Weight-Based Dosage Calculation: mg/kg, mcg/kg/min, and Patient-Specific Dosing

1) Convert pounds to kilograms (and verify the weight unit before you calculate)

Weight-based dosing is only as accurate as the weight you start with. Many errors happen when a weight in lb is accidentally treated as kg, or when an old weight is used. Before doing any math, confirm the order’s weight unit and the most recent documented weight (and whether it is actual, ideal, or adjusted body weight).

Step-by-step: lb to kg

• Formula: kg = lb ÷ 2.2
• Keep more digits during calculation; round at the end per policy.
• Sanity check: kg should be less than lb (because 1 kg ≈ 2.2 lb).

Example: Patient weight = 44 lb.

kg = 44 ÷ 2.2 = 20 kg

Quick verification habit: write the weight with units in your setup (e.g., 20 kg) and circle it. If the order lists weight in kg already, still confirm it is plausible for the patient and current (especially pediatrics and ICU).

Common weight pitfalls to actively check

• Stated vs. measured weight: use measured weight when available.
• Bed scale vs. admission weight: in critical care, fluid shifts can change weight; follow protocol for which weight to use.
• Unit mismatch: if the chart shows 70 and the patient is an adult, ask: 70 kg or 70 lb?

2) mg/kg per dose vs. mg/kg/day (divided dosing) and interpreting frequency

Weight-based orders commonly appear in two formats:

• mg/kg/dose: the calculated amount is given each time the medication is administered.
• mg/kg/day: the calculated total daily amount must be divided across the number of doses per day (based on frequency).

A. mg/kg per dose

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(ordered mg/kg/dose) × (patient kg) = mg per dose

Example: Order: cefazolin 25 mg/kg/dose IV q8h. Weight: 20 kg.

25 mg/kg/dose × 20 kg = 500 mg per dose

Interpretation: q8h means 3 doses/day, but you do not divide the 500 mg further because the order is already “per dose.”

B. mg/kg/day divided dosing

Step 1 (daily total):

(ordered mg/kg/day) × (patient kg) = total mg/day

Step 2 (per-dose amount):

mg/day ÷ (doses per day) = mg per dose

Example: Order: amoxicillin 45 mg/kg/day PO divided q12h. Weight: 20 kg.

Total mg/day = 45 mg/kg/day × 20 kg = 900 mg/day

q12h = 2 doses/day → 900 mg/day ÷ 2 = 450 mg per dose

Frequency decoding (to avoid “divide when you shouldn’t”)

• q6h = 4 doses/day
• q8h = 3 doses/day
• q12h = 2 doses/day
• q24h = 1 dose/day
• TID = 3 doses/day; BID = 2 doses/day; QID = 4 doses/day

Key rule: Only divide by doses/day when the order is written as mg/kg/day (or mcg/kg/day). Do not divide when it is written as mg/kg/dose.

3) Infusion-style weight dosing: mcg/kg/min (convert time, convert mcg↔mg, and derive pump settings)

In critical care, continuous infusions are often ordered in mcg/kg/min. Your job is to translate that into a pump rate (commonly mL/hr) using the medication concentration (e.g., mg/mL).

Core pathway (most common): mcg/kg/min → mcg/min → mcg/hr → mg/hr → mL/hr

Step-by-step template:

1) (mcg/kg/min) × (kg) = mcg/min (for this patient)  [patient-specific dose rate]

2) mcg/min × 60 min/hr = mcg/hr

3) Convert mcg/hr to mg/hr when needed (1 mg = 1000 mcg)

4) mg/hr ÷ (mg/mL) = mL/hr  [pump setting]

Worked example: derive mL/hr from an order and a bag concentration

Order: Dopamine infusion at 5 mcg/kg/min. Weight: 70 kg. Bag label: Dopamine 400 mg in 250 mL.

