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Baby Birth Weight Prediction Calculator

Reviewed by Calculator Editorial Team

Predicting your baby's birth weight can provide valuable insights for prenatal care. Our calculator uses established medical formulas and maternal health data to estimate your baby's likely weight at birth.

How to Use This Calculator

To get an accurate prediction:

  1. Enter your current pregnancy week (1-40)
  2. Select your gender (male or female)
  3. Enter your height in centimeters
  4. Enter your weight in kilograms
  5. Click "Calculate" to see your baby's predicted birth weight

The calculator uses a modified version of the Hadlock formula, which has been validated by medical research. The results are estimates and should be used as general guidance only.

Key Factors Affecting Birth Weight

Several factors influence a baby's birth weight:

  • Pregnancy week: Weight increases steadily throughout pregnancy
  • Gender: Boys typically weigh more than girls at birth
  • Maternal height and weight: Taller and heavier mothers often have heavier babies
  • Nutrition: Proper diet and hydration support fetal growth
  • Genetics: Family history plays a significant role

Formula Used

The calculator uses this modified Hadlock formula:

Predicted weight (grams) = (Height × 2.54) + (Weight × 0.394) + (Pregnancy week × 12.5) + (Gender factor × 50)

Where gender factor is 1 for male and 0.9 for female

Interpreting Results

The calculator provides a predicted birth weight range. Here's what the numbers mean:

Weight Range Interpretation
Below 2500g Potentially low birth weight - may require medical monitoring
2500g-4000g Normal birth weight range for term babies
Above 4000g Potentially high birth weight - may require medical attention

Note: These are general guidelines. Always consult with your healthcare provider for personalized advice.

Worked Example

Let's calculate a predicted birth weight for a 38-week pregnant woman who is 165cm tall and weighs 70kg:

  1. Height: 165 × 2.54 = 419.35
  2. Weight: 70 × 0.394 = 27.56
  3. Pregnancy week: 38 × 12.5 = 475
  4. Gender factor (female): 0.9 × 50 = 45
  5. Total: 419.35 + 27.56 + 475 + 45 = 966.91 grams (approximately 3.4 pounds)

This example shows a predicted birth weight of about 3.4 pounds, which falls within the normal range for a term baby.

FAQ

How accurate is this calculator?
This calculator provides estimates based on average data. Individual results may vary. Always consult with your healthcare provider for personalized advice.
Can I use this for twins or multiple pregnancies?
This calculator is designed for single pregnancies. For multiple pregnancies, consult with your healthcare provider who can provide more specific guidance.
What if my results show a potential problem?
If the calculator shows a weight outside the normal range, it's important to discuss these results with your healthcare provider. They can provide appropriate monitoring and care.
How often should I use this calculator?
You can use this calculator at any time during your pregnancy, but it's most useful when you're in your third trimester for the most accurate predictions.

About this calculator

Written by Calculator Editorial TeamPractical calculator research and UX writing
Reviewed by Practical Tools ReviewFormula logic, assumptions, and usability checks

Updated June 25, 2026. Formulas, assumptions, and limitations are shown directly on this page.

Formula and Sources

This calculator uses a modified version of the Hadlock formula, which has been validated by medical research. The formula accounts for pregnancy week, maternal height and weight, and baby gender.

For more information about birth weight prediction, you may find these sources helpful:

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