Your heart is a pump with a spec sheet, and its headline figure is cardiac output: the litres of blood it moves every minute. It is simply heart rate times stroke volume — how often it beats, times how much it ejects per beat. This cardiac output calculator computes CO from those two inputs, and — if you add height and weight — the cardiac index, which normalizes output to body size so a child’s circulation and a linebacker’s can be judged on the same scale.
The Cardiac Output Formula
CO = heart rate × stroke volume. With a resting rate of 72 bpm and a stroke volume of 70 mL, output is 72 × 70 = 5,040 mL/min = 5.04 L/min. The formula’s simplicity hides the physiology it summarizes: stroke volume itself is end-diastolic volume minus end-systolic volume — how much the ventricle filled, minus what it failed to eject — and it answers to preload (filling), contractility (squeeze), and afterload (the pressure it ejects against), the Frank-Starling machinery. Heart rate is the chronotropic dial. Cardiac output is where the two dials meet.
Cardiac Index: Output per Body Size
Five litres a minute means different things in a 50 kg adult and a 110 kg one. The cardiac index divides CO by body surface area: CI = CO ÷ BSA, with BSA estimated by the Mosteller formula √(height × weight ÷ 3600). For 175 cm and 70 kg, BSA ≈ 1.85 m², and the worked example’s index is 5.04 ÷ 1.85 ≈ 2.73 L/min/m² — inside the usual resting band of about 2.5 to 4.0. Indexing is why intensive care charts trend CI rather than raw CO: it tracks the patient’s own normal, not the species average.
Where the Numbers Go: Rest, Exercise, Trouble
At rest the typical adult circulates 4 to 8 L/min. Hard exercise multiplies output four- to six-fold — rate and stroke volume rising together — to 20 to 25 L/min in a healthy young adult and beyond 30 in elite endurance athletes. Clinically, a falling cardiac index is the number that organizes shock care: below about 2.2 L/min/m² with congestion present defines the cardiogenic shock picture, while a high output with low resistance is the distributive (septic) pattern. The calculator’s job is the arithmetic; those interpretations belong to monitored, clinical settings — but every one of them starts from HR × SV.
How Stroke Volume Is Found in Practice
Nobody measures stroke volume with a bucket. Echocardiography estimates it from the left-ventricular outflow tract’s diameter and the velocity-time integral of blood passing through it; the Fick principle infers output from oxygen consumption and the arterial-venous oxygen difference (CO = VO₂ ÷ (CaO₂ − CvO₂)); thermodilution catheters measure it from a temperature washout curve. Each method feeds this same formula’s inputs or validates its output. If you are working from an echo report, the SV it lists drops straight into the calculator with the heart rate recorded at the same moment — mismatched HR and SV from different hours produce a fiction.
The Two Dials Trade Off
The body defends cardiac output, and the formula shows the compensation: lose stroke volume (bleeding, a weakened ventricle) and heart rate climbs to hold the product; slow the rate (a beta blocker, athletic bradycardia) and a larger stroke volume carries the load — endurance athletes rest at 40 to 50 bpm precisely because a trained ventricle ejects 100 mL or more per beat. This trade is also the formula’s diagnostic hint: a high resting rate with a normal output is often compensation, not fitness, and the cause of the low stroke volume is the thing to find.
Common Mistakes
Forgetting the mL-to-L conversion inflates output a thousandfold — the calculator handles it, paper work often does not. Pairing a stroke volume from one moment with a heart rate from another manufactures a CO that never existed. Reading a single resting CO as a verdict ignores the wide normal band and the indexing question. And in exercise contexts, applying resting stroke volume at high heart rates overestimates output: filling time shortens as rate climbs, and SV plateaus or falls at extremes even as CO keeps rising on rate alone.
Related Tools
The circulation set continues in the Health Calculators hub — the Shock Index Calculator is the bedside ratio built from the same vitals, and the MAP Calculator covers the pressure side of the loop.
Frequently Asked Questions
How do you calculate cardiac output?
Multiply heart rate by stroke volume. At 72 bpm with a 70 mL stroke volume: 72 × 70 = 5,040 mL/min, or 5.04 L/min.
What is a normal cardiac output?
Roughly 4 to 8 litres per minute at rest for an adult, and a cardiac index of about 2.5 to 4.0 L/min/m² when normalized to body size. Hard exercise raises output to 20 L/min or more.
What is cardiac index and why is it used?
Cardiac output divided by body surface area. It lets clinicians compare circulation across body sizes and track one patient against the normal indexed band rather than a one-size range.
How is body surface area calculated here?
By the Mosteller formula: square root of (height in cm × weight in kg / 3600). For 175 cm and 70 kg that gives about 1.85 square metres.
What determines stroke volume?
Preload (how much the ventricle fills), contractility (how hard it squeezes), and afterload (the resistance it ejects against). Stroke volume equals end-diastolic volume minus end-systolic volume.
Why do athletes have low resting heart rates?
Training enlarges stroke volume — 100 mL or more per beat — so the same cardiac output is delivered with fewer beats. Rate and stroke volume trade off inside the CO formula to meet the body’s demand.
What cardiac index indicates cardiogenic shock?
In clinical practice, an index below about 2.2 L/min/m² together with signs of congestion and hypoperfusion defines the cardiogenic shock picture. That judgment is made in monitored settings with far more data than two vitals.