Fluid Deficit Calculator

The WHO and ETAT/AAP frameworks use three bands based on acute body weight loss. Mild dehydration is 3 to 5 percent (clinically dry mucous membranes, mild thirst, normal vitals). Moderate is 6 to 9 percent (sunken eyes, decreased turgor, tachycardia, decreased urine output, capillary refill 2 to 3 seconds). Severe is 10 percent or more (cool extremities, marked tachycardia, weak pulses, capillary refill over 3 seconds, lethargy or coma); at this point the patient is at or in hypovolemic shock. The calculator uses the midpoint of each band (4%, 7.5%, 10%) to estimate the fluid deficit in ml as weight in kg times percent times 10, because a 1 percent body-weight loss equals roughly 10 ml/kg of total body water.

The 1957 Holliday-Segar method estimates daily maintenance fluid for children by body weight: 100 ml/kg/day for the first 10 kg, 50 ml/kg/day for the next 10 kg (11 to 20), and 20 ml/kg/day for each kilogram above 20. A 25 kg child needs (10 x 100) + (10 x 50) + (5 x 20) = 1,600 ml/day, or about 67 ml/hour. This calculator applies Holliday-Segar automatically for infants and pediatric patients, then switches to 30 ml/kg/day for adults (the lower end of the ASPEN 30 to 35 ml/kg/day band). The total IV rate combines deficit plus maintenance over 24 hours plus any ongoing losses (vomiting, diarrhea, NG output, drains).

Children, and infants especially, sit much closer to lethal extremes in both directions. Total body water is a larger fraction of body weight (about 75% in newborns versus 60% in adults), so the same percent dehydration represents a larger absolute volume swing relative to body size. Their renal concentrating ability is immature, so they cannot mask early hypovolemia by holding urine the way an adult can. Blood pressure stays normal until late in shock, so tachycardia, capillary refill, mental status, and urine output are the early warning signs. Overcorrection is just as dangerous: hyponatremic encephalopathy from large volumes of hypotonic fluid is a documented preventable cause of pediatric arrest, which is why the 2018 AAP guideline recommends isotonic crystalloid (LR or 0.9% NS) for replacement, not D5 1/4 NS. In practice this means weight-based rates only, hourly reassessment, and a low threshold to involve pediatric ICU.

Both are isotonic crystalloids and either works for most adult and pediatric replacement. Lactated Ringer's (LR) is closer to plasma in composition (sodium 130, chloride 109, potassium 4, calcium 1.5, lactate 28 mmol/L) and avoids the hyperchloremic metabolic acidosis that follows large volumes (more than 2 to 3 liters) of 0.9% NS, which is why guidelines (Surviving Sepsis, ATLS, PALS, SMART trial 2018) increasingly favor balanced crystalloids for volume resuscitation and shock. NS is still standard in DKA (the chloride load matters less when the patient is already acidotic from ketones, and lactate can muddy lactate-clearance monitoring), TBI (avoid lactate which can confuse cerebral metabolism), and when LR is contraindicated (calcium-binding products like ceftriaxone in the same line, or hyperkalemia where the 4 mmol/L of K in LR adds load). D5 1/2 NS is a classic pediatric maintenance fluid but not a resuscitation fluid; in 2018 the AAP moved replacement squarely to isotonic.

Escalate immediately for any of the following: a true shock state that does not respond to two 20 ml/kg isotonic boluses (consider third bolus, vasopressors, central access, and PICU/ICU admission); severe dehydration with altered mental status, lethargy, or coma; acute kidney injury (creatinine rise, oliguria less than 0.5 ml/kg/hr in adults or less than 1 ml/kg/hr in children for 6 hours despite resuscitation); electrolyte derangements that need monitored replacement (sodium under 125 or over 155, potassium under 2.5 or over 6.5, profound metabolic acidosis with pH under 7.2); ongoing losses that outpace replacement (high-output GI losses, burns over 20% BSA, pancreatitis with third-spacing); pulmonary edema or hypoxia developing during resuscitation; and any patient with CHF, cirrhosis, or advanced CKD where standard rates risk volume overload. Infants under 1 year with severe dehydration always warrant a same-day pediatric consult.