Paediatric Dehydration Fluid Deficit Calculator
Paediatric Dehydration Fluid Deficit Calculator
Fluid deficit in millilitres from percentage dehydration and weight, with the replacement schedules that are actually published — and the finding that clinical estimation of percentage dehydration is unreliable, which is the input this arithmetic depends on.
Fluid deficit from percentage dehydration
% dehydration + weight → mL deficitA 12 kg child estimated at 5% dehydration
Formula
equivalently: deficit (mL) = % dehydration × 10 mL for every kilogram
- × 10
- where the formula comes from. 1% of body weight is 10 g per kilogram, and 1 g of water is 1 mL, so 1% dehydration is 10 mL/kg. Nothing in the constant is empirical; it is a unit conversion
- % dehydration
- the fraction of body weight lost as water. Measured where a pre-illness weight exists; estimated, unreliably, where it does not. BSPED assumes 5% for mild and moderate DKA and 10% for severe rather than estimating it at all
- weight (kg)
- the pre-illness weight, which is the denominator the percentage refers to. Using the current weight underestimates the deficit slightly, by the percentage itself
- what this is not
- maintenance, which is added separately by the Holliday-Segar method; ongoing losses, which are a third figure; and resuscitation volume, which is 10 mL/kg boluses of glucose-free isotonic crystalloid and is not part of any deficit calculation except in DKA, where the non-shock bolus is subtracted
- the tonicity
- isotonic. Sodium 131 to 154 mmol/L per NICE NG29, and isotonic solutions with potassium chloride and dextrose per the AAP 2018 guideline for children 28 days to 18 years. Hypotonic maintenance fluid is why that recommendation exists
- over how long
- 5% over 24 hours alongside maintenance, with anything above 5% spread over subsequent days — except in DKA, where the whole deficit goes over 48 hours
Worked example
A 12 kg child estimated at 5% dehydration
Deficit = 5 × 12 × 10 = 600 mL
Check the constant: 1% of 12 kg is 120 g of water, which is 120 mL, so 5% is 600 mL. The × 10 is a unit conversion, not an empirical coefficient
Routine maintenance for a 12 kg child by Holliday-Segar is 1,000 + (2 × 50) = 1,100 mL/day, so the deficit is about a third of the total 24-hour prescription of 1,700 mL and not the bulk of it
At 5%, the deficit goes over 24 hours alongside maintenance: 600 ÷ 24 = 25 mL/hour of deficit plus 1,100 ÷ 24 = 46 mL/hour of maintenance, giving about 71 mL/hour to start
The fluid is isotonic — sodium 131 to 154 mmol/L, with glucose and, once electrolytes are known, potassium
Had the estimate been 10% rather than 5%, the deficit would be 1,200 mL, and it would not be given in 24 hours: 5% in the first day and 5% in the second
Had this been diabetic ketoacidosis, 5% would have been assumed from the pH rather than estimated, the 1,200 mL of a severe (10%) deficit would go over 48 hours, and the initial 10 mL/kg non-shock bolus of 120 mL would be subtracted from it
And the 5% itself is the weak link. If a pre-illness weight of 12.6 kg existed, the measured loss would be 600 g — 4.8%, not 5% — and that number would be a measurement rather than an estimate with a likelihood ratio between 2 and 4
Where the percentage comes from, and how well it performs
| Source | What it says |
|---|---|
| BSPED, diabetic ketoacidosis guideline v3 | ‘It is not possible to accurately clinically assess the degree of dehydration to work out the deficit. Clinical methods are unreliable.’ Assume 5% for pH 7.1 to 7.29 and 10% for pH below 7.1 |
| Royal Children’s Hospital, intravenous fluids | ‘Clinician estimates of % dehydration have been shown to be unreliable’ — the guideline uses a flat hourly rate by weight instead |
| Royal Children’s Hospital, dehydration | ‘Serial weights are the best measure of acute changes in fluid status. Clinical signs can help estimate the severity of dehydration but are often imprecise’ |
| Is this child dehydrated? systematic review, JAMA 2004 | ‘Due to the imprecision and inaccuracy of available tests, it was difficult to estimate the exact level of dehydration’. Best individual signs for 5%: capillary refill positive LR 4.1 (1.7 to 9.8), skin turgor 2.5 (1.5 to 4.2), respiratory pattern 2.0 (1.5 to 2.7); combinations better than any single sign; laboratory tests of ‘modest utility’ |
| StatPearls, paediatric fluid management | Percentage body weight loss ‘is considered the gold standard for assessing dehydration’ but ‘is rarely available in the emergency department setting’ |
| NICE NG29 | Gives no formula for the deficit from a percentage at all — recommendation 1.5.1 asks only that prescriptions be adjusted for existing deficits, ongoing losses and abnormal distribution |
