Acid Base Calculator

Q: What is a normal ABG result?

A normal ABG has a pH between 7.35 and 7.45 , a PaCO₂ between 35 and 45 mmHg , and an HCO₃⁻ between 22 and 26 mEq/L . When all three values fall in these ranges, the calculator will show Normal Acid-Base Status with no disorder detected.

Q: What does acidemia vs acidosis mean?

Acidemia means the blood pH is below 7.35 — the blood is actually acidic right now. Acidosis is the process or condition that pushes pH down. You can have acidosis with a normal pH if the body has compensated. The same logic applies to alkalemia (pH above 7.45) vs alkalosis (the process pushing pH up).

Q: What is Winter's formula?

Winter's formula predicts the expected PaCO₂ during metabolic acidosis . The formula is: Expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2 If the measured PaCO₂ falls outside this range, compensation is not appropriate, and a mixed disorder may be present. This calculator applies Winter's formula automatically when it detects metabolic acidosis.

Q: When should I pick acute vs chronic respiratory mode?

Pick Acute if the respiratory problem started minutes to hours ago, such as a sudden asthma attack or anxiety-driven hyperventilation. Pick Chronic if it has been going on for days or longer, like COPD. If you are not sure, use Auto — Show Both and the calculator will display the expected compensation for both scenarios side by side.

Q: What does a normal anion gap acidosis mean?

A normal anion gap acidosis (also called hyperchloremic metabolic acidosis ) means bicarbonate is low but chloride is high. Common causes include diarrhea , renal tubular acidosis , and saline infusion . The delta-delta ratio will typically be below 0.4 in this case.

Q: What does the mixed disorder flag mean?

A mixed disorder means more than one acid-base problem is happening at the same time. The calculator flags this when the measured compensation does not match the expected range. For example, a patient could have metabolic acidosis and respiratory acidosis together. Mixed disorders are common in critically ill patients and need careful clinical review.

Q: Can I change the normal anion gap baseline?

Yes. The default baseline is 12 mEq/L , which is the most common reference value. However, some labs use 10 or 14 as their normal. You can edit the Normal AG Baseline field to match your lab's reference range. This value is used in both the anion gap interpretation and the delta-delta calculation.

Acid Base Calculator

Updated Jul 10, 2026 By Jehan Wadia

Introduction

An acid-base calculator helps you read and understand arterial blood gas (ABG) results. ABG tests measure three key values in your blood: pH, PaCO₂, and HCO₃⁻ (bicarbonate). These numbers tell doctors if your blood is too acidic, too alkaline, or just right. When one value is off, your body tries to fix it — this is called compensation.

This tool takes your ABG values and does the math for you. It finds the primary disorder, checks if compensation is appropriate, and flags possible mixed acid-base disorders. If you turn on the anion gap option, it also calculates the anion gap, applies an albumin correction, and runs a delta-delta analysis to look for hidden problems. Every result comes with a step-by-step breakdown so you can see exactly how each number was found.

Enter your pH, PaCO₂, and HCO₃⁻ below. You can switch between US units (mmHg, mEq/L) and SI units (kPa, mmol/L) at any time. Choose acute or chronic respiratory mode, or let the calculator show both. Then press Calculate to get your full acid-base interpretation in seconds.

How to Use Our Acid Base Calculator

Enter your arterial blood gas (ABG) values and electrolyte levels below. The calculator will identify the primary acid-base disorder, check if compensation is appropriate, compute the anion gap, and flag any mixed disorders.

Unit System: Choose US Units (mmHg, mEq/L, g/dL) or SI Units (kPa, mmol/L, g/L). The calculator will convert your values automatically if you switch.

pH: Enter the arterial blood pH from your ABG report. The normal range is 7.35 to 7.45. For a deeper look at how pH is calculated from hydrogen ion concentration, see our pH Calculator.

PaCO₂: Enter the partial pressure of carbon dioxide. The normal range is 35 to 45 mmHg (or 4.7 to 6.0 kPa in SI units).

HCO₃⁻: Enter the bicarbonate level. The normal range is 22 to 26 mEq/L.

Respiratory Mode: Select "Assume Acute" for sudden onset, "Assume Chronic" for long-term conditions, or "Auto — Show Both" if you are unsure. This setting controls which compensation formulas are applied.

Include Anion Gap Analysis: Turn this switch on to calculate the anion gap and delta-delta ratio. Turn it off if you only need the basic ABG interpretation. For a standalone anion gap tool, try our Anion Gap Calculator.

Na⁺: Enter the sodium level. The normal range is 135 to 145 mEq/L. This value is needed for the anion gap calculation.

Cl⁻: Enter the chloride level. The normal range is 100 to 110 mEq/L. This value is also needed for the anion gap calculation.

Albumin: Enter the serum albumin level. The normal range is 3.5 to 5.0 g/dL. This is used to correct the anion gap in patients with low albumin. The albumin correction concept is similar to the adjustment used in our Corrected Calcium Calculator, where low albumin also affects lab interpretation.

