Blood pH & Henderson-Hasselbalch Blood Gas Calculator

What Does This Blood Gas Calculator Do?

This free tool takes the three numbers found on almost every arterial blood gas (ABG) report — pH, PaCO2, and HCO3 — and runs them through the same equation doctors, nurses, and respiratory therapists use every day to make sense of a patient's acid-base status. It's built for students learning acid-base physiology for the first time, as well as anyone in nursing, medical, respiratory therapy, or paramedic training who wants to check their own hand-worked practice problems instantly.

Type in your three values and the calculator does three things at once: it uses the Henderson-Hasselbalch equation to work out what pH those numbers predict, it walks through the standard bedside logic to name the primary acid-base disorder, and it checks whether the body's compensation looks appropriate using formulas like Winter's formula. No sign-up, no downloads — just enter the numbers and see the full breakdown right away.

What Is the Henderson-Hasselbalch Equation for Blood Gas?

Human blood has to stay within an extremely narrow pH window — roughly 7.35 to 7.45 — to keep enzymes, proteins, and cell membranes working the way they should. The body manages this using several buffer systems, and the most important one by far is the carbonic acid/bicarbonate buffer, which is made of dissolved carbon dioxide (measured clinically as PaCO2) on one side and bicarbonate (HCO3) on the other.

The Henderson-Hasselbalch equation describes exactly how these two pieces relate to pH. Applied to blood, it's written as: pH = 6.1 + log10 (HCO3 / (0.03 x PaCO2)). Here, 6.1 is the pKa of carbonic acid in plasma, HCO3 is the bicarbonate concentration in mEq/L, PaCO2 is the arterial carbon dioxide pressure in mmHg, and 0.03 is the solubility coefficient that converts PaCO2 into the dissolved CO2 concentration. This single formula is the mathematical backbone of every arterial blood gas interpretation taught in medicine today.

Because the ratio of HCO3 to PaCO2 is what actually sets the pH, the body can keep pH normal in two very different ways: by keeping both values normal, or by moving both values in the same direction so their ratio stays close to normal. That second situation is called compensation, and it's a major part of what this calculator helps you spot.

How to Read an Arterial Blood Gas (ABG) Step by Step

Interpreting a blood gas doesn't have to feel overwhelming once you break it into a fixed sequence of questions, which is exactly what this calculator automates behind the scenes.

• Step 1 — Look at the pH. Is it below 7.35 (acidemia), above 7.45 (alkalemia), or within the normal range?
• Step 2 — Look at PaCO2. A high PaCO2 (above 45 mmHg) points toward a respiratory cause of acidosis. A low PaCO2 (below 35 mmHg) points toward a respiratory cause of alkalosis.
• Step 3 — Look at HCO3. A low HCO3 (below 22 mEq/L) points toward a metabolic cause of acidosis. A high HCO3 (above 26 mEq/L) points toward a metabolic cause of alkalosis.
• Step 4 — Match the direction of PaCO2 or HCO3 to the direction of the pH change to name the primary disorder.
• Step 5 — Check whether the other value (the one that isn't the primary problem) has moved in the expected direction to compensate, and by the expected amount, using a compensation formula.

The Four Primary Acid-Base Disorders

Every simple acid-base disturbance falls into one of four categories, and this calculator names the correct one automatically based on the pattern in your three numbers.

• Respiratory acidosis: pH is low, PaCO2 is high. Caused by anything that reduces how well the lungs clear CO2 — opioid overdose, severe COPD, neuromuscular weakness, or airway obstruction.
• Respiratory alkalosis: pH is high, PaCO2 is low. Caused by breathing faster or deeper than the body needs — anxiety and hyperventilation, pain, fever, high altitude, or early sepsis.
• Metabolic acidosis: pH is low, HCO3 is low. Caused by producing too much acid or losing too much bicarbonate — diabetic ketoacidosis, lactic acidosis, kidney failure, or severe diarrhea.
• Metabolic alkalosis: pH is high, HCO3 is high. Caused by losing acid or gaining bicarbonate — prolonged vomiting, certain diuretics, or excess antacid use.

