Fluid Balance (Intake & Output) Calculator
Enter each recorded volume in millilitres. This calculator adds the intake column, adds the output column, and subtracts output from intake to give the fluid balance — showing the working at every step.
Fluid balance
Total the intake and output columns
Enter each recorded volume in millilitres. The calculator totals both columns and subtracts output from intake.
Intake (mL)
Output (mL)
Simple answer
How do you calculate fluid balance?
Add every recorded intake volume, add every recorded output volume, then subtract the output total from the intake total.
The order decides the sign. A positive result means the intake total was larger; a negative result means the output total was larger. Both are valid arithmetic results.
How the calculation works
- Every intake volume you enter is added into one total.
- Every output volume you enter is added into one total.
- The output total is subtracted from the intake total, once.
- The sign of the answer records which of the two totals was larger.
All volumes must be entered in the same unit. If a volume is recorded in litres, convert it to millilitres before entering it — 1 L is 1000 mL.
Clinical use and safety
When to use this calculator — and its limits
The arithmetic can be correct while the clinical decision is still wrong. These boundaries show what the calculator can and cannot establish.
When to use
- Use when intake and output volumes have already been recorded and you need the totals and the difference between them.
- Enter every volume in millilitres. Convert a volume recorded in litres before entering it — 1 L is 1000 mL.
Key limitations
- The arithmetic assumes the volumes entered are complete and correct. A correct total does not show that everything was recorded.
- Losses that are not measured and recorded are not part of an output total and are not estimated here.
- Which items a chart counts as intake or output, and how they are recorded, is set by the chart in use and by local policy.
- The sign of the balance is a comparison of two totals over the period entered, not a trend and not an assessment.
What MedMaths does not decide
- Whether a particular fluid balance is expected, acceptable or concerning for any patient.
- Whether fluids should be given, adjusted, restricted or withheld.
- What should be recorded on a fluid balance chart, or how often it should be reviewed.
Evidence and method checks
- Clinical Nursing Skills — 19.2 Nursing Assessment for Fluid and Electrolytes — OpenStax
- Intravenous fluid therapy in adults in hospital (CG174) — Recommendations — NICE
Formula logic and known-value examples are checked as part of the MedMaths review process.
See how MedMaths checks calculator methods →Learn the maths behind it
- Adding Up Total Fluid Intake and Output — unit matching and decimal alignment when building each total.
- How to Calculate Fluid Balance — why the order matters and what the sign means.
- Adding & Subtracting Clinical Quantities — the lesson, with four levels of practice.
Common checks and fluid balance FAQs
Review the mistakes that change a balance figure, and what the sign of the result does and does not mean.
Common checks and fluid balance FAQs
Review the mistakes that change a balance figure, and what the sign of the result does and does not mean.
Common mistakes to check
- Mixing the columns: intake and output are totalled separately, then compared once.
- Subtracting output more than once: the output total is taken away from the finished intake total, not from each intake volume.
- Unmatched units: 1 L is 1000 mL. Adding 1 to a column of millilitre volumes is a hundredfold-scale error.
- Dropping the minus sign: reporting −500 mL as 500 mL reverses which total was larger.
- Misaligned decimals: 0.5 is five tenths — write it as 0.50 before adding.
What does a negative fluid balance mean?
Mathematically, it means the output total was larger than the intake total, by the amount shown. It is an ordinary arithmetic result rather than an error. What it means for a patient is a clinical judgement made against the whole picture and local policy, and this calculator does not make it.
Continue calculating
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Open calculatorReferences and sources
Open the sources used for the fluid balance definition and clinical monitoring guidance on this page.
References and sources
Open the sources used for the fluid balance definition and clinical monitoring guidance on this page.
These references support the fluid balance definition, what is recorded as intake and output, and clinical monitoring guidance used on this page.
OpenStax — Clinical Nursing Skills: 19.2 Nursing Assessment for Fluid and Electrolytes
Open-textbook source for the fluid balance definition and what is recorded as intake and output.
NICE — Intravenous fluid therapy in adults in hospital (CG174): Recommendations
Guidance supporting fluid balance chart monitoring and reassessment for patients continuing on IV fluids.