Free Water Deficit Calculator

Free Water Deficit Calculator for Nephrology. The output describes a hypothetical water balance relative to 140, not a chosen safe correction target, replacement schedule or infusion rate.

For shared guidance on choosing a tool and documenting results, read the clinical calculator workflow.

How this calculator works

Estimated deficit relative to sodium 140 mmol/L = estimated total body water x (current sodium/140 - 1). Total body water is weight times a selected adult approximation of 0.60, 0.50 or 0.45. The tool refuses sodium below 140 rather than reporting a negative replacement volume.

When to use this calculator

An adult static body-water estimate used within a complete assessment of hypernatremia, not pediatric fluid management.

Inputs used

  • Current serum sodium (mmol/L)
  • Weight used in equation (kg; US lb)
  • Estimated body-water fraction

Clinical interpretation

The output describes a hypothetical water balance relative to 140, not a chosen safe correction target, replacement schedule or infusion rate.

Worked example

For 70 kg, fraction 0.50 and sodium 160, estimated body water is 35 L and deficit is 5 L. At sodium 140 it is zero. This is not an instruction to administer 5 L.

Limitations and safety notes

Body composition, fluid compartments, hyperglycemia, acuity, hemodynamics and ongoing intake/losses may invalidate a simple estimate. Clinical monitoring and individualized correction planning remain necessary; no universal hourly or daily rate is prescribed.

Frequently asked questions

Is the displayed deficit the amount of fluid to infuse?

No. Actual fluid choice and delivery depend on resuscitation needs, sodium content, ongoing balance and serial measurements, not this static estimate alone.

What does this result leave unresolved?

Body composition, fluid compartments, hyperglycemia, acuity, hemodynamics and ongoing intake/losses may invalidate a simple estimate. Clinical monitoring and individualized correction planning remain necessary; no universal hourly or daily rate is prescribed.

References

  • https://doi.org/10.1056/NEJM200005183422006
  • https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2809955

Editorial review and citation methodology

Maintained by the Quick Medical Calculator Editorial Team. Content record date: September 9, 2026. A content date is not evidence of independent clinical review. Individual clinical sign-off is not recorded on this page.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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