What a weight-based order actually specifies
A weight-based order gives you a rate, not a dose. 45 mg/kg/day is a rule for turning a patient's weight into milligrams; it becomes a dose only after you multiply. Two multiplications and one division separate the order from the syringe, and each is a place where a decimal point can move.
The first distinction to get right is whether the order is written per day or per dose. These are the same words with a factor of two, three or four between them. 45 mg/kg/day divided q12h for a 20 kg child means 900 mg across the day and 450 mg in each of two administrations. 45 mg/kg q12h means 900 mg each time and 1,800 mg across the day — twice the intended exposure. If the order is ambiguous, it is not an order; clarify it before you calculate.
The second step is the one nursing programmes call desired over have: divide the milligrams you want by the milligrams in each millilitre of what you actually hold, and you have the volume to draw up. That is all this calculator does, with the units handled for you and both ceilings checked. For a dose written per square metre instead, use the body surface area calculator first and bring the milligrams back here.
The two formulas, and why the units cancel
Write every step with its units attached and the arithmetic checks itself.
Step one, the dose. mg/kg/day × kg = mg/day. The kilograms cancel, leaving milligrams per day. Divide by administrations per day and the days cancel too, leaving milligrams per administration. If your answer still has a stray unit in it, you have multiplied where you should have divided.
Step two, the volume. Your stock preparation is a concentration: 250 mg in 5 mL is 50 mg per mL. Dividing the desired milligrams by that concentration cancels the milligrams and leaves millilitres. The classroom version — desired ÷ have × quantity — is the identical calculation with the division written out: 450 ÷ 250 × 5 = 9 mL, exactly the same as 450 ÷ 50.
Solid dose forms need no separate formula. Enter the tablet or capsule strength as the stock amount and 1 as the stock volume, and the volume output becomes a count of tablets. A result of 1.5 means one and a half tablets, which is only giveable if the tablet is scored and is not a modified-release product.
Concentration is where most real errors hide, because a label states a strength and a volume that are easy to read as a single number. Amoxicillin 250 mg/5 mL is 50 mg per mL, not 250 mg per mL. Reading it as the latter gives a fifth of the intended dose.
Worked example: amoxicillin 45 mg/kg/day for a 20 kg child
The order reads: amoxicillin 45 mg/kg/day divided every 12 hours. The child weighs 20 kg. The bottle on the shelf says 250 mg/5 mL. The formulary lists a maximum of 1,000 mg per dose.
- Daily dose. 45 mg/kg/day × 20 kg = 900 mg per day.
- Doses per day. Every 12 hours is 24 ÷ 12 = 2 administrations.
- Dose per administration. 900 ÷ 2 = 450 mg.
- Stock concentration. 250 mg ÷ 5 mL = 50 mg/mL.
- Volume to draw up. 450 mg ÷ 50 mg/mL = 9 mL per dose.
- Check the ceiling. 450 ÷ 1,000 = 45% of the maximum single dose. Safe to proceed.
Sanity-check the volume before you draw it. Nine millilitres is close to two teaspoons of syrup, which is a plausible dose for a 20 kg child. If the same arithmetic had produced 90 mL you would know immediately that a decimal point had moved — and the dose ladder in the results panel is there for exactly this reason: it shows what the order gives at 10 kg and 40 kg, so a mistyped weight stands out against the row you expected.
Now run the ceiling case. The same 45 mg/kg/day in a 60 kg adolescent is 2,700 mg per day, or 1,350 mg per dose — above a 1,000 mg single-dose maximum. The arithmetic is correct and the dose is not giveable. Weight-based rules are written for children, and they stop being appropriate somewhere near adult size, which is why the ceiling exists at all.
How to read the result and decide it is safe
Read four things, in this order.
The percentage of the maximum. Anything above 100% stops the process. Below 100%, the number still carries information: a dose at 95% of a ceiling leaves no room for the next prescriber to add a paracetamol-containing combination product, and a dose at 20% may mean the indication calls for a higher band.
The volume. Doses under 0.1 mL cannot be measured accurately in a standard 1 mL oral syringe and need a dilution. Oral liquid volumes above roughly 20 mL are usually a sign that a more concentrated preparation exists — a 400 mg/5 mL amoxicillin suspension rather than 250 mg/5 mL, for instance.