Step 1: patient-specific mcg/min

5 mcg/kg/min × 70 kg = 350 mcg/min

Step 2: convert to mcg/hr

350 mcg/min × 60 = 21,000 mcg/hr

Step 3: convert to mg/hr

21,000 mcg/hr ÷ 1000 = 21 mg/hr

Step 4: compute concentration (mg/mL) and then mL/hr

Concentration = 400 mg ÷ 250 mL = 1.6 mg/mL

mL/hr = 21 mg/hr ÷ 1.6 mg/mL = 13.125 mL/hr

Round the pump rate per institutional policy (often to the nearest tenth for many drips): 13.1 mL/hr (if appropriate for your pump/policy).

Alternative pathway: if the concentration is given as mcg/mL

Sometimes the label is already in mcg/mL. Then you can skip the mcg→mg conversion:

(mcg/hr) ÷ (mcg/mL) = mL/hr

Mini-example: Order 0.1 mcg/kg/min, weight 20 kg, concentration 4 mcg/mL.

0.1 × 20 = 2 mcg/min

2 mcg/min × 60 = 120 mcg/hr

120 mcg/hr ÷ 4 mcg/mL = 30 mL/hr

Common infusion conversion checkpoints

• Minutes to hours: multiply by 60 (not divide).
• mcg to mg: divide by 1000.
• Keep units visible: write them in-line so they cancel correctly.

4) Safety checks: dose range verification, maximum doses, and ideal/adjusted body weight (when protocol specifies)

Weight-based dosing must be paired with safety checks. The goal is to confirm that the calculated dose is both mathematically correct and clinically reasonable within ordered parameters and facility protocols.

A. Verify ordered dose is within a safe range

Many orders include a recommended range (or your facility references a range). Compare your calculated dose to the range using the same unit basis.

Example (per-dose range): Range 10–15 mg/kg/dose. Weight 18 kg. Provider order: 300 mg per dose.

Low end = 10 mg/kg × 18 kg = 180 mg/dose

High end = 15 mg/kg × 18 kg = 270 mg/dose

Ordered 300 mg exceeds the high end (270 mg) → hold and clarify per policy.

B. Check maximum doses (dose caps)

Some medications have a maximum single dose or maximum daily dose regardless of weight. Apply the cap after calculating the weight-based dose.

Example: Order basis 15 mg/kg/dose, weight 80 kg, but max single dose 1000 mg.

15 × 80 = 1200 mg/dose → exceeds max

Use 1000 mg/dose (and clarify if the order conflicts with the cap).

C. Use the correct “weight type” when specified: actual vs. ideal vs. adjusted

Some protocols specify which weight to use:

• Actual body weight (ABW): common in pediatrics and many general meds.
• Ideal body weight (IBW): sometimes used for drugs that distribute poorly into adipose tissue or for certain ventilator/ICU protocols.
• Adjusted body weight (AdjBW): sometimes used when dosing should partially account for excess adipose tissue.

Practice point: do not substitute ABW/IBW/AdjBW based on guesswork. Use what the order set, protocol, or pharmacist specifies. If the order says “dose using IBW,” confirm IBW is documented or calculated per your facility method before proceeding.

D. Infusion guardrails

• Confirm the order’s unit basis matches the pump library entry (e.g., mcg/kg/min vs mcg/min).
• Re-check concentration: verify the bag label matches what you used (total drug amount and total volume).
• For titratable drips, verify min/max rate limits and ensure your calculated mL/hr corresponds to the ordered mcg/kg/min.

5) Case-based exercises: extract key data, calculate, and verify dose and rate

In real medication administration, you rarely receive a “clean” math problem. You must extract the weight, dosing basis, frequency, and concentration from the order and label, then calculate and verify.

Exercise 1: mg/kg/dose with lb→kg conversion

Order: Ketorolac 0.5 mg/kg/dose IV q6h PRN pain. Chart: Weight 66 lb. Policy note: Max single dose 15 mg.

