Foetal Weight Estimate Calculator

Work out foetal weight estimate instantly with clear inputs, formula shown and shareable results.

About Foetal Weight Estimate Calculator

Applies Johnson's formula, which multiplies the symphysis-fundal height minus an engagement constant by 155 to estimate foetal weight.

Formula

Johnson's formula

Estimated weight (g) = (fundal height in cm - 12 if the head is not engaged, or 11 if engaged) x 155

Worked example

With the default values below, Foetal Weight Estimate Calculator returns Estimated foetal weight of 3,410. Change any input above and every figure here is recalculated in your browser.

Inputs used

Symphysis-fundal heightaccepts 15–45 cm: 34 cm
Presenting partone of Not engaged (subtract 12), Engaged (subtract 11): Not engaged (subtract 12)

Results

Estimated foetal weight: 3,410 g
Estimated weight: 3.410 kg
Estimated weight: 7.52 lb

Step by step

1Values used
2Johnson's formula
3Estimated foetal weight= 3,410 g
4Estimated weight= 3.410 kg
5Estimated weight= 7.52 lb

Assumptions and limits

The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Educational estimate only. Not a substitute for professional clinical judgement, and must not be used to make treatment or dosing decisions.
This is an estimate for general information and not professional medical advice. See the full disclaimer for the limits that apply to this kind of calculator.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Educational estimate only. Not a substitute for professional clinical judgement, and must not be used to make treatment or dosing decisions.

Frequently Asked Questions

How accurate is Johnson's formula?

It is a clinical estimate with wide error margins; ultrasound biometry using Hadlock formulae is far more precise.

Does maternal size affect it?

Yes. Obesity, fibroids and polyhydramnios all distort fundal height and therefore the estimate.