Deficit in mL, by weight and percentage
| Weight | 3% | 5% | 7% | 10% |
|---|---|---|---|---|
| 5 kg | 150 mL | 250 mL | 350 mL | 500 mL |
| 10 kg | 300 mL | 500 mL | 700 mL | 1,000 mL |
| 12 kg | 360 mL | 600 mL | 840 mL | 1,200 mL |
| 20 kg | 600 mL | 1,000 mL | 1,400 mL | 2,000 mL |
| 30 kg | 900 mL | 1,500 mL | 2,100 mL | 3,000 mL |
| 50 kg | 1,500 mL | 2,500 mL | 3,500 mL | 5,000 mL |
Replacement schedules, quoted from the guidelines that publish them
| Setting | Deficit | Over | Fluid |
|---|---|---|---|
| General paediatric rehydration (RCH) | 5% over 24 hours, remainder over following days | 24 hours plus maintenance | Sodium chloride 0.9% with glucose 5%, potassium added once electrolytes known |
| Gastroenteritis, oral route (NICE CG84) | 50 mL/kg | 4 hours, as well as maintenance | Oral rehydration salt solution |
| Gastroenteritis, intravenous after shock (NICE CG84) | 100 mL/kg added to maintenance | Not specified in the recommendation | Isotonic crystalloid |
| Gastroenteritis, intravenous without shock (NICE CG84) | 50 mL/kg added to maintenance | Not specified in the recommendation | Isotonic crystalloid |
| Shock (NICE NG29 1.3.1) | Not a deficit — 10 mL/kg bolus | Less than 10 minutes | Glucose-free crystalloid, sodium 131 to 154 mmol/L |
| Shock in a term neonate (NICE NG29 1.3.2) | Not a deficit — 10 to 20 mL/kg bolus | Less than 10 minutes | Glucose-free crystalloid, sodium 131 to 154 mmol/L |
| Diabetic ketoacidosis (BSPED v3) | 5% assumed for pH 7.1 to 7.29; 10% for pH below 7.1 | 48 hours alongside maintenance | Isotonic, per the DKA guideline; the 10 mL/kg non-shock bolus is subtracted from the deficit |
Precise arithmetic on an imprecise number
The formula is trivial and exactly right. One per cent of body weight is ten grams per kilogram, one gram of water is one millilitre, so one per cent of dehydration is ten millilitres per kilogram and the deficit is the percentage times the weight times ten. Two published worked examples land on it to the millilitre: a ten-kilogram child who now weighs 9.5 kg is five per cent dehydrated with a 500 mL deficit, and a twenty-kilogram child assumed to be five per cent dehydrated in ketoacidosis has a deficit of 1,000 mL. There is no coefficient to get wrong.
The number fed into it is a different matter, and this is the finding rather than a footnote. The systematic review of dehydration assessment in children concluded that the imprecision and inaccuracy of the available tests made it difficult to estimate the level of dehydration at all; the best individual physical signs for five per cent dehydration carried positive likelihood ratios of 4.1 for prolonged capillary refill, 2.5 for abnormal skin turgor and 2.0 for an abnormal respiratory pattern, and laboratory tests were of modest utility. The guidelines have acted on this. The British paediatric ketoacidosis guideline states that it is not possible to assess the degree of dehydration accurately and that clinical methods are unreliable, and therefore assumes a percentage from the blood pH instead of examining for one. The Royal Children’s Hospital intravenous fluids guideline says the same and abandoned percentage-based formulas entirely in favour of a flat hourly rate by weight. NICE NG29 offers no percentage-to-deficit formula at all.
Which leaves the pre-illness weight as the only measurement in the vicinity. Serial weights are described as the best measure of acute change in fluid status, and percentage body weight loss as the gold standard for assessing dehydration; the problem is availability, not validity. A weight from a clinic four weeks ago, from a personal child health record, or from a previous admission converts this page from an estimate into a subtraction, and it is worth several minutes of looking for.
The tonicity is settled and was not always. Hypotonic maintenance fluid caused hospital-acquired hyponatraemia in children, and both sides of the Atlantic now say the same thing: NICE NG29 asks for crystalloids containing sodium in the range 131 to 154 mmol per litre, and the American Academy of Pediatrics’ 2018 guideline makes isotonic maintenance a strong recommendation at evidence quality A for patients from 28 days to 18 years, with appropriate potassium chloride and dextrose. Both exclude the neonate under 28 days, and the AAP recommendation also excludes renal, hepatic, cardiac and neurosurgical disease, voluminous watery diarrhoea and severe burns — which is a reminder that a prescription this simple is only simple in the uncomplicated child.
Frequently asked questions
How do you calculate a fluid deficit in a child?
