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ICU Drip Calculations: mcg/kg/min to mL/hr, Step by Step

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ICU drip calculations turn an ordered dose into a pump rate with one formula: mL/hr = dose (mcg/kg/min) × weight (kg) × 60 ÷ concentration (mcg/mL). Find the concentration first, keep every unit lined up, and check the answer by running the math backward.

This post walks through the formula, worked examples for norepinephrine, dopamine and dobutamine, insulin in units per hour and heparin in units per kilogram per hour, a quick checklist for catching errors, and a dosage-calculation practice question. Smart pumps and your unit protocol come first; this is the math behind them.

The mcg/kg/min to mL/hr formula

Every weight-based drip calculation is the same three moves: work out how much drug is in each mL, work out how much drug the patient needs per hour, and divide. The 60 converts minutes to hours. The dopamine label prints exactly this formula for its premixed bags.

Ordered asmL/hr equals
mcg/kg/mindose × weight (kg) × 60 ÷ concentration (mcg/mL)
mcg/mindose × 60 ÷ concentration (mcg/mL)
units/h or mg/hdose ÷ concentration (units/mL or mg/mL)
units/kg/hdose × weight (kg) ÷ concentration (units/mL)
Reverse: mL/hr to mcg/kg/minrate × concentration (mcg/mL) ÷ 60 ÷ weight (kg)
  1. Concentration: total drug in the bag ÷ total volume, converted to the dose unit (mg to mcg: × 1,000).
  2. Hourly need: multiply the dose by weight if it is per kg, and by 60 if it is per minute.
  3. Rate: hourly need ÷ concentration = mL/hr.
  4. Check: run it backward and compare with the usual dose range.

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Step 1: know the concentration

Most ICU errors start here, not in the arithmetic. Read the bag, not your memory: the same drug comes in more than one strength, and units must match the dose (mcg for mcg/kg/min, units for units/h).

Premixed strengths from the DailyMed labels. Your pharmacy may use others.
Drug and bagConcentration
Norepinephrine 4 mg in 250 mL16 mcg/mL
Dopamine 400 mg in 250 mL1,600 mcg/mL (also 3,200 mcg/mL)
Dobutamine 250 mg in 250 mL1,000 mcg/mL (also 2,000 and 4,000)
Regular insulin 100 units in 100 mL1 unit/mL
Heparin 25,000 units in 250 mL100 units/mL (25,000 in 500 mL is 50 units/mL)

Norepinephrine: mcg/kg/min and mcg/min

Order: norepinephrine 0.1 mcg/kg/min for an 80 kg patient, bag 4 mg in 250 mL (16 mcg/mL).

  • Hourly need: 0.1 × 80 × 60 = 480 mcg/h.
  • Rate: 480 ÷ 16 = 30 mL/hr.
  • Same patient ordered in mcg/min: 0.1 × 80 = 8 mcg/min; 8 × 60 ÷ 16 = 30 mL/hr.
  • Reverse check: 30 × 16 ÷ 60 ÷ 80 = 0.1 mcg/kg/min.

The label doses norepinephrine in mcg per minute (8 to 12 mcg/min to start, 2 to 4 mcg/min typical maintenance) and asks for a large vein. Many ICUs order it per kilogram instead. Know which one your order and pump library use, because 8 mcg/min and 8 mcg/kg/min differ by a factor of the patient's weight.

Dopamine and dobutamine: weight-based drips

Dopamine

Order: dopamine 5 mcg/kg/min for a 70 kg patient, bag 400 mg in 250 mL (1,600 mcg/mL). Hourly need: 5 × 70 × 60 = 21,000 mcg/h. Rate: 21,000 ÷ 1,600 = 13.1 mL/hr. The label starts adults at 2 to 5 mcg/kg/min, titrates in 5 to 10 mcg/kg/min steps, and caps at 50. Its own example: 60 kg at 2 mcg/kg/min on the 1,600 bag runs at 4.5 mL/hr.

Dobutamine

Order: dobutamine 5 mcg/kg/min for a 70 kg patient, bag 250 mg in 250 mL (1,000 mcg/mL). Rate: 5 × 70 × 60 ÷ 1,000 = 21 mL/hr. The label table agrees: at 70 kg, each 1 mcg/kg/min on that bag is 4.2 mL/hr, and 4.2 × 5 = 21. The usual range is 2 to 20 mcg/kg/min.

