Hemoglobin Saturation Calculator

Calculator Inputs

Arterial PO₂

PO₂ / PCO₂ Unit

Arterial PCO₂

Blood pH

Temperature (°C)

Hemoglobin (g/dL)

FiO₂ (%)

Atmospheric Pressure (mmHg)

Water Vapor Pressure (mmHg)

Respiratory Quotient

Hill Coefficient

Measured SaO₂ (%)

Compare entered measured saturation with the predicted saturation

Example Data Table

Case	PO₂	pH	PCO₂	Temp °C	Hb g/dL	Predicted SaO₂
Normal adult	95 mmHg	7.40	40 mmHg	37	15.0	~97%
Mild hypoxemia	70 mmHg	7.38	42 mmHg	37	14.0	~93%
Acidosis shift	60 mmHg	7.25	55 mmHg	38.5	13.2	Lower than standard
Alkalosis shift	60 mmHg	7.52	30 mmHg	36	13.2	Higher than standard

Formula Used

The calculator estimates hemoglobin saturation with a Hill equation form:

SaO₂ = (PO₂ⁿ / (PO₂ⁿ + P50ⁿ)) × 100

Here, n is the Hill coefficient and P50 is the oxygen pressure giving 50% saturation. The calculator adjusts P50 using pH, temperature, and carbon dioxide to reflect left or right shifts in oxygen affinity.

Estimated bicarbonate uses:

HCO₃⁻ = 0.03 × PCO₂ × 10^(pH − 6.1)

Alveolar oxygen uses the alveolar gas equation:

PAO₂ = FiO₂ × (P_atm − P_H₂O) − (PACO₂ / RQ)

Oxygen content is estimated by:

CaO₂ = 1.34 × Hb × SaO₂ + 0.0031 × PO₂

How to Use This Calculator

Enter arterial oxygen pressure and carbon dioxide pressure in either mmHg or kPa.
Provide blood pH, temperature, and hemoglobin concentration.
Add inspired oxygen fraction, atmospheric pressure, water vapor pressure, and respiratory quotient.
Optionally enter a measured saturation value for direct comparison.
Submit the form to view saturation, P50, oxygen content, and graph output.
Use CSV or PDF export buttons to save the calculated results.

Frequently Asked Questions

1. What does this calculator estimate?

It estimates hemoglobin oxygen saturation from arterial oxygen pressure and related blood gas variables. It also reports oxygen content, bicarbonate, alveolar oxygen, and affinity shifts.

2. Why does pH affect saturation?

Lower pH usually shifts the oxygen dissociation curve rightward. That reduces hemoglobin affinity, making oxygen unloading easier and saturation lower at the same PO₂.

3. What is P50?

P50 is the oxygen partial pressure at which hemoglobin is 50% saturated. A higher P50 suggests lower affinity, while a lower P50 suggests higher affinity.

4. Why include temperature and PCO₂?

Both temperature and carbon dioxide shift hemoglobin affinity. Higher values generally promote a right shift, changing the saturation predicted from the same oxygen pressure.

5. Is this a clinical diagnosis tool?

No. It is an educational and estimation tool based on common physics and physiology relationships. Clinical decisions should rely on validated instruments and professional judgment.

6. What does the A–a gradient show?

The alveolar–arterial gradient compares calculated alveolar oxygen with measured arterial oxygen. Larger differences can suggest impaired gas transfer or ventilation-perfusion mismatch.

7. Why might measured saturation differ from predicted saturation?

Differences can come from dyshemoglobins, measurement error, unusual physiology, altered affinity, or model simplifications. The comparison helps identify when the estimate and observation diverge.

8. Can I export the result summary?

Yes. The page includes CSV and PDF export options for the main calculated outputs, so results can be saved, shared, or reviewed offline.