The dose per administration against what is dispensable. A calculated 1.37 tablets is not administrable. Most formularies allow rounding to a convenient dose within a defined tolerance; find the local rule rather than inventing one.
The weight itself. If the dose ladder row at half the entered weight looks like the dose you were expecting, you have entered pounds where kilograms were wanted — a 2.2-fold error that reads as entirely plausible at every downstream step.
Common paediatric liquid concentrations and what 1 mL contains
| Label states | Concentration (mg/mL) | Volume for a 200 mg dose |
|---|---|---|
| 125 mg / 5 mL | 25 | 8.0 mL |
| 160 mg / 5 mL | 32 | 6.25 mL |
| 200 mg / 5 mL | 40 | 5.0 mL |
| 250 mg / 5 mL | 50 | 4.0 mL |
| 400 mg / 5 mL | 80 | 2.5 mL |
| 100 mg / mL | 100 | 2.0 mL |
| 1 g / 10 mL | 100 | 2.0 mL |
The last two rows are the same concentration written two different ways — a reminder that a gram in ten millilitres and a hundred milligrams in one millilitre are identical products on paper.
Errors this calculation is designed to catch
- Pounds entered as kilograms. Multiplies every dose by 2.2. Weigh in kilograms and record in kilograms only; ISMP has long recommended that weights be documented in kilograms alone to eliminate this failure.
- Confusing mg/kg/day with mg/kg/dose. A factor of two to four, in the dangerous direction, and both phrasings appear on real orders.
- Misreading the concentration. 250 mg/5 mL is 50 mg/mL. Treating it as 250 mg/mL gives one fifth of the dose.
- Ignoring the total from all sources. A daily maximum applies to the drug, not to the order. Paracetamol in a combination analgesic counts against the same ceiling.
- Rounding a tablet count. 1.37 tablets is not a dose. Modified-release and enteric-coated tablets must not be split at all.
- Using a dry or estimated weight. Weight-based dosing assumes the number on the chart is real and current. An estimate carried forward from admission is not.
- Dosing an obese patient on total body weight without checking. Some drugs are dosed on ideal or adjusted body weight instead; the formulary, not the calculator, decides which.
What this calculator does not know
It does not know your drug. It has no formulary, so the maximum single and daily doses are whatever you type in; entering nothing means nothing is checked. It does not know the indication, and many drugs carry different mg/kg bands for different infections or severities. It does not know renal or hepatic function, so it will not tell you that a dose needs to be reduced or an interval extended — for that, start from an estimate of clearance such as Cockcroft-Gault or the CKD-EPI eGFR and apply the renal dosing table for the drug.
It also assumes a single stock preparation and a uniform dose at every administration. Loading doses, tapering regimens, alternating-day schedules and doses that differ morning and evening all need the arithmetic run more than once.
Finally, it is arithmetic and nothing more. A number produced here is a proposal to be checked against the order, the label, the patient and the formulary, and in most settings against a second qualified person. Injectable and high-alert medicines require an independent double check regardless of how simple the sum looks.
Check the answer before you check the arithmetic
Experienced practitioners run the estimate first. Round the weight and the dose to easy numbers and see roughly where you should land: 45 mg/kg/day at 20 kg is about 1,000 mg a day, so about 500 mg twice daily, so about 10 mL of a 50 mg/mL syrup. Any exact answer that is not near that estimate is wrong, whatever the calculator says — and this habit catches the whole class of errors where a single keystroke moved a decimal point.
Where this fits among the other dosing calculations
Weight-based dosing is the default for children and for many adult drugs, but it is one of four scales in routine use. Per square metre is the oncology convention and needs a BSA first. Age-based rules such as Clark's and Young's rules survive mainly in teaching and in settings without a weight; the Clark's rule calculator covers them, and they are a fallback, not a preference. Flat dosing is used where the exposure-response relationship is shallow enough that size does not matter.
Once the milligrams are settled, the remaining question is how to give them. An intermittent infusion becomes a pump rate through the mL/hour calculator, a gravity set needs the drip rate calculator, and a titrated ICU drip ordered per kilogram per minute goes through the mcg/kg/min converter. Each of them starts from a concentration, which is why getting the stock strength right here pays off twice.