Normal AG Baseline: Enter your lab's normal anion gap reference value. The default is 12. Change this number only if your lab uses a different baseline.

Press Calculate to see your results. Press Reset to clear all fields and start over.

What Is an Acid-Base (ABG) Calculator?

An acid-base calculator helps you figure out if the blood is too acidic, too alkaline, or just right. It uses numbers from a blood test called an arterial blood gas (ABG). Doctors take a small sample of blood from an artery, usually in the wrist, and measure three key values: pH, PaCO₂, and HCO₃⁻ (bicarbonate).

What Do the Values Mean?

pH tells you how acidic or alkaline the blood is. Normal blood pH is between 7.35 and 7.45. A pH below 7.35 means the blood is too acidic (called acidemia). A pH above 7.45 means the blood is too alkaline (called alkalemia). You can explore how pH relates to hydrogen ion concentration using our pH Calculator.

PaCO₂ is carbon dioxide in the blood. The lungs control this value. Normal is 35 to 45 mmHg. When CO₂ goes up, the blood becomes more acidic. When it drops, the blood becomes more alkaline.

HCO₃⁻ (bicarbonate) is a substance the kidneys control. Normal is 22 to 26 mEq/L. It acts as a buffer that helps keep pH steady. Low bicarbonate makes the blood more acidic. High bicarbonate makes it more alkaline.

The Four Main Acid-Base Disorders

Metabolic acidosis — bicarbonate is too low, so the blood is acidic.
Metabolic alkalosis — bicarbonate is too high, so the blood is alkaline.
Respiratory acidosis — CO₂ is too high because the lungs are not removing enough of it.
Respiratory alkalosis — CO₂ is too low because a person is breathing too fast.

Compensation and Mixed Disorders

The body always tries to fix pH on its own. If the lungs cause a problem, the kidneys try to help, and the other way around. This is called compensation. This calculator checks whether the compensation matches what is expected. If it does not match, a mixed disorder may be present, meaning more than one problem is happening at the same time.

What Is the Anion Gap?

The anion gap (AG) is a simple math formula: Na⁺ minus (Cl⁻ plus HCO₃⁻). It helps find the cause of metabolic acidosis. A high anion gap points to causes like kidney failure, diabetic ketoacidosis, or poisoning. You can assess kidney function with our GFR Calculator or Creatinine Clearance Calculator. A normal anion gap often points to bicarbonate loss, such as from diarrhea. Low albumin can hide a high anion gap, so this calculator also corrects for albumin levels. For a quick standalone check, you can also use our dedicated Anion Gap Calculator.

What Is the Delta-Delta Ratio?

The delta-delta ratio compares the change in the anion gap to the change in bicarbonate. It helps tell if a patient has just one type of metabolic acidosis or if a second hidden problem, like metabolic alkalosis, is also present. A ratio between 1.0 and 2.0 suggests a pure anion gap acidosis. Values outside that range suggest something extra is going on. If you need to work with ratios in other contexts, our Ratio Calculator can help. For additional lab value analysis, consider tools like the Serum Osmolality Calculator or Bun Creatinine Ratio Calculator, which are often evaluated alongside ABG results. You can also explore related chemistry concepts with our Molarity Calculator and Dilution Calculator.

Formulas used

Anion Gap (Albumin-Corrected)

AG_{corr} = \left(Na^+ - Cl^- - HCO_3^-\right) + 2.5 \times (4.0 - \text{Albumin})

Winter's Formula (Metabolic Acidosis Expected PaCO₂)

PaCO_2 = 1.5 \times HCO_3^- + 8 \pm 2

Expected PaCO₂ in Metabolic Alkalosis

PaCO_2 = 0.7 \times HCO_3^- + 20 \pm 5

Expected HCO₃⁻ in Acute Respiratory Acidosis

HCO_3^- = 24 + 1 \times \frac{\Delta PaCO_2}{10}

Expected HCO₃⁻ in Chronic Respiratory Acidosis

HCO_3^- = 24 + 3.5 \times \frac{\Delta PaCO_2}{10}

Expected HCO₃⁻ in Acute Respiratory Alkalosis

HCO_3^- = 24 - 2 \times \frac{\Delta PaCO_2}{10}

Expected HCO₃⁻ in Chronic Respiratory Alkalosis

HCO_3^- = 24 - 5 \times \frac{\Delta PaCO_2}{10}

Delta-Delta Ratio

\frac{\Delta AG}{\Delta HCO_3^-} = \frac{AG_{corr} - AG_{normal}}{24 - HCO_3^-}

Frequently asked questions

What is a normal ABG result?

A normal ABG has a pH between 7.35 and 7.45, a PaCO₂ between 35 and 45 mmHg, and an HCO₃⁻ between 22 and 26 mEq/L. When all three values fall in these ranges, the calculator will show Normal Acid-Base Status with no disorder detected.

What does acidemia vs acidosis mean?

