Postpartum Blood Loss Calculator
Quantitative blood loss after birth is more accurate than visual estimation. The gravimetric method weighs blood-soaked items and subtracts their dry weight; since one gram of blood is taken as one milliliter, the difference in grams equals the volume in milliliters. Add any directly measured volume, such as from a calibrated drape, for the total. This calculator does that arithmetic and compares the total against the postpartum hemorrhage threshold, which defaults to 1,000 milliliters per the current reVITALize definition used by ACOG. It is an educational aid, not a bedside measurement or medical advice.
Gravimetric blood loss formula
Gravimetric volume = soaked weight - dry weight (1 g = 1 mL)
Total blood loss = gravimetric volume + directly measured volume
Status = at or above threshold means meets hemorrhage definition
Percent of threshold = total / threshold * 100
One gram of blood is treated as one milliliter because blood density is near 1 g/mL. The default 1,000 mL threshold follows the current reVITALize definition adopted by ACOG.
Blood loss context
- Postpartum hemorrhage is defined as 1,000 mL or more, or loss with signs of low blood volume, within 24 hours.
- Older teaching used 500 mL for vaginal birth; the threshold here is editable.
- Quantitative measurement is more accurate than visual estimation.
- One gram of blood is taken as one milliliter of volume.
- This tool is educational arithmetic only and is not medical advice.
Postpartum blood loss: frequently asked questions
How is blood loss estimated by the gravimetric method?
The gravimetric method weighs blood-soaked items and subtracts their known dry weight. Because one gram of blood is taken as roughly one milliliter, the difference in grams gives the volume in milliliters. Adding any directly measured volume, such as from a calibrated drape, gives total quantitative blood loss.
What counts as postpartum hemorrhage?
The current U.S. obstetric definition (reVITALize, used by ACOG) is cumulative blood loss of 1,000 milliliters or more, or blood loss with signs and symptoms of low blood volume, within 24 hours of birth, regardless of delivery route. Older teaching used 500 milliliters for vaginal birth; this calculator defaults to the 1,000 milliliter threshold and lets you change it.
Why is quantitative blood loss preferred over visual estimation?
Visual estimation tends to underestimate true blood loss, especially when it is heavy. Weighing soaked materials and measuring collected volume gives a more accurate figure, which supports earlier recognition of hemorrhage. Many hospitals have adopted quantitative measurement for this reason.
What is the one gram equals one milliliter assumption?
Blood has a density very close to one gram per milliliter, so the weight of absorbed blood in grams is treated as an equal volume in milliliters. This is the standard simplifying assumption in gravimetric quantitative blood loss measurement.
Is this calculator medical advice?
No. It performs the gravimetric arithmetic and a threshold comparison for general information and education. Actual measurement and management of blood loss must be done by clinicians at the bedside. Always rely on the care team.
Official sources
- American College of Obstetricians and Gynecologists: postpartum hemorrhage guidance.
- U.S. National Library of Medicine, MedlinePlus: postpartum hemorrhage.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.