Understanding Respiratory and Renal Compensation

Once one system causes a primary problem, the other system tries to correct the pH back toward normal — this is compensation, and it's never perfect. If the lungs cause the primary problem, the kidneys compensate over hours to days by changing how much bicarbonate they keep or excrete. If the kidneys or metabolism cause the primary problem, the lungs compensate within minutes by changing the breathing rate to blow off more or less CO2.

The key clinical rule is that compensation never fully normalizes pH and never overshoots into the opposite direction — if it does, that's a sign of a second, independent disorder rather than a single one being compensated. This calculator checks that automatically for you using the standard formulas below.

Winter's Formula for Metabolic Acidosis

When the primary problem is a metabolic acidosis, the lungs are expected to blow off CO2 to help correct the pH. Winter's formula predicts exactly how much: Expected PaCO2 = (1.5 x HCO3) + 8, give or take 2 mmHg.

If the measured PaCO2 falls inside that predicted window, the respiratory compensation is considered appropriate. If it's higher than expected, a second respiratory acidosis is likely also present. If it's lower than expected, a second respiratory alkalosis is likely also present. This calculator applies Winter's formula automatically whenever a metabolic acidosis is identified.

Compensation Formulas for the Other Three Disorders

Metabolic alkalosis: the expected respiratory compensation is a rise in PaCO2 of roughly 0.7 mmHg for every 1 mEq/L rise in HCO3 above 24, though the lungs rarely push PaCO2 much past the mid-50s even with a severe alkalosis.

Respiratory acidosis: renal compensation raises HCO3 by roughly 1 mEq/L for every 10 mmHg rise in PaCO2 in the acute setting (minutes to hours), and by roughly 3.5-4 mEq/L for every 10 mmHg rise once the kidneys have had 3-5 days to fully adjust in the chronic setting.

Respiratory alkalosis: renal compensation lowers HCO3 by roughly 2 mEq/L for every 10 mmHg fall in PaCO2 acutely, and by roughly 4-5 mEq/L for every 10 mmHg fall once compensation is chronic (typically after 2-3 days).

This calculator applies the correct formula automatically based on which primary disorder it identifies from your entered values, and shows whether your HCO3 fits an acute pattern, a chronic pattern, both, or neither.

Worked Example: Reading a Blood Gas

Suppose a blood gas shows pH 7.28, PaCO2 30 mmHg, and HCO3 14 mEq/L. The pH is low, so this is acidemia. HCO3 is low, which explains the acidemia, so the primary disorder is a metabolic acidosis. Using Winter's formula, the expected PaCO2 is (1.5 x 14) + 8 = 29, with an acceptable range of 27 to 31 mmHg. The measured PaCO2 of 30 mmHg falls right inside that window, so this is a metabolic acidosis with appropriate respiratory compensation — not a mixed disorder. Try entering these numbers into the calculator above to see the full breakdown.

Who Uses a Blood Gas & Henderson-Hasselbalch Calculator

Nursing students, medical students, respiratory therapy students, and paramedics-in-training reach for a tool like this constantly, since ABG interpretation is one of the most heavily tested and most practically important skills in clinical education. Rather than reaching for a textbook table every time, this calculator lets you punch in a set of values from a practice question or a case study and instantly see whether your own reasoning matches the correct answer.

Nurses, respiratory therapists, and physicians already comfortable with ABGs also use quick calculators like this one as a fast second check on a hand-worked interpretation, especially when running through Winter's formula or a compensation range under time pressure. It's worth repeating clearly: this tool performs the arithmetic and pattern-matching only. It cannot see the patient, their history, their oxygenation, or any of the other findings a real clinical decision depends on, and it should never be used in place of a trained professional's judgment for an actual patient.

A Note on Accuracy and Responsible Use

The Henderson-Hasselbalch equation, Winter's formula, and the compensation rules used here are the same standard formulas taught in medical, nursing, and respiratory therapy programs and used in clinical reference guides worldwide. Reference ranges for pH, PaCO2, and HCO3 can vary slightly between institutions and textbooks, and this calculator uses the most commonly cited adult arterial ranges.

This tool is provided strictly for education and quick reference. It does not replace a full clinical assessment, and it does not account for oxygenation (PaO2), the patient's history, medications, or any other lab or physical exam finding. Any real acid-base result should always be interpreted by a qualified doctor, nurse, or respiratory therapist with the complete clinical picture in front of them.