How is dehydration percentage calculated?

Dehydration % = (pre-illness weight − current weight) / pre-illness weight × 100. A 10 kg child who now weighs 9.5 kg has lost 0.5 kg, giving 0.5 / 10 × 100 = 5% dehydration.

What are the clinical severity thresholds?

Standard pediatric guidelines classify dehydration as mild (less than 5% body weight loss), moderate (5–9%), and severe (10% or more). Some sources use slightly different cutoffs; always follow local institutional protocols.

How is the fluid deficit in mL derived?

Each kilogram of weight lost approximates 1 liter (1000 mL) of fluid. So fluid deficit (mL) = weight loss (kg) × 1000. A 0.5 kg loss equals a 500 mL deficit.

What is oral rehydration solution (ORS)?

ORS is a glucose-electrolyte solution recommended by the WHO for mild to moderate dehydration. It contains sodium, potassium, chloride, bicarbonate or citrate, and glucose in concentrations that optimize intestinal water absorption. Commercial formulations (Pedialyte, Rehydralyte) or WHO-ORS sachets are standard.

When should IV fluids be used instead of ORS?

Intravenous rehydration is indicated for severe dehydration (≥10% weight loss), shock, altered consciousness, inability to tolerate oral intake, or persistent vomiting that prevents ORS. A clinician must evaluate these situations.

Is this calculator accurate without a known pre-illness weight?

Pre-illness weight is essential for an accurate percentage. If unknown, clinical signs (skin turgor, mucous membranes, sunken fontanelle, capillary refill) are used alongside growth chart reference weights. This tool requires a known baseline weight for a numeric result.

Pediatric Dehydration Percentage Calculator

Enter pre-illness and current weight to calculate dehydration percentage, classify severity, and estimate fluid deficit with rehydration guidance.

Patient Weights

Weight Unit

Pre-illness Weight (kg)

Weight before onset of illness

Current Weight (kg)

Weight at time of assessment

Medical disclaimer: This tool is for educational reference only. Results must be verified by a licensed clinician before any clinical decision, treatment, or fluid administration.

Enter pre-illness and current weights to calculate dehydration.

Summary

This tool estimates pediatric dehydration as a percentage of body weight lost (pre-illness weight minus current weight, divided by pre-illness weight, multiplied by 100). Severity is classified per standard pediatric clinical thresholds: mild (<5%), moderate (5–9%), and severe (≥10%). The calculated fluid deficit (mL) equals the weight loss in kg multiplied by 1000. Guidance on oral rehydration solution (ORS) or intravenous fluid replacement is displayed per severity tier. All output is for educational reference only and must be verified by a licensed clinician before any clinical decision.

How it works

Enter the child's pre-illness (baseline) weight in kilograms or pounds.
Enter the child's current weight after illness onset.
Dehydration % is computed as (pre-illness weight − current weight) / pre-illness weight × 100.
The fluid deficit in mL equals weight loss in kg × 1000.
Severity is classified: mild <5%, moderate 5–9%, severe ≥10%.
Rehydration guidance is displayed for the classified severity tier.

Use cases

Estimate dehydration severity in a child presenting with vomiting and diarrhea.
Calculate fluid deficit in mL to guide oral rehydration therapy planning.
Quickly classify dehydration as mild, moderate, or severe for triage decisions.
Cross-check a bedside dehydration assessment during a pediatric consultation.
Teach nursing students about weight-based dehydration percentage calculations.
Track rehydration progress by comparing serial weights during inpatient care.

Frequently Asked Questions

Last updated: 2026-06-18 · Reviewed by Nham Vu