A 12-kilo child at 5% dehydration has lost 600 ml, and over four hours needs about 783 counting maintenance. Work out the volume of oral rehydration solution.
Total fluid for the period
—
Total fluid for the period—
Per hour—
Deficit already lost—
Maintenance for the period—
WHO plan B—
In 5 ml teaspoons, per hour—
How this was worked out
Indicative result. The calculation does not replace clinical assessment. Whether rehydration should be oral or intravenous, and under what supervision, is a medical decision; severe dehydration or a child who cannot keep fluids down needs immediate care.
The formula
total = weight × % dehydration × 10 + maintenance for the period
Where it comes from
Dehydration is estimated as a percentage of body weight lost, and each percentage point is ten millilitres per kilo: 5% in twelve kilos is six hundred millilitres. To that deficit you add what the body needs anyway over those hours, worked out with the Holliday-Segar rule: one hundred millilitres per kilo for the first ten kilos, fifty for the next ten and twenty for the rest.
How to work it out by hand
Deficit: weight × percentage of dehydration × 10
Daily maintenance: 100 ml/kg for the first 10 kg, 50 for the next 10, 20 for the rest
Take the share of maintenance matching the hours of replacement
Add deficit, maintenance and any losses still happening
What is worth knowing
The WHO oral rehydration solution has been the reduced-osmolarity one since 2003: 75 millimoles of sodium and 75 of glucose per litre, 245 mOsm/l in total. The reduction against the older formula shortens diarrhoea and cuts vomiting, and the ratio between sodium and glucose is not accidental: glucose drags sodium across the intestinal wall, and water with it. That is why a soft drink is no substitute: it has far too much sugar and almost no sodium, and can make diarrhoea worse.
Frequently asked questions
How much solution does a dehydrated child need?
It depends on weight and degree: at 5% it is about 50 ml per kilo of deficit, plus maintenance for those hours.
What is WHO plan B?
The moderate dehydration regimen: 75 ml per kilo of oral solution over four hours, in small frequent sips.
Can I use sports drinks or soft drinks?
Not as a substitute. They carry too much sugar and too little sodium, and can make diarrhoea worse.
How do I give it if the child vomits?
Little and often: five-millilitre teaspoons every few minutes. A whole glass at once usually ends in another vomit.
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