The greatest diameter of the bleed on CT | cm | 0.1–20 |
b_cm (B) | The largest diameter perpendicular to A | cm | 0.1–20 |
c_cm (C) | The number of slices showing the bleed multiplied by the slice thickness | cm | 0.1–20 |
volume_ml | Estimated hemorrhage volume | mL | 1 decimal place |
How the formula works
- Measure A, the longest diameter of the bleed, in centimeters.
- Measure B, the longest diameter at right angles to A.
- Work out C: count the slices that show the bleed and multiply by the slice thickness in centimeters.
- Multiply A × B × C.
- Divide by 2.
- Round to 1 decimal place.
The exact volume of an ellipsoid is 4/3 × π × (A/2) × (B/2) × (C/2), which works out to π/6 × A × B × C. The formula rounds π/6 to one half. That's where the "÷ 2" comes from.
Units
- Work in centimeters. CT viewers often show millimeters. Convert to centimeters before using the formula: 60 mm is 6 cm.
- Slice thickness in centimeters too. Convert the slice thickness the same way, so 6 slices at 5 mm give C = 3 cm.
- The answer is in milliliters. Mixing millimeters in by mistake makes the result far too large.
Worked examples
These use Magent's own expected values for the calculator.
Example 1: a 2 cm cube (A = 2, B = 2, C = 2)
- 2 × 2 × 2 = 8
- 8 ÷ 2 = 4.0 mL
Example 2: A = 6, B = 4, C = 3
- 6 × 4 × 3 = 72
- 72 ÷ 2 = 36.0 mL
Example 3: A = 3, B = 2, C = 1
- 3 × 2 × 1 = 6
- 6 ÷ 2 = 3.0 mL
Example 4: the smallest accepted dimension (A = 0.1, B = 2, C = 2)
- 0.1 × 2 × 2 = 0.4
- 0.4 ÷ 2 = 0.2 mL
Example 2 shows why a single long diameter can mislead: the bleed is 6 cm long, but its volume depends on all three measurements together.
ABC/2 reference table
These values are computed from the formula and rounded to 1 decimal, as the calculator reports them. Each row is one set of A, B and C. They show how the volume grows; they aren't targets or severity grades.
| A (cm) | B (cm) | C (cm) | A × B × C | Volume (mL) |
|---|---|---|---|---|
| 2 | 2 | 2 | 8 | 4.0 |
| 3 | 2 | 1 | 6 | 3.0 |
| 4 | 3 | 2 | 24 | 12.0 |
| 5 | 4 | 3 | 60 | 30.0 |
| 6 | 4 | 3 | 72 | 36.0 |
| 6 | 5 | 4 | 120 | 60.0 |
| 7 | 5 | 5 | 175 | 87.5 |
Doubling any one measurement doubles the volume. Doubling all three multiplies it by eight.
History of ABC/2
The method comes from Rashmi U. Kothari, Thomas Brott, Joseph P. Broderick and colleagues, The ABCs of measuring intracerebral hemorrhage volumes, published in Stroke in 1996 (volume 27, issue 8, pages 1304–1305).
The paper's abstract describes A as the greatest hemorrhage diameter by CT, B as the diameter perpendicular to A, and C as the approximate number of CT slices with hemorrhage multiplied by the slice thickness. The authors compared this bedside method with computer-assisted planimetric image analysis in 118 patients. They concluded that ICH volume can be estimated in less than 1 minute with the formula.
When it's used
A 2026 study in the European Stroke Journal notes that ABC/2 can be carried out by any clinician within a few minutes, and that it can still give a quick first estimate of hemorrhage volume as long as clinicians know it is a rough estimation.[1]
Where it's used
The same study compared ABC/2 with semi-automated and fully automated volume measurement in 300 consecutive adults with spontaneous hemorrhage above the tentorium, the sheet of tissue that separates the cerebrum from the cerebellum.[1] It found good agreement between different people doing ABC/2, but poor agreement between ABC/2 and its reference method.[1]
Who uses it
Published work frames ABC/2 as a clinician's bedside estimate that sits alongside software-based segmentation, which the 2026 study treated as the reference standard.[1]
Limits of the calculation
- It's an ellipsoid approximation, not a measured volume.
- The 2026 study explains that ABC/2 assumes an ellipsoid shape, while hemorrhages are often irregular, so it overestimates volume in irregularly shaped bleeds.[1]
- The calculator doesn't read a scan or pick A, B or C. Whoever enters the numbers chooses them.
- It isn't the ICH Score. That is a separate point score.
- It doesn't diagnose a hemorrhage or say what a volume means.
- Each measurement must be 0.1–20 cm. Values outside that are rejected, not estimated.
Clinical decisions belong to a qualified clinician who knows the patient.
How it differs from related calculators
- ICH Score is a 0–6 point total from Glasgow Coma Scale, age, hemorrhage volume, intraventricular blood and infratentorial origin. A volume such as the one ABC/2 gives is one of its inputs; ABC/2 itself gives a volume, not points.
Glossary
- Intracerebral hemorrhage (ICH): bleeding into the brain tissue itself.
- CT slice: one cross-sectional image from a computed tomography scan.
- Slice thickness: how much of the body each CT slice covers, usually shown in millimeters.
- Ellipsoid: a three-dimensional oval, like an egg or a rugby ball.
- Planimetry: measuring an area by tracing its outline, slice by slice, often with software.
Common mistakes
- Mixing millimeters and centimeters. Enter 60 mm as 6 cm.
- Forgetting to multiply the slice count by the slice thickness. C is a length in centimeters, not a number of slices.
- Measuring B at the wrong angle. B is the largest diameter perpendicular to A.
- Leaving out the ÷ 2. Example 2 would come out as 72 instead of 36.0.
- Treating ABC/2 as the ICH Score. One gives milliliters, the other gives points.
Source
Kothari RU, Brott T, Broderick JP, et al. The ABCs of measuring intracerebral hemorrhage volumes. Stroke. 1996;27(8):1304-1305. DOI: 10.1161/01.STR.27.8.1304
Further reading
- Colmer NHC, Fakhry R, de Haan Y, et al. Validity of intracerebral haemorrhage volume assessment: comparison of fully automated segmentation analysis with manual ABC/2 and semi-automated measurement. Eur Stroke J. 2026;11(1):aakaf020.
Frequently asked questions
How do you calculate ABC/2?
Multiply the largest diameter (A), the diameter at right angles to it (B) and the slice count times slice thickness (C), all in centimeters, then divide by 2. For 6, 4 and 3 cm, that's 36.0 mL.
What ICH volume is considered large?
This page doesn't give a cutoff. It calculates the volume; what a volume means depends on the clinical context.
Why divide by 2?
The volume of an ellipsoid is π/6 × A × B × C. Since π/6 is close to one half, the formula uses one half.
How do I get C?
Count the CT slices that show the bleed and multiply by the slice thickness in centimeters. Six 5 mm slices give 3 cm.
Is ABC/2 accurate?
It's an approximation. A 2026 study found it overestimates volume when a bleed is irregularly shaped.[1]
Who developed ABC/2?
Rashmi Kothari and colleagues, in a 1996 paper in Stroke.