How is net fluid balance calculated?

Net fluid balance is total fluid input minus total fluid output over a period, usually 24 hours. Inputs include IV fluids, oral and enteral intake, medication and flush volumes and blood products. Outputs include urine, drains, nasogastric losses, stool and estimated insensible losses.

What are insensible losses?

Insensible losses are fluid lost through the skin and breathing that cannot be directly measured. They are commonly estimated at around 0.5 mL per kilogram per hour at rest, roughly 800 to 1000 mL per day in an adult. Fever increases them by about 12 percent for every degree Celsius above 37.

Should I include insensible loss in the chart?

Routine ward fluid balance charts often omit insensible losses and track only measured ins and outs. This tool lets you toggle the estimate on so you can see a more physiologically complete balance, which is especially useful in ICU, sepsis or burns where insensible losses are large.

Does a positive balance mean fluid overload?

Not necessarily. A single positive balance can be appropriate during resuscitation, while cumulative positive balance over several days is more concerning for overload. Always interpret the number alongside daily weights, observations, oedema and the overall clinical picture.

What units does the calculator use?

All volumes are entered in millilitres over a 24-hour period. The net result is shown in millilitres, with a plus sign for a net gain and a minus sign for a net loss. Blank fields are treated as zero so you only enter what is relevant.

24-Hour Fluid Balance Calculator

Daily fluid balance, ins minus outs

Accurate fluid balance underpins safe care in surgery, sepsis, heart failure and critical illness. This calculator tallies every measurable input and output over a 24-hour period and returns the net balance, with an optional estimate of insensible losses so the figure reflects real physiology rather than just what was charted.

How it works

The core calculation is simple addition:

net balance = total inputs − total outputs
total inputs  = IV + oral/enteral + IV meds/flushes + blood products
total outputs = urine + drains + NG/vomit + stool + insensible loss

The optional insensible loss term estimates fluid lost through skin and breathing:

insensible (mL/24h) = 0.5 × weight_kg × 24 × fever_multiplier

The fever multiplier adds about 12.5 percent per degree Celsius above 37, reflecting the rise in evaporative loss with pyrexia.

Notes and example

For a 70 kg patient receiving 2000 mL IV, 500 mL oral and 250 mL of medication, who passes 1800 mL of urine at a normal temperature, the measured balance is +950 mL — but adding ~840 mL of insensible loss brings the true balance close to even. That difference matters when titrating diuretics or maintenance fluids.

This estimate does not capture sweating, open wounds, stoma output or evaporative loss from burns; in those settings actual losses are far higher. Use the result as one input alongside daily weights, observations and clinical examination.