Deficit in millilitres equals the percentage dehydration multiplied by the weight in kilograms multiplied by ten. A 12 kg child estimated at 5% dehydration has a deficit of 5 × 12 × 10 = 600 mL. The ten comes from the fact that 1% of body weight is 10 g per kilogram and 1 g of water is 1 mL, so it is a unit conversion rather than an empirical constant.
How accurate is a clinical estimate of percentage dehydration?
Poor, and the guidelines say so. The BSPED diabetic ketoacidosis guideline states that it is not possible to assess the degree of dehydration accurately and that clinical methods are unreliable; the Royal Children’s Hospital intravenous fluids guideline abandoned percentage-based formulas for the same reason. In the systematic review, the best individual signs for 5% dehydration had positive likelihood ratios of only 4.1 (capillary refill), 2.5 (skin turgor) and 2.0 (respiratory pattern), and combinations of signs performed better than any single one.
Is a pre-illness weight better than examining the child?
Substantially. Percentage body weight loss is described as the gold standard for assessing dehydration, and serial weights as the best measure of acute changes in fluid status; the difficulty is that a recent weight is rarely available in an emergency department. Where one exists — a clinic record, a personal child health record, a previous admission — subtract the current weight from it and divide by it, and the percentage becomes a measurement.
What fluid should be used to replace a deficit in a child?
An isotonic crystalloid. NICE NG29 recommendation 1.4.3 asks for solutions containing sodium in the range 131 to 154 mmol/L, and the AAP 2018 clinical practice guideline recommends isotonic solutions with appropriate potassium chloride and dextrose for patients 28 days to 18 years, as a strong recommendation at evidence quality A. Hypotonic fluids are inappropriate for treating volume depletion in children, and hospital-acquired hyponatraemia from hypotonic maintenance fluid is the reason the recommendation exists.
Over how long should the deficit be given?
A 5% deficit over 24 hours alongside the maintenance rate, with anything above 5% spread over successive days, is the Royal Children’s Hospital schedule. NICE CG84 for gastroenteritis adds 100 mL/kg to maintenance after shock has been treated, or 50 mL/kg without shock, and gives 50 mL/kg of oral rehydration salt solution over 4 hours where the enteral route is tolerated. Diabetic ketoacidosis is different: BSPED replaces the deficit over 48 hours.
Does the deficit include maintenance fluid?
No. The deficit is added to maintenance, which is calculated separately by the Holliday-Segar method — 100 mL/kg/day for the first 10 kg, 50 for the next 10 and 20 for each kilogram above 20. Ongoing losses from vomiting, diarrhoea, a stoma or a drain are a third figure again. For a 12 kg child, maintenance of 1,100 mL/day is nearly twice a 5% deficit of 600 mL.
Related calculators
References
- National Institute for Health and Care Excellence. Intravenous Fluid Therapy in Children and Young People in Hospital. NICE guideline NG29; 2015, updated 2020 — recommendation 1.3.1 (10 mL/kg bolus, glucose-free crystalloid, sodium 131 to 154 mmol/L), 1.3.2 (term neonates, 10 to 20 mL/kg), 1.4.1 (Holliday-Segar) and 1.4.3 (isotonic maintenance).
- Clinical practice guideline: maintenance intravenous fluids in children. Pediatrics. 2018;142(6):e20183083 — isotonic solutions with appropriate potassium chloride and dextrose for patients 28 days to 18 years; evidence quality A, strong recommendation, with the exclusion list.
- British Society for Paediatric Endocrinology and Diabetes. BSPED Guideline for the Management of Children and Young People under the Age of 18 Years with Diabetic Ketoacidosis, version 3 — clinical methods for assessing dehydration are unreliable; 5% assumed for pH 7.1 to 7.29 and 10% for pH below 7.1; deficit over 48 hours; the non-shock bolus subtracted.
- Royal Children’s Hospital Melbourne. Clinical Practice Guidelines: Intravenous Fluids and Dehydration — clinician estimates of percentage dehydration are unreliable; serial weights are the best measure; 5% deficit over 24 hours plus maintenance, with the 10 kg / 9 kg worked example.
- Is this child dehydrated? JAMA. 2004 — systematic review; likelihood ratios for capillary refill, skin turgor and respiratory pattern, and the conclusion on the imprecision of estimating the level of dehydration. Read through the DARE structured abstract, NCBI Bookshelf NBK70977.
- National Institute for Health and Care Excellence. Diarrhoea and Vomiting Caused by Gastroenteritis in Under 5s. NICE clinical guideline CG84; 2009 — 50 mL/kg oral rehydration salt solution over 4 hours, and the 100 mL/kg and 50 mL/kg intravenous deficit figures.
- Pediatric Fluid Management. StatPearls, NCBI Bookshelf NBK560540 — percentage body weight loss as the gold standard but rarely available, and hypotonic fluids as inappropriate for treating volume depletion in children.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.