Step A: convert weight

kg = 66 ÷ 2.2 = 30 kg

Step B: calculate mg per dose

0.5 mg/kg × 30 kg = 15 mg per dose

Step C: safety check

• Calculated dose equals max single dose (15 mg) → acceptable if no other contraindications and policy allows.
• Because it is mg/kg/dose, do not divide by 4 even though q6h allows up to 4 doses/day.

Exercise 2: mg/kg/day divided dosing with oral suspension label

Order: Clindamycin 30 mg/kg/day PO divided q8h. Weight: 16 kg. Label: 75 mg/5 mL.

Step A: total mg/day

30 mg/kg/day × 16 kg = 480 mg/day

Step B: doses per day

q8h = 3 doses/day

Step C: mg per dose

480 mg/day ÷ 3 = 160 mg/dose

Step D: convert mg dose to mL dose using label concentration

75 mg/5 mL = 15 mg/mL

mL per dose = 160 mg ÷ 15 mg/mL = 10.666... mL

Round per policy for oral syringes (example): 10.7 mL.

Exercise 3: mcg/kg/min infusion to mL/hr (with mg/mL concentration)

Order: Norepinephrine start 0.05 mcg/kg/min. Weight: 82 kg. Bag label: Norepinephrine 4 mg in 250 mL. Protocol: Verify starting dose is within 0.01–0.1 mcg/kg/min.

Step A: range check (ordered vs protocol)

Ordered 0.05 is within 0.01–0.1 → proceed.

Step B: patient-specific mcg/min

0.05 mcg/kg/min × 82 kg = 4.1 mcg/min

Step C: mcg/hr

4.1 × 60 = 246 mcg/hr

Step D: convert to mg/hr

246 mcg/hr ÷ 1000 = 0.246 mg/hr

Step E: concentration and pump rate

Concentration = 4 mg ÷ 250 mL = 0.016 mg/mL

mL/hr = 0.246 mg/hr ÷ 0.016 mg/mL = 15.375 mL/hr

Round per pump policy (example): 15.4 mL/hr.

Exercise 4: identify when a maximum dose overrides mg/kg/day

Order: Medication X 12 mg/kg/day IV divided q12h. Weight: 90 kg. Protocol: Max total daily dose 800 mg/day.

Step A: calculate weight-based daily dose

12 × 90 = 1080 mg/day

Step B: apply max daily dose

Use 800 mg/day (cap)

Step C: divide by frequency

q12h = 2 doses/day → 800 ÷ 2 = 400 mg/dose

Exercise 5: extract data from a mixed-format order and label (infusion titration)

Order: “Start infusion at 2 mcg/kg/min, titrate by 1 mcg/kg/min q10 min to maintain target; max 10 mcg/kg/min.” Weight: 24 kg. Bag label: Drug Y 200 mg in 100 mL.

Step A: compute concentration

200 mg ÷ 100 mL = 2 mg/mL

Step B: starting rate (2 mcg/kg/min)

2 mcg/kg/min × 24 kg = 48 mcg/min

48 × 60 = 2880 mcg/hr

2880 mcg/hr ÷ 1000 = 2.88 mg/hr

mL/hr = 2.88 mg/hr ÷ 2 mg/mL = 1.44 mL/hr

Step C: titration step size (1 mcg/kg/min)

1 mcg/kg/min × 24 kg = 24 mcg/min

24 × 60 = 1440 mcg/hr = 1.44 mg/hr

mL/hr change = 1.44 mg/hr ÷ 2 mg/mL = 0.72 mL/hr

Step D: maximum rate (10 mcg/kg/min)

10 × 24 = 240 mcg/min

240 × 60 = 14,400 mcg/hr = 14.4 mg/hr

mL/hr = 14.4 ÷ 2 = 7.2 mL/hr

Verification checklist for this case:

• Starting mL/hr corresponds to ordered mcg/kg/min.
• Each titration step corresponds to a consistent mL/hr change.
• Maximum mL/hr aligns with max mcg/kg/min and stays within pump library limits.