Pediatric Dose Calculator
Weight-based mg/kg dosing arithmetic with per-dose and daily ceilings. It supplies the arithmetic only — every number in it must come from approved labelling.
Pediatric Dose Calculator: with the default inputs, dose per administration is 300.
A weight measured today on a scale. Never an estimate, and never a remembered figure.
Per dose, not per day. Take it from the product's approved labelling for this exact indication, route and formulation.
The per-dose ceiling from the labelling. Leave at 0 if the labelling gives none — but check, because most weight-based paediatric doses have one.
The per-day ceiling from the labelling. Leave at 0 if there is none.
milligrams, after any ceiling has been applied.
- Total per day
- 900milligrams.
- Dose per administration before ceilings
- 300
- Total per day before ceilings
- 900
- Effective dose actually delivered
- 15mg/kg per dose after ceilings — lower than the target if a ceiling bit.
- Did a ceiling apply?
- No ceiling was entered
- Weight in kilograms
- 20
Assumptions
- This is an estimate for education. It does not diagnose anything and does not replace clinical judgement. It is not a prescribing tool.
- The calculator holds no drug data. The mg/kg figure and both ceilings are inputs and must come from the product's approved labelling or a paediatric formulary for the exact indication, route, formulation and age.
- The dose per kilogram is treated as a per-dose figure, not a per-day figure.
- A single-dose ceiling caps each administration; a daily ceiling caps the day and reduces each dose proportionally to stay within it.
- Weight-based dosing is not appropriate for neonates, whose dosing depends on postmenstrual age, and often stops applying once a child reaches adult size.
| Weight (kg) | Dose from mg/kg (mg) | After ceilings (mg) | Effective mg/kg |
|---|---|---|---|
| 5 | 75 | 75 | 15 |
| 10 | 150 | 150 | 15 |
| 20 | 300 | 300 | 15 |
| 30 | 450 | 450 | 15 |
| 40 | 600 | 600 | 15 |
| 50 | 750 | 750 | 15 |
| 70 | 1,050 | 1,050 | 15 |
Where the last two columns diverge, the ceiling is doing the work rather than the weight. That is the point at which a mg/kg rule has stopped being the right rule.
How this is worked out
The formula
Dose per administration = dose per kg × weight in kg then capped at the single-dose maximum, if there is one Total per day = dose per administration × doses per day then capped at the daily maximum, if there is one, and the per-dose amount is reduced to keep the day within that ceiling Effective dose delivered = final dose ÷ weight in kg
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Weight
- A weight measured today on a scale. Never an estimate, and never a remembered figure.in kg, lb · from 0 to 200 · defaults to 20
- Dose per kilogram
- Per dose, not per day. Take it from the product's approved labelling for this exact indication, route and formulation.from 0 to 2000 · defaults to 15
- Doses per day
- A number.from 1 to 24 · whole numbers only · defaults to 3
- Maximum single dose
- The per-dose ceiling from the labelling. Leave at 0 if the labelling gives none — but check, because most weight-based paediatric doses have one.from 0 to 100000 · defaults to 0
- Maximum daily dose
- The per-day ceiling from the labelling. Leave at 0 if there is none.from 0 to 200000 · defaults to 0
What you get back
- Dose per administrationmain answer
- milligrams, after any ceiling has been applied.
- Total per day
- milligrams.
- Dose per administration before ceilings
- Total per day before ceilings
- Effective dose actually delivered
- mg/kg per dose after ceilings — lower than the target if a ceiling bit.
- Did a ceiling apply?
- Weight in kilograms
What this assumes
- This is an estimate for education. It does not diagnose anything and does not replace clinical judgement. It is not a prescribing tool.
- The calculator holds no drug data. The mg/kg figure and both ceilings are inputs and must come from the product's approved labelling or a paediatric formulary for the exact indication, route, formulation and age.
- The dose per kilogram is treated as a per-dose figure, not a per-day figure.
- A single-dose ceiling caps each administration; a daily ceiling caps the day and reduces each dose proportionally to stay within it.
