How does the WHO Plan B dose work?

The WHO treatment Plan B for some (mild to moderate) dehydration gives approximately 75 mL of ORS per kilogram of body weight over the first four hours. The calculator multiplies weight by 75 and divides the total over four hours to give an hourly target, offered in frequent small sips.

How is the NICE approach different?

NICE estimates a fluid deficit of 50 mL/kg for clinical dehydration and replaces it over four hours, given on top of normal maintenance fluids. This calculator shows the deficit volume and, when maintenance is included, adds the Holliday-Segar maintenance requirement so you see the full target.

What is the Holliday-Segar maintenance rule?

Daily maintenance fluid is 100 mL/kg for the first 10 kg, plus 50 mL/kg for the next 10 kg, plus 20 mL/kg for each kilogram above 20 kg. The four-hour share is one sixth of that daily figure, which the NICE-plus-maintenance option adds to the deficit.

When is oral rehydration not appropriate?

Oral rehydration is unsuitable for severe dehydration or shock, persistent vomiting that prevents intake, ileus, or reduced consciousness, all of which need intravenous fluids and urgent assessment. ORS is for mild to moderate dehydration in a child who can drink.

Is this calculator a prescription?

No. It reproduces published WHO and NICE volume rules for education and bedside cross-checking. Actual fluid management depends on ongoing losses, clinical reassessment, and the responsible clinician's judgement, and should follow local paediatric guidance.

Oral Rehydration Solution Dosing Calculator

Oral rehydration solution (ORS) replaces water and electrolytes lost through diarrhoea and is the first-line treatment for mild to moderate dehydration in children. The volume given is weight-based, and the two most widely used schemes are the WHO Plan B and the NICE deficit-plus-maintenance approach.

How it works

The WHO Plan B for some dehydration uses a single weight rule over four hours:

WHO volume (4 h) = 75 mL/kg x weight

The NICE approach replaces an estimated deficit and, optionally, adds maintenance:

deficit (4 h)    = 50 mL/kg x weight
maintenance/day  = 100 mL/kg (first 10 kg)
                 + 50 mL/kg (next 10 kg)
                 + 20 mL/kg (over 20 kg)
4 h maintenance  = maintenance/day / 6

Each total is divided across four hours and given as frequent small sips to maximise absorption and reduce vomiting.

Notes and cautions

ORS is for the child who can drink and has mild to moderate dehydration. Severe dehydration, shock, persistent vomiting, ileus, or reduced consciousness require intravenous fluids and urgent assessment instead. Ongoing stool and vomit losses must be replaced in addition to the volumes here, and the child should be reassessed regularly. This calculator reproduces published WHO and NICE volume rules for education and cross-checking only; final fluid management follows local paediatric guidance and the responsible clinician’s judgement.