Fluid balance: recording intake and output
- Calculation
Fluid balance is what goes in minus what comes out over a period; with daily weights, it shows whether the patient is retaining or losing fluid.
What it is
Fluid balance compares, over a shift or 24 hours, everything the patient takes in with everything they lose that can be measured. A positive balance means more came in than went out; a negative one, the opposite. It is read alongside daily weights, the best indicator of rapid fluid changes, and the clinical assessment (oedema, crackles, mucous membranes, blood pressure). Insensible losses through the skin and breathing are not measured in a container, so the measured balance does not include them.
Formulas
- Total intake = oral fluids + IV fluids + enteral feeds + tube flush water (as the protocol counts them).
- Total output = urine + vomit + liquid stool + drains + tube suction (whatever can be measured).
- Balance = total intake − total output.
- Hourly urine output (mL/h) = urine for the period (mL) ÷ hours in the period.
- Urine output by weight (mL/kg/h) = hourly urine output ÷ weight in kg.
- Daily weight: 1 kg gained or lost in 24 hours ≈ 1 litre of fluid.
How to calculate it
- Set the period (shift or 24 hours) and the cut-off time — adding different periods gives a false balance.
- Record every intake when it happens, with its route: oral, IV, enteral and flushes — they are forgotten by end of shift.
- Measure every output in a graduated container: urine, vomit, liquid stool, drains and suction — guessing misleads.
- Add intake and output separately — so each column is checked before you subtract.
- Subtract output from intake and write the sign (+ or −) — the sign says whether fluid is retained or lost.
- Work out the hourly urine output and compare it with the call limit — a low output can hide inside a total.
- Compare with the daily weight (same scale, same time, similar clothing) and the clinical assessment.
Worked examples with made-up numbers
| Case | Intake (mL) | Output (mL) | Result |
|---|---|---|---|
| 1. 8-hour shift | Oral 360 + IV 800 = 1160 | Urine 950 + vomit 150 = 1100 | 1160 − 1100 = +60 mL (positive) |
| 2. 8-hour shift with enteral feeding | IV 1200 + enteral 480 + flushes 120 = 1800 | Urine 1350 + drain 230 = 1580 | 1800 − 1580 = +220 mL (positive) |
| 3. 8-hour shift with diarrhoea | Oral 700 + IV 500 = 1200 | Urine 1100 + liquid stool 400 + tube 250 = 1750 | 1200 − 1750 = −550 mL (negative) |
| 4. 24-hour balance | Oral 1250 + IV 900 = 2150 | Urine 1800 + drain 120 = 1920 | 2150 − 1920 = +230 mL (positive) |
| 5. Hourly urine output over 8 h | Not applicable | Urine 200 in 8 h | 200 ÷ 8 = 25 mL/h: below 30 mL/h, call |
Common mistakes
- Forgetting tube flushes, the fluid used to dilute medications or secondary infusions.
- Estimating urine in a nappy or the bed instead of measuring it per protocol.
- Adding periods with different cut-off times.
- Writing the balance without a sign, or subtracting the wrong way round (output − intake).
- Trusting a 'normal' balance when the weight rises or oedema and crackles appear.
Double-checking
- Recheck each sum separately before subtracting, and re-add from the chart, not from memory.
- Compare the result with the previous shift's balance and the daily weight: a sudden change needs a second look.
- At handoff, state the balance with its sign and the hourly urine output.
- Call for urine output below 30 mL/h or 0.5 mL/kg/h, a weight change over 1 kg in 24 hours, or new crackles, oedema or breathlessness.
Test yourself
Think of the answer, then tap the question to see it.
Intake of 1160 mL and output of 1100 mL in a shift: what is the balance?
1160 − 1100 = +60 mL, a positive balance. Educational example, not a clinical order.
A patient passes 200 mL of urine in 8 hours: what is the hourly output and what do you do?
200 ÷ 8 = 25 mL/h, below 30 mL/h: assess the patient and call the provider.
What does a weight gain of 1 kg in 24 hours mean?
That the patient has probably retained about 1 litre of fluid. It is a significant change and is reported.
Why does the measured balance not include insensible losses?
Because they are lost by evaporation from the skin and lungs and cannot be collected in a container; so the balance is read with the weight and the clinical picture.
Elsewhere in DurgiPedia
Connections
Educational content for students; it does not replace your institution's protocols, supervised clinical training or a prescriber's order. Dose-calculation examples use made-up numbers: they are educational, not a clinical order.
Sources: Nursing Fundamentals, 15.6 Fluids and Electrolytes: Applying the Nursing Process (Open RN (Chippewa Valley Technical College), LibreTexts, 2021); Nursing Skills, 23.3 Intravenous Therapy Assessment (Open RN (Chippewa Valley Technical College), LibreTexts, 2021).
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Representative model, for educational purposes only.