Acidemia means the blood pH is below 7.35 — the blood is actually acidic right now. Acidosis is the process or condition that pushes pH down. You can have acidosis with a normal pH if the body has compensated. The same logic applies to alkalemia (pH above 7.45) vs alkalosis (the process pushing pH up).

What is Winter's formula?

Winter's formula predicts the expected PaCO₂ during metabolic acidosis. The formula is:

Expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2

If the measured PaCO₂ falls outside this range, compensation is not appropriate, and a mixed disorder may be present. This calculator applies Winter's formula automatically when it detects metabolic acidosis.

When should I pick acute vs chronic respiratory mode?

Pick Acute if the respiratory problem started minutes to hours ago, such as a sudden asthma attack or anxiety-driven hyperventilation. Pick Chronic if it has been going on for days or longer, like COPD. If you are not sure, use Auto — Show Both and the calculator will display the expected compensation for both scenarios side by side.

Why does the albumin level matter for the anion gap?

Albumin is a protein in your blood that carries a negative charge. When albumin is low, the anion gap looks lower than it really is. The albumin correction formula adds 2.5 mEq/L to the anion gap for every 1 g/dL that albumin falls below 4.0. This prevents you from missing a high anion gap in patients with low albumin.

What does a high anion gap mean?

A high anion gap (usually above 12) means there are extra acids in the blood that are not chloride. Common causes include:

Diabetic ketoacidosis (DKA)
Lactic acidosis
Kidney failure
Toxic ingestions (methanol, ethylene glycol, salicylates)

The mnemonic MUDPILES is often used to remember these causes.

What does a normal anion gap acidosis mean?

A normal anion gap acidosis (also called hyperchloremic metabolic acidosis) means bicarbonate is low but chloride is high. Common causes include diarrhea, renal tubular acidosis, and saline infusion. The delta-delta ratio will typically be below 0.4 in this case.

How do I read the delta-delta ratio result?

The delta-delta ratio compares the rise in the anion gap to the drop in bicarbonate:

Below 0.4 — Non-anion gap (hyperchloremic) acidosis
0.4 to 0.8 — Mixed AG and non-AG acidosis
1.0 to 2.0 — Pure anion gap acidosis
Above 2.0 — AG acidosis plus a hidden metabolic alkalosis

Values between 0.8 and 1.0 are borderline and need clinical judgment.

What does the mixed disorder flag mean?

A mixed disorder means more than one acid-base problem is happening at the same time. The calculator flags this when the measured compensation does not match the expected range. For example, a patient could have metabolic acidosis and respiratory acidosis together. Mixed disorders are common in critically ill patients and need careful clinical review.

Can I change the normal anion gap baseline?

Yes. The default baseline is 12 mEq/L, which is the most common reference value. However, some labs use 10 or 14 as their normal. You can edit the Normal AG Baseline field to match your lab's reference range. This value is used in both the anion gap interpretation and the delta-delta calculation.

What is the difference between US units and SI units?

US units use mmHg for PaCO₂, mEq/L for electrolytes, and g/dL for albumin. SI units use kPa for PaCO₂, mmol/L for electrolytes, and g/L for albumin. The calculator converts values automatically when you switch. The pH value stays the same in both systems.

What does fully compensated mean?

Fully compensated means the body has adjusted enough to bring the pH back into the normal range (7.35–7.45), even though PaCO₂ or HCO₃⁻ is still abnormal. The original disorder is still present, but the compensation has corrected the pH. The calculator identifies this when the pH is normal but both PaCO₂ and HCO₃⁻ are abnormal in the same direction.

Why is my PaCO₂ value flagged with a warning?

The calculator shows a warning if any value falls far outside the expected clinical range. For PaCO₂, a warning appears if the value is below 5 mmHg or above 150 mmHg. This does not block the calculation. It simply reminds you to double-check the number for typos or unit errors.

What does the deviation chart show?

The Deviation from Normal chart shows how far each value (pH, PaCO₂, HCO₃⁻) sits from its normal midpoint. The green shaded band represents the normal range. Bars going to the right mean the value is higher than normal. Bars going to the left mean it is lower. Longer bars mean bigger deviations.

Can this calculator diagnose a medical condition?

No. This calculator is an educational and screening tool. It interprets ABG numbers using standard formulas, but it cannot replace a doctor's clinical judgment. ABG results must always be considered alongside the patient's symptoms, history, and other lab work. Always consult a healthcare professional for diagnosis and treatment.

What does the compensation table show?

The compensation table lists all six standard acid-base scenarios (metabolic acidosis, metabolic alkalosis, acute and chronic respiratory acidosis, acute and chronic respiratory alkalosis). For each one, it shows the formula used, the expected range, the measured value, and whether they match. The row marked Primary is the one that applies to your result.

How is respiratory compensation different from metabolic compensation?

Respiratory compensation happens when the lungs adjust breathing to change CO₂ levels. It starts within minutes and works fast. Metabolic compensation happens when the kidneys adjust bicarbonate levels. It takes hours to days to fully develop. This is why the acute vs chronic setting matters for respiratory disorders — the kidneys need time to respond.

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