Acid-Base Calculators

All Acid-Base calculators

• Anion Gap, Delta Gap and Delta Ratio CalculatorCalculate Anion Gap, Delta Gap and Delta Ratio.Use calculator →
• Winter's Formula CalculatorExpected pCO2 in Metabolic Acidosis.Use calculator →

Arterial blood gas results pack more decision-making power into a few numbers than almost any other bedside test, yet mixed acid-base disorders slip past even experienced clinicians. This page explains how the anion gap, delta gap, delta ratio and Winter's formula turn raw electrolytes into answers, when to reach for each one, how to read what they say, and the traps that catch the unwary.

Medically reviewed by Dr. Taimoor Asghar, physician.

In short: Free acid-base calculators: anion gap, delta gap, delta ratio and Winter's formula, with step-by-step ABG interpretation for clinicians and students. Browse the calculators below, each with an interpretation guide.

What acid-base medicine covers, and the clinical questions these tools answer

The body defends arterial pH within a narrow band, roughly 7.35 to 7.45, because enzyme function, oxygen delivery and cardiac excitability all depend on it. Two organ systems share the work: the lungs adjust carbon dioxide within minutes, and the kidneys adjust bicarbonate over hours to days. When one side fails or is overwhelmed, the other compensates in a predictable direction and, importantly, to a predictable degree. Acid-base medicine is the disciplined study of these patterns: naming the primary disorder, checking whether compensation is appropriate, and hunting for a second disorder hiding underneath.

In practice the questions are concrete. A patient arrives tachypnoeic with a bicarbonate of 12: is this a metabolic acidosis, and if so, is the anion gap raised? A raised gap points toward lactate, ketones, renal failure or a toxin; a normal gap points toward bicarbonate loss through the gut or the kidney. When the gap is raised, the delta gap and delta ratio ask the next question: does the pattern fit a single disorder, or is a second metabolic process concealed? Winter's formula then closes the loop on the respiratory side, predicting what the pCO2 should be if compensation is behaving itself.

The two calculators in this library map directly onto those questions. The anion gap calculator works through the gap, the delta gap and the delta ratio from sodium, chloride and bicarbonate, showing each step so students can follow the arithmetic. Winter's formula takes the bicarbonate and returns the expected pCO2 with its accepted range, flagging when the measured value falls outside it.

When clinicians and students reach for these calculators

The emergency department is the most common setting: undifferentiated tachypnoea, altered consciousness, severe sepsis, or a collapse where the cause is not yet clear. Diabetic ketoacidosis is the classic teaching case, because the delta gap during treatment can unmask a normal anion gap acidosis left behind as ketones clear, and because an unexpectedly high pCO2 hints at respiratory fatigue or a coexisting problem.

In intensive care the tools guide ventilation. A patient with a metabolic acidosis should be blowing off carbon dioxide down to the predicted level, and failure to do so changes the ventilator strategy or prompts a search for a respiratory depressant. Nephrologists use the same framework for the acidosis of chronic kidney disease, renal tubular acidosis and the response to alkali therapy. Toxicologists lean on the gap when methanol, ethylene glycol or salicylates are on the differential, and anaesthetists use it when an unexplained acidosis appears on the table.

For students and junior doctors, acid-base interpretation is a rite of passage precisely because it rewards method. Every gas gets the same questions in the same order, and the calculators remove the arithmetic so the reasoning stays visible. Working one gas through both tools, aloud, on a ward round teaches the method faster than any lecture.

How to interpret results, and what they change in practice

Start with the gap itself: sodium minus the sum of chloride and bicarbonate, normally around 8 to 12 depending on the laboratory and its assay. A raised gap means unmeasured anions are present, and the shortlist is short: lactate from shock or tissue hypoxia, ketoacids from diabetic, alcoholic or starvation ketoacidosis, retained acids in renal failure, and the toxic alcohols. A gap that is normal while bicarbonate is low means bicarbonate has been lost or replaced by chloride, through diarrhoea, a fistula, or a renal tubular acidosis, and the workup heads in a different direction.

The delta gap and delta ratio then test whether one disorder is enough. The delta gap compares the rise in the anion gap with the fall in bicarbonate; the delta ratio divides one by the other. A ratio below about 1 suggests a concurrent normal anion gap acidosis, a ratio between 1 and 2 fits a straightforward raised-gap acidosis, and a ratio above 2 suggests a metabolic alkalosis riding alongside, as seen with vomiting in a patient with ketoacidosis. These are guides, not laws, and they assume starting values near the normal range.

Winter's formula predicts the pCO2 in a pure metabolic acidosis: multiply the bicarbonate by 1.5, add 8, and allow 2 mmHg either way. If the measured pCO2 sits within that window, compensation is appropriate. If it is higher, a respiratory acidosis is superimposed and the patient is retaining carbon dioxide; if lower, a respiratory alkalosis is driving ventilation beyond compensation. Each answer changes management: fluids and insulin for ketoacidosis, decisions about bicarbonate therapy, dialysis for severe renal acidosis, or airway and ventilation support when the lungs are failing the compensation.

Limitations and pitfalls

Winter's formula applies only to a primary metabolic acidosis. It has no meaning in primary respiratory disorders or in chronic, fully compensated states, and applying it there produces nonsense. A low albumin falsely lowers the anion gap, so the gap should be corrected upward in hypoalbuminaemia before any delta arithmetic, and lactate should be measured directly rather than inferred from the numbers. The delta calculations assume a baseline gap near 12 and a baseline bicarbonate near 24, which fails in patients whose baselines were never normal.

Timing matters. A gas drawn after fluids, bicarbonate or intubation reflects treatment as much as disease, and venous samples read differently from arterial ones, particularly for pCO2. Chronic respiratory compensation follows different rules and takes days to develop, so acute formulas mislead in stable chronic lung disease. Most importantly, no calculator names the cause: a raised gap demands a lactate, ketones, renal function and a toxin and medication history, and the numbers are only as good as the sample they came from.

How to use this library

Enter the electrolytes from the same blood draw as the gas, note whether the sample is arterial or venous, and read the step-by-step working rather than the headline number alone. Use the anion gap page when bicarbonate is low or the diagnosis is unclear, and Winter's formula whenever a metabolic acidosis is confirmed, to check the respiratory response. Keep this page bookmarked for teaching: the same five questions, in the same order, for every gas, will catch the mixed disorder that a glance at the pH alone would miss.

Related specialities

• Nephrology
• Critical Care
• ICU Care
• Emergency Medicine

Frequently asked questions