Step 1
Measured labs
Start with the values actually reported by the laboratory.
Laboratory calculations
Lab Calculators & Corrections
Use the measured laboratory values to calculate the specific gap, osmolality or correction your clinical question requires. The calculation is only one layer; interpretation still belongs to the reporting method, reference information and clinical context.
Measured values first
Units stay visible
Derived ≠ measured
Core lab calculation pathway
Measured data → derived value → interpretation
Step 2
Choose the equation
Match the calculation to the clinical question and source.
Step 3
Derived value
Keep the result labelled as calculated, corrected or estimated.
Step 4
Interpret in context
Use the laboratory method, range, trend and clinical picture.
Keep the label attached: “corrected”, “calculated”, “anion gap” and “osmol gap” describe different derived values. Do not present them as replacement laboratory measurements.
Calculate now
Choose the value you need to calculate
Each calculator focuses on one arithmetic task. If your question is what the result means rather than how to calculate it, use the matching Guide below.
Lab-derived value
Na − (Cl + HCO₃)
→ Anion gap
Calculate the potassium-free anion gap from sodium, chloride and bicarbonate, with optional albumin adjustment.
Use when: You have the relevant electrolytes and need the arithmetic gap for comparison with the reporting laboratory range.
Calculate anion gapLab-derived value
Na + glucose + urea/BUN
→ Osmolality / osmol gap
Calculate serum osmolality from sodium, glucose and urea/BUN, with optional measured osmolality for an osmol gap.
Use when: You need calculated osmolality, or measured minus calculated osmolality as an osmol gap.
Calculate osmolalityLab-derived value
Measured Na + glucose adjustment
→ Corrected sodium
Estimate sodium adjusted for hyperglycaemia using the Katz 1.6 or Hillier 2.4 correction factor.
Use when: A current source specifically asks for a glucose-adjusted sodium estimate in hyperglycaemia.
Calculate corrected sodiumLab-derived value
Total calcium + albumin adjustment
→ Corrected calcium
Calculate the legacy simplified albumin-adjusted calcium estimate from measured total calcium and albumin.
Use when: You need to reproduce or interpret a legacy/external corrected-calcium value rather than replace ionised calcium measurement.
Calculate corrected calciumCalculated and corrected values are estimates
These tools perform defined arithmetic from laboratory inputs. They do not replace the measured laboratory result, the reporting laboratory's reference information, serial trends, or the wider clinical assessment.
Understand
Use the Guide when the hard part is interpretation
The calculator gives the number. These Guides explain what the number represents, why different methods disagree and where interpretation becomes clinically limited.
Interpretation
How to Interpret the Anion Gap
Use the local laboratory range, albumin context, GOLDMARK and normal-gap patterns after the arithmetic has been calculated.
Read GuideOsmolality context
Calculated Serum Osmolality and Osmol Gap Explained
Understand measured versus calculated osmolality, formula differences, ethanol assumptions and why the osmol gap is a clue rather than a diagnosis.
Read GuideHyperglycaemia
Corrected Sodium in Hyperglycaemia
Understand what corrected sodium is trying to estimate, why Katz and Hillier factors differ and what the value cannot tell you on its own.
Read GuideCalcium interpretation
Corrected Calcium vs Ionised Calcium
Compare an albumin-adjusted estimate with directly measured ionised calcium and understand why the two values can disagree.
Read GuideCommon lab-calculation mistakes
The equation can be right while the interpretation is wrong
Treating a calculated value as a measured result
Anion gap, calculated osmolality and corrected values are derived from laboratory inputs. Keep the calculation name attached to the result.
Using a universal cutoff
Reference intervals and interpretation can depend on the reporting laboratory, equation and clinical context. Use the relevant local or authoritative source.
Mixing units or formula conventions
Glucose, urea/BUN, calcium and albumin may be reported in different units. Match the calculator mode to the actual laboratory report before entering values.
Letting one derived number answer the whole clinical question
A gap or corrected estimate can support assessment, but it does not replace the measured data, trends, examination or the wider clinical picture.
Used in clinical areas
See these calculations alongside other tools commonly used in the same clinical setting.
Related calculation topics
Continue when the laboratory calculation connects to kidney function, fluids or a medication calculation.
Lab calculator FAQs
Answers about choosing and interpreting derived laboratory calculations without turning the hub into a diagnostic protocol.
Lab calculator FAQs
Answers about choosing and interpreting derived laboratory calculations without turning the hub into a diagnostic protocol.
Which lab calculator should I use?
Choose the calculator that matches the derived or corrected value explicitly required by the clinical question or source you are using.
- Use Anion Gap when you have sodium, chloride and bicarbonate/total CO₂ and need the arithmetic gap.
- Use Serum Osmolality when you need calculated osmolality or, with a measured value, an osmol gap.
- Use Corrected Sodium when a current source asks for a glucose-adjusted sodium estimate in hyperglycaemia.
- Use Corrected Calcium when you specifically need to reproduce or interpret an albumin-adjusted calcium estimate.
Are calculated or corrected laboratory values direct measurements?
No. They are equation-based values derived from measured laboratory inputs and should remain labelled as calculated, corrected or estimated values.
- A corrected sodium does not replace the measured sodium or serum osmolality.
- A corrected calcium is not the same as directly measured ionised calcium.
- An osmol gap depends on both the measured laboratory osmolality and the formula used for the calculated value.
Why can my result differ from another calculator or laboratory?
Different equations, unit conventions, correction factors and laboratory methods can produce different results.
- Corrected sodium commonly uses either the Katz 1.6 or Hillier 2.4 factor.
- Serum osmolality equations vary in their glucose, urea/BUN and ethanol terms.
- Anion-gap reference intervals vary with the laboratory method and whether potassium is included.
Can MedMaths diagnose an acid-base disorder, toxic alcohol exposure or electrolyte disorder?
No. MedMaths performs the selected arithmetic and explains the calculation; it does not diagnose a condition or replace clinical assessment.
- Use the calculated value with the measured laboratory data, clinical history, examination and the relevant diagnostic guidance.
- If a result is unexpected or clinically important, verify the inputs, units, equation and source before acting on it.
Formula origins in this topic
The equations used in these calculators were developed by published researchers. These researchers are not affiliated with MedMaths and have not reviewed or endorsed this site.
Smithline-Gardner Calculated Serum Osmolality Formula
Equation attributed to: Neil Smithline and Kenneth D. Gardner Jr
Katz hyperglycaemia sodium correction (1.6)
Formula originally described by: Murray A. Katz
Hillier hyperglycaemia sodium correction (2.4 overall estimate)
Based on research published by: Teresa A. Hillier, Robert D. Abbott and Eugene J. Barrett