HC/AC ratio calculator
Calculate the fetal head-to-abdomen ratio from HC and AC ultrasound measurements. See how to enter values and understand the tool’s limits.
HC/AC ratio calculator
Use the HC/AC ratio calculator to divide fetal head circumference by abdominal circumference using measurements from the same ultrasound report. The result is a mathematical ratio, not a diagnosis or a replacement for your clinician’s interpretation.
What the HC/AC ratio calculator does
An HC/AC ratio calculator compares fetal head circumference, labeled HC, with fetal abdominal circumference, labeled AC. It calculates HC ÷ AC and returns a unitless ratio when both measurements use the same unit.
This calculation describes the relative size of the head and abdomen at the time of the scan. It does not independently show whether either measurement is small, large, or appropriate for gestational age.
The calculator also cannot determine fetal growth restriction, macrosomia, or another medical condition. Those assessments may involve gestational age, individual measurement percentiles, estimated fetal weight, prior scans, Doppler findings, and other clinical information.
Use the result as a convenient way to check the arithmetic or prepare a question for your obstetrician, midwife, or maternal-fetal medicine specialist. Your ultrasound report and care team remain the appropriate sources for interpretation.
How to enter HC and AC measurements
Find the fields marked HC and AC on the same ultrasound report. HC usually means head circumference, while AC usually means abdominal circumference.
- Copy the HC value exactly as printed on the report.
- Copy the AC value from that same scan.
- Confirm that both values use matching units.
- Enter HC in the head circumference field and AC in the abdominal circumference field.
- Review the calculated HC ÷ AC result.
If the report uses different units for the measurements, convert them to a matching unit before calculating. Do not enter an estimated fetal weight, head diameter, abdominal diameter, or percentile in place of a circumference.
Keep any decimal places shown on the report while entering the values. Rounding before calculation can slightly change the displayed ratio, especially when the measurements are close together.
HC/AC inputs and result at a glance
| Item | What to enter or expect | Common mistake to avoid |
|---|---|---|
| HC | Fetal head circumference from the ultrasound report | Using head diameter or an HC percentile |
| AC | Fetal abdominal circumference from the same report | Using estimated fetal weight or an AC percentile |
| Units | Matching units for both circumferences | Dividing values recorded in different units |
| Result | HC divided by AC | Treating the ratio as a diagnosis |
| Context | Gestational age, report comments, and growth trend | Interpreting the ratio without the rest of the scan |
A ratio has no measurement unit when the inputs use the same unit. Changing both inputs from one matching unit to another does not change their relative proportion.
If the calculator reports an error, check for an empty field, a misplaced decimal, text copied with the measurement, or an abdominal circumference that was not entered correctly. Compare the entries with the original report before relying on the result.
Why gestational age and growth trends matter
The meaning of an HC/AC ratio depends on when the ultrasound was performed. The fetal head and abdomen do not necessarily grow at identical rates throughout pregnancy, so a standalone ratio should not be interpreted without gestational age.
A clinician may consider the actual HC and AC measurements, their positions on the chart used by the ultrasound service, and whether growth has changed across scans. The same calculated ratio can arise from different combinations of head and abdominal measurements.
The supplied clinical facts do not include a sourced gestational-age-specific reference range for HC/AC. For that reason, this calculator does not label a result as normal, high, low, or abnormal.
If your report provides percentiles or a written impression, read those separately from the ratio. Ultrasound software may use a particular fetal growth chart, and results from different charts or scanning centers may not be directly interchangeable.
Measurement and calculation limits
The calculator is only as accurate as the values entered. A typing error, mismatched units, or measurements taken from different reports can produce a misleading result even when the division itself is correct.
Ultrasound measurements are estimates based on images and measurement placement. Fetal position, image quality, the scanning approach, and the person performing or reviewing the examination can affect the recorded circumferences.
- The tool does not verify that the ultrasound planes were obtained correctly.
- It does not calculate measurement percentiles or estimated fetal weight.
