Enter a partial pressure of oxygen (pO₂) value in mmHg and this Oxygen Saturation Calculator computes your estimated blood oxygen saturation (SpO₂) using the Severinghaus equation. You'll see the calculated SO₂ percentage along with a saturation classification — useful for clinical reference and educational purposes.
Oxygen Saturation (SO₂)
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Saturation Category
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Desaturated Fraction
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This calculator uses the Severinghaus equation: SO₂ = (23,400 × (pO₂³ + 150 × pO₂)⁻¹ + 1)⁻¹. It was published by J.W. Severinghaus in the Journal of Applied Physiology (1979) and is widely used for estimating blood oxygen saturation from pO₂ values.
Normal blood oxygen saturation (SpO₂) for a healthy adult is typically 95–100%. Values below 90% are generally considered hypoxic and may require clinical attention. Values between 90–94% are borderline and should be monitored carefully.
pO₂ (partial pressure of oxygen) measures the amount of oxygen dissolved in blood plasma, expressed in mmHg. Oxygen saturation (SO₂ or SpO₂) reflects how much of the hemoglobin is carrying oxygen. The relationship between them follows the oxyhemoglobin dissociation curve — a sigmoidal shape meaning saturation rises steeply as pO₂ increases from about 20–60 mmHg, then levels off.
Yes. The Severinghaus equation provides an estimate and does not account for variations in body temperature, blood pH, or 2,3-DPG (2,3-diphosphoglycerate) concentration. These factors shift the oxyhemoglobin dissociation curve and affect the true saturation. For clinical decisions, always use direct measured values from pulse oximetry or arterial blood gas analysis.
SaO₂ refers to arterial oxygen saturation measured directly via arterial blood gas (ABG) analysis — considered the gold standard. SpO₂ is the peripheral oxygen saturation estimated non-invasively by a pulse oximeter. Both aim to measure the same thing, but SpO₂ carries a small margin of error (typically ±2%).
Using the Severinghaus equation, a pO₂ of approximately 60 mmHg corresponds to roughly 90% oxygen saturation. This is a clinically important threshold — below 60 mmHg, saturation drops steeply due to the shape of the oxyhemoglobin dissociation curve.
The Severinghaus equation was derived from adult data and is generally applied to adult patients. For neonates and pediatric patients, fetal hemoglobin (HbF) and other physiological differences affect the dissociation curve. This calculator should not be used as a clinical decision tool for pediatric care without consulting appropriate references.
At high altitudes, atmospheric pressure decreases, reducing the pO₂ in inhaled air. This lowers arterial pO₂ and consequently oxygen saturation. For example, at the summit of Everest (~8,849 m), arterial pO₂ can drop to around 25–35 mmHg, resulting in saturations well below 90% even in acclimatized climbers.