Rex says
Same equations the textbooks use — just way less awkward. Pop in your stats and I'll tell you what the number actually means for you.
Try a scenario
Click to load — tweak from there.Inputs
Result
Replacement rate to add
60.0
Combined total infusion rate
135.0
Projected 24-hour output at this rate
1,440
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How to use this
- 1Enter measured output over the period (mL).
- 2Enter measurement period (hr).
- 3Enter current maintenance rate (optional) (mL/hr).
- 4Enter fraction of output to replace (%).
- 5Read your replacement rate to add on the right — it updates as you type.
- 6Hit Share to keep the scenario or send it to someone.
About this calculator
Nasogastric suction, high-output ostomies, and surgical drains remove fluid and electrolytes that maintenance fluid orders do not account for, and failing to replace them leads to progressive volume depletion and electrolyte derangement, classically hypokalemic, hypochloremic metabolic alkalosis from gastric losses. Standard practice replaces measured output, typically on a delayed schedule (replacing the prior period's output over the current period) using a fluid similar in electrolyte composition to what was lost — normal saline with potassium chloride is a common choice for gastric output given its high chloride and potassium content. This calculator totals a shift's measured output and converts it into an hourly replacement rate to add on top of the separately ordered maintenance fluids; it does not select the replacement fluid composition, which should match the source (gastric, biliary, pancreatic, ileostomy) and be confirmed against institutional protocol and electrolyte trends. This tool is for education and workflow support only — not medical advice. Always verify results with a clinician and use clinical judgment alongside your institution's protocols.
Worked example
Using the values the calculator loads with:
Inputs
- Measured output over the period: 480 mL
- Measurement period: 8 hr
- Current maintenance rate (optional): 75 mL/hr
- Fraction of output to replace: 100 %
Results
- Replacement rate to add: 60
- Combined total infusion rate: 135
- Projected 24-hour output at this rate: 1,440
What each field means
Inputs
- Measured output over the period (mL)
- The measured output over the period used in the calculation, measured in mL. Starts at 480 mL so you have a working example on load.
- Measurement period (hr)
- The measurement period used in the calculation, measured in hr. Starts at 8 hr so you have a working example on load. Accepted range: 1–24 hr.
- Current maintenance rate (optional) (mL/hr)
- The current maintenance rate (optional) used in the calculation, measured in mL/hr. Starts at 75 mL/hr so you have a working example on load.
- Fraction of output to replace (%)
- The fraction of output to replace used in the calculation, measured in %. Starts at 100 % so you have a working example on load. Accepted range: 0–150 %.
Results
- Replacement rate to add
- Returned as a decimal number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Combined total infusion rate
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Projected 24-hour output at this rate
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
What fluid should replace NG output?
Gastric output is rich in chloride, hydrogen, and potassium, so 0.45% or 0.9% normal saline with added potassium chloride is a common choice, but this varies by institution and the specific source of output (gastric vs. biliary vs. pancreatic vs. ileostomy each has a different electrolyte profile). Confirm with your pharmacy or protocol rather than assuming one fluid fits all drains.
Should I replace 1:1 or on a delay?
Many protocols replace the prior shift's or prior period's measured output over the following period, rather than trying to real-time match losses, since output can be sporadic. Replacing over the following 4-8 hours as this calculator models is a common, practical approach.
Why might I replace less than 100% of output?
In a fluid-overloaded patient, or one whose losses are decreasing, replacing at 50-75% of measured output can be appropriate to avoid overcorrection, especially if serum electrolytes are being trended and adjusted concurrently. This should be a clinical decision, not a default.
Does this replace maintenance fluids or add to them?
It adds to them. NG or drain output replacement addresses ongoing abnormal losses; maintenance fluids address baseline physiologic needs independent of those losses. Both are typically ordered separately and summed to get total infusion rate, as this calculator does.
Accuracy and limitations
- Population formulas describe averages; individual bodies vary, especially at the extremes of height, age, or muscle mass.
- Results do not account for medical conditions, medication, or pregnancy.
- Use this as a directional reference, not a diagnosis — talk to a clinician before acting on it.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). NG Tube Output Fluid Replacement Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/ng-tube-fluid-replacement
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/ng-tube-fluid-replacement" target="_blank" rel="noopener">NG Tube Output Fluid Replacement Calculator — RevenueLab</a> (2026).</p>
Source: [NG Tube Output Fluid Replacement Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/ng-tube-fluid-replacement) (2026).