Medication dosage by weight (mg/kg)
Many medication doses are prescribed as a mass of drug per kilogram of body weight (mg/kg). This calculator
performs the core arithmetic used in weight-based dosing and optionally converts the dose (mg) into a liquid volume (mL)
if a formulation concentration (mg/mL) is provided.
Safety disclaimer (read before using)
This tool is educational only. It does not consider diagnosis, age group, indication, renal/hepatic function,
drug interactions, contraindications, formulation limits, rounding rules, device calibration, or professional guidance.
Always confirm dosing with a qualified healthcare professional and the official medication label.
Step 1: Convert weight into kilograms
The dose rule is written in mg/kg, so weight must be in kilograms. If weight is entered in pounds (lb),
convert to kilograms using:
\[
1\ \text{lb} = 0.45359237\ \text{kg}
\]
\[
W_{\text{kg}} = W_{\text{lb}}\times 0.45359237
\]
If weight is already in kilograms, then \(W_{\text{kg}} = W_{\text{kg, entered}}\).
Step 2: Compute the dose in milligrams (mg)
If the prescribed guideline is \(\text{mg/kg}\), the raw per-dose amount is:
\[
\text{Dose}_{\text{raw}}(\text{mg}) = (\text{mg/kg})\times W_{\text{kg}}
\]
This is the most common weight-based calculation: multiply the dose rule by the patient’s weight in kilograms.
Step 3: Apply optional maximum caps (single-dose and daily)
Some medications specify a maximum allowed amount regardless of weight. If a maximum single-dose cap is provided,
the calculator uses the smaller of the raw dose and the cap:
\[
\text{Dose}_{\text{after single cap}} = \min\!\left(\text{Dose}_{\text{raw}},\ \text{Max single dose}\right)
\]
If a dosing frequency is entered (doses/day) and a max daily cap is also provided, the calculator can enforce the
daily maximum by limiting the per-dose amount to:
\[
\text{Per-dose limit from daily cap} = \frac{\text{Max daily dose (mg/day)}}{\text{doses/day}}
\]
\[
\text{Dose}_{\text{used}} = \min\!\left(\text{Dose}_{\text{after single cap}},\ \text{Per-dose limit from daily cap}\right)
\]
When a cap is triggered, the calculator displays a warning badge so it is clear that the computed dose was reduced to
satisfy the limit.
Step 4: Convert dose (mg) to volume (mL) using concentration
Liquid medications are often labeled with a concentration such as “40 mg/mL.” If the concentration is known, the volume
corresponding to the dose is:
\[
\text{Volume (mL)} = \frac{\text{Dose}_{\text{used}}(\text{mg})}{\text{Concentration}(\text{mg/mL})}
\]
The calculator shows volume only when the concentration is provided and greater than zero.
Step 5: Total daily dose (if doses/day is entered)
If the medication is taken multiple times per day, the total daily amount is:
\[
\text{Daily dose (mg/day)} = \text{Dose}_{\text{used}}(\text{mg})\times \text{doses/day}
\]
This number is useful for checking against a maximum daily limit when one is specified.
How to interpret the visualizations
-
Three-step flow ribbon (weight → mg → mL): summarizes how the calculation progresses and shows the final per-dose
amount. Hovering reveals exact numbers (and whether caps were applied).
-
Dose vs weight chart: shows how the per-dose amount changes as weight changes for the same mg/kg rule. If caps are
active, dashed lines indicate the cap thresholds. Hover to read values, use the mouse wheel to zoom, and drag to pan.
Clicking can set the weight to quickly explore “what-if” scenarios.
-
Cap warning badge: appears if the raw dose exceeds a single-dose cap or if the per-dose dose must be reduced to
respect a max daily limit.
-
Optional schedule tiles: repeat the same per-dose mg and mL amount across the day to illustrate dosing frequency.
They are not a prescription schedule.
Common unit pitfalls
-
kg vs lb: mg/kg requires kilograms. Always confirm the entered weight unit.
-
mg/mL vs mL: concentration must be in mg/mL for the division formula above.
-
Rounding: real dosing often requires rounding to measurable increments (syringe markings, tablet strengths),
which this calculator does not standardize.
The calculator’s outputs are best used to understand the arithmetic relationship between body weight, mg/kg dosing rules,
liquid concentration, and safety caps. For real medication use, dosing must be verified with professional guidance and
official labeling.