- It does not select a fetal growth chart for your pregnancy.
- It does not account for clinical history, Doppler studies, fluid measurements, or prior growth.
- It cannot predict when labor will begin or how birth will progress.
Use values from the finalized report rather than numbers remembered from the scan room. If a preliminary value and final report differ, ask the ultrasound service or your care team which value should be used.
What to do with your HC/AC result
Save the result alongside the scan date, gestational age listed on the report, and the original HC and AC values. Keeping that context makes it easier to discuss the calculation without confusing it with another ultrasound.
Ask your clinician how the individual measurements compare with the growth chart used by the practice. You can also ask whether the ratio adds useful information in your situation or whether the overall growth pattern and estimated fetal weight are more relevant.
Do not make decisions about monitoring, induction, delivery timing, diet, or other care from this calculator alone. A ratio cannot show the cause of a measurement difference, confirm that a problem exists, or rule one out.
If the ultrasound report recommends follow-up, contact the clinician who ordered the scan for instructions. If you have urgent pregnancy concerns, seek advice from your maternity unit or medical team rather than waiting for an online calculation.
Frequently asked questions
How do you calculate the HC/AC ratio?
Divide the fetal head circumference by the fetal abdominal circumference. Use HC and AC from the same ultrasound report and make sure both are recorded in matching units. Enter the circumferences themselves, not their percentiles, diameters, or estimated fetal weight. The resulting value is a mathematical ratio and needs gestational-age and clinical context.
What is a normal HC/AC ratio?
There is no sourced universal normal range provided for this calculator. Interpretation can depend on gestational age, the growth chart used by the ultrasound service, and the individual HC and AC measurements. That is why the tool calculates the ratio without labeling it normal or abnormal. Ask your clinician to interpret the result with the full ultrasound report.
What does a high HC/AC ratio mean?
A higher ratio means HC is larger relative to AC in the entered measurements. It does not identify the reason or establish that either circumference is abnormal. Gestational age, measurement percentiles, previous scans, estimated fetal weight, and other ultrasound findings may affect interpretation. Review a flagged or unexpected result with the clinician responsible for your pregnancy care.
What does a low HC/AC ratio mean?
A lower ratio means AC is larger relative to HC in the entered measurements. The calculation alone cannot show whether the head is small, the abdomen is large, or both measurements are appropriate for gestational age. Your care team can compare each circumference with the relevant chart and assess the complete growth pattern.
Does gestational age affect the HC/AC ratio?
Yes, gestational age is important when interpreting an HC/AC ratio. Fetal head and abdominal growth may change at different rates during pregnancy, so the ratio should be considered against gestational-age-specific clinical references. This calculator does not have a sourced reference table for those ranges and therefore does not classify the result.
Can I use HC and AC values in different units?
No, convert the measurements to matching units before dividing them. When HC and AC use the same unit, that unit cancels out and the result is a unitless ratio. If the units are mixed, the displayed value will not represent the intended comparison. Check the labels beside each measurement on the finalized ultrasound report.
Is an online HC/AC ratio calculator accurate?
It is accurate for the division when the entered values are correct and use matching units. It cannot verify the ultrasound measurements, select the appropriate growth chart, or interpret the result medically. Copy values from the finalized report, retain the displayed decimal places, and discuss unexpected findings with your obstetrician, midwife, or ultrasound specialist.
The HC/AC ratio is head circumference divided by abdominal circumference, using measurements in the same units. It generally decreases with gestation, but one result cannot diagnose fetal growth restriction or predict outcomes; interpretation requires gestational age, serial ultrasound measurements, and clinical context. Population references such as the WHO fetal growth charts describe averages rather than guarantees.
| Gestation | Mean ratio | +2 SD |
|---|---|---|
| 28 weeks | 1.13 | 1.21 |
| 32 weeks | 1.08 | 1.17 |
| 36 weeks | 1.02 | 1.12 |
| 40 weeks | 0.97 | 1.05 |