Total Body Water (TBW) Calculator

Total Body Water (TBW) Calculator for Endocrinology. TBW is the estimated amount of water already in the body. It is not the amount to drink or infuse. This page does not calculate a sodium-correction plan or identify fluid depletion.

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

How this calculator works

Watson male TBW in liters is 2.447 + 0.3362W + 0.1074H - 0.09516A. The female equation is -2.097 + 0.2466W + 0.1069H. The male age coefficient is 0.09516, correcting a transposition in the previous implementation.

When to use this calculator

Check a Watson estimate for a specified adult clinical or educational calculation. Record whether the weight was pre- or post-dialysis when that distinction matters; a formula cannot repair an inappropriate measurement time.

Inputs used

  • Adult weight in kg
  • Height in cm
  • Age in years
  • Published sex coefficient

Clinical interpretation

TBW is the estimated amount of water already in the body. It is not the amount to drink or infuse. This page does not calculate a sodium-correction plan or identify fluid depletion.

Worked example

For 80 kg, 180 cm and age 40, the male calculation is 2.447 + 26.896 + 19.332 - 3.8064 = 44.8686 L. The female result at the same height and weight is 36.873 L; age does not enter that equation.

Limitations and safety notes

Anthropometry does not directly measure fluid compartments. Edema, altered body composition and dialysis populations can produce clinically important error. A software check cannot establish suitability for a particular patient.

Frequently asked questions

Should age change the female result?

Not in the published Watson female equation. Adding an age adjustment would create a different model.

References

  • https://pubmed.ncbi.nlm.nih.gov/6986753/
  • https://ajcn.nutrition.org/article/S0002-9165%2823%2943745-8/pdf
  • https://www.kidney.org/sites/default/files/12-85-0946_ica_crn-pocket-guide_v6_chp_1.pdf

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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