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Winters’ formula in kPa

Expected pCO₂ in metabolic acidosis for blood gas analysers reporting kPa

Sources last checked: 2026-10-13

Winters’ formula in kPa

Expected pCO₂ in metabolic acidosis for blood gas analysers reporting kPa

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The formula was derived in mmHg; this page only converts its result (1 mmHg = 0.133322 kPa). Same limits as the mmHg version: primary metabolic acidosis, steady state.

Source: Albert MS, Dell RB, Winters RW. Quantitative displacement of acid-base equilibrium in metabolic acidosis. Ann Intern Med 1967;66(2):312-322.Open

What is the Winters’ formula in kPa?

This page gives the Winters’ formula prediction in kilopascals, the unit printed by blood gas analysers in the UK, Scandinavia, the Netherlands and many intensive care units. The formula itself was derived in mmHg by Albert, Dell and Winters (1967); here the result is multiplied by the defined factor 0.1333 kPa per mmHg.

When to use it

  • Primary metabolic acidosis on a blood gas reported in kPa
  • Checking whether respiratory compensation is adequate without converting the measured pCO₂ first
  • Teaching acid–base interpretation in European kPa-based settings

When not to use it

  • Respiratory acid–base disorders or metabolic alkalosis
  • Acute acidosis in the first hours, before compensation has settled

How it is calculated

Expected pCO₂ (kPa) = (1.5 × HCO₃⁻ + 8) × 0.1333 ± 0.27 (1 mmHg = 0.1333 kPa; ± 2 mmHg = ± 0.27 kPa)

Clinical pearls and limitations

  • The formula was derived in mmHg; this page only converts its result (1 mmHg = 0.133322 kPa). Same limits as the mmHg version: primary metabolic acidosis, steady state.
  • A practical kPa shortcut is expected pCO₂ ≈ 0.2 × HCO₃⁻ + 1.07 kPa — the same equation rescaled.
  • The ± 2 mmHg tolerance becomes ± 0.27 kPa; values outside it suggest a mixed disorder.
  • Normal arterial pCO₂ is about 4.7–6.0 kPa, so ‘normal’ values in severe acidosis are actually too high.

Frequently asked questions

How do I convert mmHg to kPa?

Multiply by 0.1333 (1 mmHg = 0.133322 kPa). To go back, multiply kPa by 7.50.

Is there a separate Winters’ formula for kPa?

No. The formula was derived in mmHg; the kPa result is the same prediction expressed in another unit.

What is the expected pCO₂ for HCO₃⁻ 12 mmol/L?

1.5 × 12 + 8 = 26 mmHg, which is about 3.47 kPa (range roughly 3.2–3.7 kPa).

Evidence and guidelines

  1. Primary source: Albert MS, Dell RB, Winters RW. Quantitative displacement of acid-base equilibrium in metabolic acidosis. Ann Intern Med 1967;66(2):312-322.
  2. Duška F et al. A pragmatic approach to complex acid base disturbances of critical illness: the “Stewart light”. Intensive Care Med 2026 — states Winters’ formula as expected PCO₂ (mmHg) = 1.5 × HCO₃⁻ + 8 ± 2.

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