How this instrument works
Stroke volume is the amount of blood the left ventricle pushes out with a single contraction. An echocardiogram or cardiac MRI measures how full the chamber gets right before it squeezes, called end-diastolic volume, and how much is left behind right after, called end-systolic volume. Subtract the second from the first and what remains is the volume that actually left the chamber on that beat.
It sits underneath a separate, more familiar figure: ejection fraction takes the identical subtraction and reports it as a percentage of the filled chamber rather than a raw milliliter count. Two ventricles can eject the same fraction of their contents while moving very different volumes of blood, because a larger chamber ejecting the same percentage still pushes out more milliliters — which is exactly why cardiologists track both numbers rather than one standing in for the other.
Multiply this figure by heart rate and the product is cardiac output, the total volume the heart moves per minute rather than per beat — another calculation this site keeps separate because it answers a different question. A typical resting adult figure runs somewhere in the 60-100 mL range, though normal varies with body size, so a single reading is descriptive rather than a verdict on heart health by itself.
- Enter End-diastolic volume (mL), the chamber's fill just before contraction.
- Enter End-systolic volume (mL), what remains in the chamber right after contraction.
- Read Stroke volume (mL) — the straight subtraction of the two.
- Multiply by heart rate separately if you want cardiac output in mL per minute.
Worked example — EDV 120 mL, ESV 50 mL
An echocardiogram reports an end-diastolic volume of 120 mL and an end-systolic volume of 50 mL. Stroke volume is the plain difference: 120 minus 50 leaves 70 mL ejected on that single beat, comfortably inside the typical resting range for an adult.
Take the same 120 mL chamber but imagine it only empties down to 90 mL instead of 50. Stroke volume drops to 30 mL, a much weaker beat from an identically sized chamber — the arithmetic is unchanged, but a smaller gap between the two readings always means less blood actually moved, regardless of how large the chamber starts out.
Questions
How is this different from ejection fraction?
Both start from the same two readings, end-diastolic and end-systolic volume, but ejection fraction divides the difference by end-diastolic volume and reports a percentage, while this figure reports the raw milliliters ejected. A dilated heart can post a normal or even large stroke volume while its ejection fraction is low, because a bigger starting chamber can move plenty of blood while still emptying an unimpressive share of itself.
What is a normal stroke volume?
Roughly 60 to 100 mL per beat for an average-sized adult at rest, though the figure scales with body and heart size, so a smaller person's normal reading sits lower without indicating anything abnormal. Clinicians generally weigh stroke volume alongside ejection fraction and chamber size rather than judging it against a single fixed cutoff.
How does stroke volume turn into cardiac output?
Multiply stroke volume by heart rate. A 70 mL stroke volume at a resting rate of 70 beats per minute gives a cardiac output of 4,900 mL, or about 4.9 liters, per minute — the total blood volume the heart circulates each minute rather than each individual beat, and a separate calculation this site keeps apart from stroke volume itself.
Why do EDV and ESV vary between an echocardiogram and an MRI?
Different imaging methods estimate chamber volume with different assumptions and margins of error, so the same heart can return a slightly different EDV or ESV depending on which technique produced the numbers. Small shifts in the input readings move the resulting stroke volume proportionally, which is one reason trends across repeated scans of the same type matter more than one isolated pair of numbers.
Does a low stroke volume always mean a weak heart?
Not on its own. A low reading can reflect a genuinely reduced pumping strength, but it can also reflect a smaller body frame, a fast heart rate that shortens filling time, or measurement variability between scans. This calculator performs the subtraction; interpreting what a given figure means for a specific person is a clinical judgment that also weighs chamber size, heart rate, and the person's overall condition.
References
- American Heart Association — Ejection Fraction Heart Failure Measurement
- NHLBI — How the Heart Works
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.