Abdominal circumference (cm)

Femur length (cm)

Updates automatically as you type.

Calculated in your browser; your inputs are not sent to us. Privacy

Results will appear here.

General estimate for planning only — not medical advice. Consult a doctor about your specific situation.

Calculation details

Every figure above is calculated locally in your browser from the assumptions shown. No inputs are sent anywhere. See the methodology section below for how this tool works.

Use this resultShare the current inputs, or ask ChatGPT to explain the calculation in context.

Ask an AI to explain this result

Review the generated output before relying on it.

This tool runs in your browser. Your calculator inputs and results are never transmitted to us or to ad/affiliate partners. Ads and sponsored links may set third-party cookies to serve and measure them, but they never receive your calculation values. If you explicitly save a scenario, its permitted fields stay in local browser storage until you clear them. See our Privacy Policy.

Use this result

Share the current inputs or ask ChatGPT to explain the calculation in context.

Include calculator values in URL

More options

Report a calculation issue

Hadlock fetal weight formula components ranked by contribution — The AC term (1.69) dominates the 2,180 g estimate.

Method & verification

How This Tool Works

fact-checked against our calculation methodology

Calculation Method

Computes Estimated fetal weight from Abdominal circumference and Femur length.

Site methodology

BoringToolsKit methodology and verification standard

No unrelated authority link is added merely to make this page look sourced. Tool-specific external sources are shown only when they directly support the method, rate, or rule.

Assumptions to Verify

These tools provide general estimates for planning and education only.
They are not medical advice and not a substitute for a qualified healthcare professional.
Always consult your doctor or another healthcare provider about your specific situation.

Privacy: The calculation runs in this browser. Shared links contain the page address only, not your inputs or results.

Worked example: AC 32.0 cm, FL 6.5 cm

The Hadlock AC+FL formula is log10(weight in grams) = 1.304 + 0.05281*AC + 0.1938*FL - 0.0044*AC*FL. Take a 34-week report reading AC 32.0 cm and FL 6.5 cm. Step by step: 0.05281*32.0 = 1.68992; 0.1938*6.5 = 1.25970; 0.0044*32.0*6.5 = 0.91520. Sum: 1.304 + 1.68992 + 1.25970 - 0.91520 = 3.33842.

Raise 10 to that power: 10^3.33842 = about 2180 g. Convert to pounds: 2180 / 453.592 = 4.81, so the page shows 2,180 g (4.8 lb). The interaction term (the -0.0044*AC*FL piece) subtracts 0.915 here, which is why you cannot just add the AC and FL contributions linearly; it corrects for the fact that head-to-femur proportions differ between fetuses of the same abdominal size.

Run the same inputs at AC 33.0 with FL unchanged and the estimate rises to about 2,394 g, roughly 214 g for a single centimeter of abdominal circumference. That sensitivity is the whole reason the sonographer measures AC three times and averages.

Where the estimate breaks down

At term, a typical Hadlock estimate carries a margin near plus or minus 15 percent. On a 3,400 g fetus that is about 510 g in either direction, so a read of 2,950 g and a read of 3,450 g are both consistent with the same baby. Studies comparing estimated to actual birth weight put mean absolute error around 10 percent, and no third-trimester formula does meaningfully better, so treat a single reading as a screening number, not a weight on a scale.

Accuracy degrades at the extremes the formula was trained on. Estimates above roughly 4,500 g systematically undershoot actual birth weight, which matters if macrosomia is the clinical question; below about 1,000 g the error band widens further. If your percentile lands under the 10th or above the 90th, the follow-up is a repeat scan or a growth series, not a one-off number.

AC is the dominant input and also the hardest to measure: off-angle images or the baby's breathing can swing it several percent, and the squared interaction with FL amplifies that error. Growth-restricted fetuses with an abnormal head-to-abdomen ratio are another known bias source, since the formula pools head and belly proportions from a normal-growth reference population.