Pregnancy Blood Volume Calculator
During pregnancy, maternal blood volume expands to support the growing uterus and placenta. This calculator estimates your baseline (non-pregnant) blood volume from body weight, then applies an editable expansion percentage to estimate pregnancy blood volume. Because published expansion figures vary by individual and gestational age, every factor here is a user-editable input with a sensible default. This is an educational planning estimate only, not a clinical measurement or a basis for any medical decision.
Blood volume formula
Baseline (mL) = weight (kg) * volume per kg (mL/kg)
Added (mL) = baseline * (expansion% / 100)
Pregnancy volume (mL) = baseline + added
Litres = millilitres / 1,000
The model is intentionally simple: scale body weight by a per-kilogram blood volume estimate, then increase it by the chosen expansion percentage. Both the 70 mL/kg figure and the expansion percentage are defaults you can change to match your own reference.
Worked example
For a 65 kg person, 70 mL/kg baseline, and 45 percent expansion:
- Baseline = 65 * 70 = 4,550.00 mL.
- Added = 4,550 * 0.45 = 2,047.50 mL.
- Pregnancy volume = 4,550 + 2,047.50 = 6,597.50 mL (6.60 litres).
Pregnancy blood volume: frequently asked questions
How much does blood volume increase in pregnancy?
Maternal blood volume rises substantially during pregnancy, with most references describing an increase on the order of 40 to 50 percent above the non-pregnant baseline by the third trimester. The exact figure varies by individual, by single versus multiple pregnancy, and by the week of gestation. This calculator uses an editable expansion percentage so you can match the figure your own clinician or reference uses.
How is non-pregnant blood volume estimated?
This tool uses the common approximation of about 70 millilitres of blood per kilogram of body weight for adults, an editable default. Multiplying your weight in kilograms by that figure gives an estimated baseline blood volume in millilitres, which is then expanded by the pregnancy expansion percentage you enter.
Is this a medical or diagnostic tool?
No. This is an educational estimator. Real blood volume is measured clinically and depends on many factors this simple model does not capture. Do not use it for dosing, transfusion planning, or any clinical decision. Always rely on your healthcare provider's measurements.
Why are the percentages editable rather than fixed?
Because no single fixed figure is universally true. Published ranges differ, and the increase depends on gestational age and individual physiology. Following our sourcing rules, we never hardcode a contested figure as if it were exact; instead we give you a sensible default you can change to match your reference.
Official sources
- U.S. National Library of Medicine, MedlinePlus: Pregnancy.
- National Heart, Lung, and Blood Institute (NIH): Blood.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.