Resuscitation Fluid & Electrolyte Deficit Calculator

Resuscitation Fluid & Electrolyte Deficit Calculator: Plan Rehydration in Seconds

This free resuscitation fluid and electrolyte deficit calculator is built for nursing students, medical students, paramedics, and clinicians who need a fast, reliable way to work out an IV fluid plan for a dehydrated patient. In plain words, it answers four practical questions at once: how much fluid does this patient need just to keep their body running normally (maintenance fluid), how much extra fluid do they need to make up for what they've already lost (the deficit), how much sodium and potassium is missing, and — if the patient is in shock — how big should an emergency fluid bolus be right now.

Instead of pulling out a textbook or doing the math by hand under pressure, you enter a weight, pick a dehydration severity, and set your target sodium and potassium levels. The calculator instantly returns the maintenance rate, the fluid deficit, the sodium and potassium deficits, a resuscitation bolus volume, and a full 24-hour replacement schedule — plus a chart comparing two common replacement strategies and a complete written breakdown of every step, so you can see exactly how each number was reached.

Every formula used here is a standard, widely taught bedside formula — the Holliday-Segar maintenance fluid rule, the percent-dehydration deficit formula, and the total-body-water electrolyte deficit model. Nothing here replaces clinical judgment, a doctor's order, or your hospital's specific protocol, but it's an excellent way to check your own math, study for an exam, or quickly sanity-check a fluid order.

What Is a Fluid Deficit, and Why Does It Matter?

When a person loses more fluid than they take in — from vomiting, diarrhea, fever, heavy sweating, poor oral intake, or bleeding — their body ends up with less total water than it needs. This shortfall is called the fluid deficit. It's usually estimated as a percentage of body weight, because water makes up a large, fairly predictable share of a person's total mass, and losing even a small percentage of that water has a real, measurable effect on how the body works.

A mild fluid deficit (around 5% of body weight) might show up as slightly dry lips or a little less urine than usual. A moderate deficit (around 10%) starts to show more obvious signs — dry mouth, sunken eyes, skin that's slower to bounce back when gently pinched. A severe deficit (15% or more) is a medical emergency: the patient may be lethargic, have very little urine output, and show signs that circulation itself is being affected. Estimating this percentage accurately — usually from a combination of physical exam findings and, when available, a comparison to the patient's normal 'well' weight — is the first and most important step in building a rehydration plan.

The Fluid Deficit Formula

The formula used here is simple and widely taught: Fluid deficit (in mL) = body weight (in kg) x percent dehydration x 10. So a 10 kg child estimated at 10% dehydration has a fluid deficit of 10 x 10 x 10 = 1,000 mL — a full liter of fluid that needs to be replaced on top of their normal daily fluid needs.

This works because 1% of body weight lost as fluid is roughly equal to 10 mL of fluid per kilogram of body weight — a handy rule that comes from the fact that 1 kg of body weight is approximately 1 liter (1,000 mL) of fluid, so 1% of that kilogram is 10 mL. Multiplying by the weight in kilograms and the percent dehydration gives the total deficit directly, in milliliters.

Maintenance Fluids: The Holliday-Segar Rule (4-2-1 Rule)

On top of replacing what's already been lost, a patient still needs their normal, ongoing daily fluid intake — this is called maintenance fluid, and it's calculated separately from the deficit. The most widely used method is the Holliday-Segar rule, sometimes remembered as the '4-2-1 rule' because of the hourly rates it produces.

The rule works in weight bands: the first 10 kg of body weight needs 100 mL per kg per day (which works out to 4 mL/kg/hour), the next 10 kg (from 11 to 20 kg) needs 50 mL per kg per day (2 mL/kg/hour), and every kilogram above 20 kg needs 20 mL per kg per day (1 mL/kg/hour). For example, a 25 kg child would need: (10 kg x 100) + (10 kg x 50) + (5 kg x 20) = 1,000 + 500 + 100 = 1,600 mL per day, or about 67 mL per hour.

This formula was originally developed for children but is still commonly referenced (with some adjustment) for adults in certain settings, and it remains one of the most-taught fluid formulas in nursing and medical education because of how reliably it estimates day-to-day water needs across a wide weight range.

Sodium and Potassium Deficit: The Total-Body-Water Method

Dehydration rarely affects water alone — electrolytes like sodium and potassium are dissolved in that same body water, and significant fluid losses often bring significant electrolyte losses along with them. This calculator estimates both using the total-body-water (TBW) deficit model: Deficit (in mEq) = body weight (kg) x TBW fraction x (target concentration − current concentration).

The TBW fraction — the percentage of body weight that is water — changes with age and sex, which is why this calculator lets you pick a category: roughly 0.6 to 0.7 for infants and children (whose bodies are proportionally more water), 0.6 for adult males, and around 0.5 for adult females and older adults (who typically carry a higher proportion of body fat, which holds less water than lean tissue). Choosing the right category matters, because it directly scales the estimated deficit.

It's worth being upfront about a limitation here: potassium is mostly stored inside cells rather than dissolved freely in body water, so a total-body-water-based potassium deficit is a rougher estimate than the sodium figure, and real potassium replacement in practice is often guided by empiric, protocol-based dosing rather than this formula alone. Still, it's a useful teaching estimate and a reasonable starting point for planning.

