Bicarbonate Deficit Calculator
Estimate the bicarbonate replacement dose from body weight, current and target HCO₃⁻, and a distribution factor. Returns the total deficit in mEq with the half-dose administration rule applied.
How the bicarbonate deficit is calculated
The bicarbonate deficit (or HCO₃⁻ deficit) estimates how much exogenous bicarbonate is needed to raise the serum bicarbonate from its current value to a chosen target. The formula is HCO₃⁻ deficit = distribution factor × weight(kg) × (target HCO₃⁻ − current HCO₃⁻). The distribution factor reflects the apparent volume into which bicarbonate distributes — conventionally 0.4–0.5 L/kg. A factor of 0.4 approximates the extracellular fluid space (bicarbonate acts mostly there in the short term), while 0.5 reflects whole-body distribution over a longer horizon. Both are estimates; clinical practice commonly uses 0.4–0.5 interchangeably and then titrates to repeat labs.
To use the calculator, enter the patient's weight, current serum bicarbonate, and the target bicarbonate (commonly 10–12 mEq/L for severe metabolic acidosis — see the discussion below), then choose a distribution factor. The result is shown in mEq of bicarbonate.
Worked example
A 70 kg patient in severe metabolic acidosis has a current bicarbonate of 8 mEq/L and a target of 12. Using the whole-body factor of 0.5: deficit = 0.5 × 70 × (12 − 8) = 0.5 × 70 × 4 = 140 mEq. Applying the half-dose rule, give roughly 70 mEq initially, then recheck the serum bicarbonate and pH before giving more. Switching to the extracellular factor of 0.4 yields 0.4 × 70 × 4 = 112 mEq, with an initial dose of ~56 mEq — illustrating how the factor selection changes the estimate.
When to give bicarbonate
The decision to give bicarbonate is one of the most contested in acid–base medicine. The calculator tells you the dose; it does not tell you whether to give it. Bicarbonate therapy is generally reserved for severe metabolic acidosis — arterial pH < 7.1 or 7.2, depending on the source — and even there the evidence is mixed. In DKA, current guidelines recommend bicarbonate only for pH < 6.9, and only as a temporizing measure while insulin and fluids address the underlying ketoacidosis. In lactic acidosis and most other HAGMAs, bicarbonate has not been shown to improve outcomes and can cause harm (hypocalcemia, sodium overload, paradoxical intracellular acidosis).
Treating the underlying cause (hypoperfusion, ketoacidosis, toxin, renal failure) is almost always more effective than neutralizing the acid. Bicarbonate generates CO₂ as it buffers, which diffuses into cells and cerebrospinal fluid faster than bicarbonate itself — potentially worsening intracellular acidosis even as the serum pH rises. Most guidelines reserve bicarbonate for arterial pH < 7.1–7.2 in the acutely unstable patient, and only as a bridge to definitive therapy. See the DKA management guide for protocol-specific thresholds.
The half-dose rule
Because the distribution factor is an estimate and the buffer system is dynamic, the calculated deficit is almost never given in full. The standard practice is to administer approximately 50% of the calculated deficit, then re-measure the serum bicarbonate and arterial pH 15–30 minutes later, and titrate the remainder to the patient's response. Giving the entire calculated dose in one push risks overshooting into metabolic alkalosis, driving potassium intracellularly (worsening hypokalemia, a particular danger in DKA where total body potassium is already depleted), and causing hypernatremia from the sodium load in the bicarbonate ampule. The half-dose rule is the safety margin between the formula's prediction and the patient's real physiology.
To revisit the underlying chemistry that produces the acid load in the first place, see the anion gap formula page; the deficit calculator complements the anion gap calculator rather than replacing it.
This calculator runs entirely in your browser — no values are transmitted or stored. Reference logic is reviewed against StatPearls and LITFL. Results are informational only and should always be interpreted in full clinical context by a qualified clinician.