- Weight-based dosing is not appropriate for neonates, whose dosing depends on postmenstrual age, and often stops applying once a child reaches adult size.
About this calculator
This is an estimate for education. It does not diagnose anything and does not replace clinical judgement. It is not a prescribing tool. Do not use it to decide what to give a child. If you are a parent working out a dose, the right source is the packaging, the dispensing label, or a pharmacist.
What this page contains, and what it does not
It contains multiplication and two ceilings. It contains no drug data whatsoever — no doses, no maximums, no drug names. That is deliberate. The mg/kg figure and the ceilings are the part that has to be right, and the only defensible source for them is the product's own approved labelling or a paediatric formulary, read for the specific indication, route, formulation and age group. A calculator that ships an embedded dose table invites you to trust a number whose provenance you cannot see; this one makes you fetch it.
Two things about those figures catch people out. The same drug is dosed differently for different indications — the mg/kg for a routine infection and for a severe one are often not the same number. And the oral and intravenous doses are frequently different, sometimes by a factor of two.
Why there is a ceiling
Weight-based dosing works because drug clearance in children scales roughly with body size. It stops working when the child reaches adult size, because at that point a mg/kg rule keeps scaling while the adult dose does not. An adolescent weighing 75 kg, given a dose calculated at 15 mg/kg, receives more than an adult would.
Approved labelling handles this by stating the ceiling explicitly. Isoniazid preventive therapy in children is "10 mg/kg (up to 300 mg daily)" — and 300 mg is the adult dose. Ceftizoxime's paediatric schedule may be increased "not to exceed the maximum adult dose for serious infection". FDA guidance on paediatric studies makes the same point from the other direction: for adolescents, weight- or surface-area-based dosing is "not always necessary", and final recommendations may use weight bands instead.
So: enter the ceiling. If you cannot find one in the labelling, that is a reason to check again rather than to assume there is none. The table under the results shows what the ceiling does across a range of weights, which makes visible the point where the mg/kg rule stops being the right rule.
Where the errors actually happen
Rarely in the multiplication. The common failure modes are all upstream of it:
- The weight. Pounds entered as kilograms gives a dose 2.2 times too large. Use a weight measured today, on a scale, in kilograms. This calculator accepts pounds and converts, but the safer habit is to record the weight in kilograms in the first place.
- Per dose against per day. A figure of 40 mg/kg/day given three times daily is 13.3 mg/kg per dose, not 40. Confusing the two is the single most common way this arithmetic goes wrong, in either direction.
- Measuring the result. Below about 10 kg, small absolute errors are large relative ones. Oral liquids are measured in millilitres with an oral syringe, not with a kitchen spoon.
- Neonates. Do not use this page for a newborn. Clearance changes week by week over the first weeks of life, and many neonatal doses depend on postmenstrual age as well as weight. It is a separate discipline.
Whatever comes out of this page, have it checked independently by another person, alongside the source you took the mg/kg figure from.
Frequently asked questions
▸How do I calculate a dose in mg/kg?
Multiply the dose per kilogram by the child's weight in kilograms, then apply whichever ceiling the labelling gives. Check whether the mg/kg figure is per dose or per day before you start — that is where most errors begin.
▸Why does this calculator not know the dose for my child's medicine?
Because a dose depends on the indication, the route, the formulation and the age, and no calculator can safely guess which combination you mean. The number belongs in the approved labelling or a paediatric formulary, and you should be reading it there.
▸Why is there a maximum dose?
Because weight-based dosing stops working once a child reaches adult size — a mg/kg rule keeps scaling upward while the adult dose does not. Approved labelling states the ceiling; isoniazid in children, for example, is 10 mg/kg up to 300 mg daily, which is the adult dose.
▸Can I use this for a newborn?
No. Neonatal dosing depends on postmenstrual age as well as weight because clearance changes over the first weeks of life, and paediatric mg/kg rules do not transfer. That needs specialist input.
▸My child's weight is in pounds — does that matter?
Enter it as pounds and the calculator converts. The danger is entering a pounds figure while the units say kilograms, which produces a dose 2.2 times too large. Record weights in kilograms wherever you can.
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