Emergency Resuscitation Bolus: The 20 mL/kg Rule

When a patient shows signs of poor circulation from significant fluid loss — low blood pressure, a fast heart rate, cool extremities, delayed capillary refill — the priority shifts from a slow, planned replacement to immediate fluid resuscitation. The standard approach is a rapid isotonic crystalloid bolus, most commonly dosed at 20 mL per kilogram of body weight, given as quickly as safely possible (often over 10 to 20 minutes), with reassessment afterward to decide whether another bolus is needed.

This calculator lets you adjust the mL/kg figure, since some protocols use a more conservative 10 mL/kg bolus, especially for patients where fluid overload is a concern, such as those with heart or kidney problems. The resuscitation bolus is separate from — and given before — the calculated 24-hour maintenance-plus-deficit plan; it addresses the immediate emergency, not the total fluid replacement over the following day.

The 8/16-Hour Rule vs. an Even 24-Hour Replacement

Once the deficit and maintenance needs are known, the next question is how fast to give them. Two common strategies are shown here. The 8/16-hour rule gives half the calculated deficit plus a proportional share of maintenance fluid in the first 8 hours, then the remaining half of the deficit plus the rest of the maintenance fluid over the next 16 hours. This front-loaded approach is commonly taught for isotonic (isonatremic) dehydration, where sodium is close to normal and there's less risk in correcting the fluid deficit relatively quickly.

The even 24-hour approach instead spreads the entire deficit and maintenance volume out at one constant rate across the full day. This slower, steadier method is the generally preferred approach for hypernatremic dehydration (high sodium), because correcting a sodium imbalance too quickly can cause water to shift rapidly into brain cells, risking dangerous cerebral swelling. The chart on this page plots both strategies side by side so the difference in hourly rate is easy to see at a glance.

Worked Example: A 14 kg Child With Moderate Dehydration

Take a 14 kg child with an estimated 10% dehydration, a measured sodium of 130 mEq/L (target 140), and a potassium of 3.0 mEq/L (target 4.0). Maintenance fluid, by the Holliday-Segar rule: the first 10 kg needs 1,000 mL/day, and the remaining 4 kg needs 4 x 50 = 200 mL/day, for a total of 1,200 mL/day (50 mL/hour).

Fluid deficit: 14 kg x 10% x 10 = 1,400 mL. Using a child's TBW fraction of 0.6: sodium deficit = 14 x 0.6 x (140 − 130) = 84 mEq, and potassium deficit = 14 x 0.6 x (4.0 − 3.0) = 8.4 mEq. Total 24-hour fluid requirement (deficit plus maintenance) = 1,400 + 1,200 = 2,600 mL. Using the 8/16-hour rule, the first 8 hours would carry 700 mL (deficit half) plus 400 mL (8-hour share of maintenance) = 1,100 mL, or about 137 mL/hour — figures this calculator produces automatically once you enter the same numbers.

Common Mistakes When Calculating Fluid and Electrolyte Deficits

The most common mistake is confusing maintenance fluid with the deficit, and giving only one when the patient actually needs both — maintenance keeps up with ongoing daily losses, while the deficit replaces what's already been lost, and a complete plan almost always needs to account for both.

A second common mistake is using the wrong total-body-water fraction for the patient's age and sex, which can meaningfully skew an electrolyte deficit estimate — infants and children carry proportionally more water than adults, and adult women and older adults typically carry less than adult men.

A third mistake is applying a fast, front-loaded replacement schedule (like the 8/16-hour rule) to a patient with hypernatremic dehydration, where a slower, even correction is generally safer to avoid rapid, dangerous shifts in brain cell water content. Always match the replacement speed to the type of dehydration, not just the volume.

Real-World Uses of This Calculator

Nursing and medical students use fluid and electrolyte calculations constantly in coursework, clinical rotations, and licensing exam preparation — this calculator is a fast way to check hand-calculated answers or build intuition for how each input changes the result. Paramedics and emergency responders benefit from a quick, reliable resuscitation bolus figure when every minute counts in the field.

Bedside clinicians and pharmacists can use it as a quick sanity check against a written fluid order, catching an obvious mis-calculation before it reaches the patient. And for anyone studying pediatrics, emergency medicine, or critical care, working through several worked examples with this tool is a genuinely effective way to build lasting familiarity with formulas that come up on exams and in practice throughout a clinical career.

Resuscitation Fluid & Electrolyte Deficit Calculator: Quick Reference Summary

Maintenance fluid (Holliday-Segar): 100 mL/kg/day for the first 10 kg, 50 mL/kg/day for the next 10 kg, 20 mL/kg/day for each kg above 20 kg. Fluid deficit: weight (kg) x %dehydration x 10 = deficit in mL. Electrolyte deficit: weight (kg) x TBW fraction x (target − current concentration) = deficit in mEq. Resuscitation bolus: weight (kg) x mL/kg (commonly 20 mL/kg for an emergency isotonic crystalloid bolus).

This calculator is intended to support learning, exam preparation, and quick clinical sanity-checks — it is not medical advice, and it doesn't replace a clinician's judgment, a physician's order, or your institution's specific fluid and electrolyte protocols. Always confirm any real patient fluid plan against current clinical guidelines and appropriate medical supervision.