The average adult has roughly 4,500 to 5,700 grams of blood in their body. That’s about 4.5 to 5.7 kilograms, or 10 to 12.5 pounds. The exact number depends on your size, sex, and a few other factors, but blood consistently makes up about 7% to 8% of total body weight.
How the Gram Estimate Is Calculated
Blood volume is usually measured in liters, so converting to grams requires one extra step: accounting for blood’s density. Whole blood is slightly denser than water, with a specific gravity of about 1.057 grams per milliliter. That means one liter of blood weighs roughly 1,057 grams rather than the 1,000 grams a liter of water would weigh.
The standard medical estimate for blood volume is 75 milliliters per kilogram of body weight for adult men and 65 milliliters per kilogram for adult women. For a 70 kg (154 lb) man, that works out to about 5,250 mL of blood, or approximately 5,550 grams. For a 60 kg (132 lb) woman, it comes to about 3,900 mL, or roughly 4,120 grams. Here’s how those numbers look across a range of body sizes:
- 60 kg (132 lb) woman: ~3,900 mL → ~4,120 grams
- 70 kg (154 lb) woman: ~4,550 mL → ~4,810 grams
- 70 kg (154 lb) man: ~5,250 mL → ~5,550 grams
- 90 kg (198 lb) man: ~6,750 mL → ~7,130 grams
Why Men Typically Have More Blood
The 75 vs. 65 mL/kg difference between men and women isn’t just about body size. Men generally carry more lean muscle tissue, which is heavily supplied with blood vessels, while women tend to have a higher proportion of body fat, which requires less blood flow. Even when a man and woman weigh exactly the same, the man will usually have several hundred more grams of blood circulating through his body.
What Makes Up Those Grams
About 55% of blood by volume is plasma, a pale yellow fluid that is mostly water with dissolved proteins, salts, and hormones. The remaining 45% is made up of cells, predominantly red blood cells, with white blood cells and platelets making up a small fraction. Red blood cells are denser than plasma, which is why whole blood’s density sits above that of water. If you separated the components, the cellular portion would account for a disproportionately large share of the total mass despite being a smaller share of the volume.
When Blood Mass Changes
Your blood mass isn’t a fixed number. It shifts in response to hydration, altitude, fitness level, and certain medical conditions. Dehydration reduces plasma volume, temporarily lowering total blood mass by hundreds of grams. Living at high altitude triggers your body to produce more red blood cells to compensate for thinner air, gradually increasing blood mass over weeks.
Pregnancy causes the most dramatic normal change. Total blood volume rises by about 45% on average, though increases anywhere from 20% to 100% have been documented. Both plasma volume and red blood cell production increase, with plasma expanding faster and further. For a woman who started with around 4,100 grams of blood, that 45% increase would bring her close to 6,000 grams by the third trimester.
The Quick Body Weight Shortcut
If you want a fast estimate without pulling out a calculator, the simplest approach is the 7% to 8% rule from the American Society of Hematology. Multiply your body weight in grams by 0.07 or 0.08. A person weighing 75 kg (75,000 grams) carries roughly 5,250 to 6,000 grams of blood. This range is broad enough to cover most healthy adults regardless of sex, and it lands close to the more precise per-kilogram calculations described above.
For context, losing about 750 to 1,500 grams of blood (roughly 15% to 30% of total volume) is enough to cause noticeable symptoms like a rapid heartbeat, dizziness, and anxiety. Losses beyond 2,000 grams in an average adult become life-threatening. A standard blood donation removes about 473 mL, which works out to approximately 500 grams, or roughly 10% of a typical adult’s total supply. Your body replaces the plasma portion within a day or two, while the red blood cells take several weeks to fully regenerate.