Acid Base Calculator (2024)

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What is acid base balance?Arterial blood gas valuesBlood gas interpretationHow to calculate anion gapAcid base calculator — an explanationA practical example

This acid base calculator helps to interpret a patient's arterial blood gas values and determines the anion gap. All you need is the results of the patient's blood sample.

In the article below, we included some information on acid-base balance, arterial blood gas interpretation, how to calculate the anion gap, and a practical example of how to use this calculator.

We try our best to make our Omni Calculators as precise and reliable as possible. However, this tool can never replace a professional doctor's assessment. All information on this website is for informational purposes only and is not intended to serve as a substitute for medical consultation. Always consult your results with a healthcare provider.

What is acid base balance?

If the body is functioning correctly, the acid-base balance is maintained automatically. It means that lungs and kidneys are working. Yet, a decrease in their function can lead to abnormalities, such as acidosis or alkalosis. If it is the lungs that deteriorate, the acidosis/alkalosis is known as respiratory. On the contrary, if the kidneys deteriorate in their function, we name the acidosis/alkalosis metabolic.

By acidosis, we mean a state where the pH of your arterial blood is below 7.35, and alkalosis is where the pH is higher than 7.45.

It is possible, however, for the person's arterial pH to stay within the correct range while the person has imbalanced acid-base levels. How? Our bodies try to compensate every time something begins to fail. Therefore, if lung function decreases, one should present with metabolic compensation. Also, in the case of kidney deterioration, there's respiratory compensation. Here, you may find interesting the blood pH calculator

Arterial blood gas values

We often use arterial blood gas (ABG) in a hospital to estimate a patient's oxygenation (sometimes with the oxygenation index calculator). Unlike saturation (with a norm SatO2 > 92%), ABG gives a direct result, providing a quick and accurate clinical evaluation of a patient's status.

A doctor begins by taking a blood sample from an artery, most commonly the radial artery or sometimes the femoral and brachial arteries. It can also be taken from veins, although that's less reliable, and oxygenation data cannot be estimated from it — it solely determines pH and CO2 levels.

An ABG gives information on the level of oxygenation, the patient's ability to remove carbon dioxide and general acidity. More specifically, it provides medical personnel with the following data:

  • pH;
  • Partial pressure of oxygen (PaO2);
  • Partial pressure of carbon dioxide (PaCO2); and
  • Bicarbonate (HCO3).

Blood gas interpretation

The norms of each component of arterial blood gas values are strictly determined and are as follows:


Normal value




75-100 mmHg


35-45 mmHg

Bicarbonate (HCO3)

22-26 mEq/L

How to calculate anion gap

The anion gap is the difference between specific positively charged ions (cations) and particular negatively charged ions (anions). We can measure those ions either from the serum, plasma, or urine.

We commonly calculate it without the potassium value, i.e., just with the sodium, chloride, and bicarbonate values. Anion gap, AG, equals the difference between the most prevalent cation, sodium, Na+, and the sum of the most common anions (chloride Cl- and bicarbonate HCO3-).

AG=Na+(Cl+HCO3)\rm AG = Na^+ - (Cl^- + HCO_3^-)AG=Na+(Cl+HCO3)

The standard value of AG is between 8-16 mEq/L.

The calculated value of the AG should be corrected for the serum albumin concentration [1].

 ⁣correctedAG=AG+2.5×(4.4albumin)\!\scriptsize \rm corrected\ AG = AG + 2.5 \times (4.4 - albumin)correctedAG=AG+2.5×(4.4albumin)

Acid base calculator — an explanation

This acid base calculator estimates both the anion gap and provides you with an arterial blood gas interpretation. Altogether, they help to determine the status of the patient — their acid-base balance. All you need to know is a few parameters:

  • pH (Norm: 7.35-7.45);
  • PCO2 — Partial pressure of carbon dioxide (Norm: 35-45 mmHg);
  • Bicarbonates (Norm: 22-26 mEq/L);
  • Chloride — Cl- level;
  • Sodium — Na+ level; and
  • Albumin level.

The acid-base calculator estimates the level of oxygenation from the first three values. The result is whether the patient is in acidosis, alkalosis, or within the normal range. This tool also suggests if those values are due to metabolic, respiratory, or combined deficiencies.

How to calculate the anion gap? Take the ion levels and put them into the equation specified above: AG = Na+ − (Cl- + HCO3-). The normal values should be between 8 and 16 mEq/L.

A practical example

It's time for us to explain how we can use this acid-base calculator in practice. Ian is an 86-year-old man with severe COPD (Chronic Obstructive Pulmonary Disease) and a couple more health conditions. Recently, he showed up at the ER (Emergency room), presenting fatigue and shortness of breath. His saturation was around 85%, so the doctor immediately ordered ABG (arterial blood gas test) and ion test.

Here are the test's results:

  1. pH: 7.1;
  2. pCO2: 56 mmHg; and
  3. Bicarbonates: 24 mEq/L.

Therefore, his pH is low and indicates acidosis. To be more specific, uncompensated respiratory acidosis.

As for the ions in blood:

  1. Chloride: 108 mEq/L;
  2. Sodium: 140 mEq/L; and
  3. Albumin: 3.3 g/dL.

It means that the patient's anion gap equals 8 mEq/L.

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