Insulin drip: units per hour

Regular insulin is usually mixed 1 unit per mL, so units/h and mL/hr are the same number. For DKA, the 2024 consensus starts a fixed rate of 0.1 units/kg/h once potassium is above 3.5. A 90 kg patient gets 9 units/h, which is 9 mL/hr. When glucose falls below 250 mg/dL, dextrose is added and the rate drops to 0.05 units/kg/h: 4.5 units/h, or 4.5 mL/hr.

The easy math hides the real risk: a different bag strength, or a rate typed in units when the pump expects mL. Recheck glucose and potassium as often as your protocol asks, usually every 1 to 2 hours for glucose in DKA.

Heparin drip: units/kg/h to mL/hr

Weight-based heparin for VTE starts with an 80 units/kg bolus and 18 units/kg/h. For an 80 kg patient: bolus 80 × 80 = 6,400 units; infusion 18 × 80 = 1,440 units/h. On a 25,000 units in 250 mL bag (100 units/mL), 1,440 ÷ 100 = 14.4 mL/hr.

Hang the 25,000 in 500 mL bag (50 units/mL) instead and the same dose needs 28.8 mL/hr. Program the pump for one bag while the other hangs and the patient gets half or double the dose. The label warns to confirm the strength before giving heparin. Dose changes after that follow your unit nomogram and aPTT or anti-Xa results.

How to check a drip rate before you start it

  • Weight in kg, from today, matching the weight the order and pump use (some protocols use a dosing weight).
  • Units match: mcg vs mg (1,000×), per minute vs per hour (60×), per kg or not.
  • The answer sits inside the usual range: a dopamine rate that works out to 150 mcg/kg/min is a decimal error, not a dose.
  • Run the math backward from the pump rate to the dose.
  • Use the pump drug library, and an independent double check for high-alert drips if your policy requires one.
  • Confirm the line: a vasopressor on its own lumen, not piggybacked where a flush could bolus it.

The drip pages in the Reference section have a converter for each of these drugs that runs both directions and takes the numbers off any bag, which makes a quick second check.

Practice: ICU dosage calculation questions

Question 1 · Cardiac Arrhythmias & ACLS

A 72-kg patient has new atrial fibrillation at 148/min without preexcitation. Data are: BP 134/82, EF 60% on a recent echo, no signs of heart failure or ischemia. The provider orders the labeled first IV bolus of diltiazem, which is supplied as 5 mg/mL. Which dose and administration are correct?

  1. 18 mg (3.6 mL) IV over 2 minutes
  2. 25 mg (5 mL) IV over 2 minutes
  3. 18 mg (3.6 mL) rapid IV push over 1 to 2 seconds
  4. 36 mg (7.2 mL) IV over 2 minutes
Show the answer and rationale

Answer: A

  • A. Correct. The label's first bolus is 0.25 mg/kg of actual body weight over 2 minutes: 0.25 × 72 = 18 mg, and 18 mg ÷ 5 mg/mL = 3.6 mL.
  • B. About 25 mg is the label's average second bolus of 0.35 mg/kg, given after 15 minutes if the first dose has not worked.
  • C. The dose is right, but a rapid push with a flush is the technique for adenosine. Diltiazem is given over 2 minutes.
  • D. This doubles the dose (0.5 mg/kg) and raises the risk of hypotension and bradycardia.

For rate control of AF or flutter without preexcitation, the 2025 AHA guideline recommends an IV beta-blocker or nondihydropyridine calcium channel blocker, and diltiazem is generally preferred to verapamil. The diltiazem label gives a first bolus of 0.25 mg/kg over 2 minutes (20 mg for an average patient), a second bolus of 0.35 mg/kg after 15 minutes if needed, and an infusion starting at 10 mg/h. Diltiazem is avoided in severe hypotension, decompensated heart failure with reduced EF and preexcited AF, because its negative inotropy and AV nodal block can cause collapse.

Sources: FDA Prescribing Information (DailyMed); AHA 2025 Part 9: Adult ALS; ACC/AHA/ACCP/HRS AF Guideline (2023)

Walk-through: this is a bolus, not a drip, but the moves are the same. Dose per kg times weight gives the dose in mg; dose divided by concentration gives the volume. The trap answers test the second half of the order: how fast to give it, and whether the number came from the first or the second dose. Get the math right, then read every word of the administration.

Two drip problems to try

  • Dobutamine 2,000 mcg/mL is running at 12 mL/hr in a 60 kg patient. What dose is that? 12 × 2,000 ÷ 60 ÷ 60 = 6.7 mcg/kg/min.
  • Norepinephrine 16 mcg/mL is ordered at 0.05 mcg/kg/min for a 100 kg patient. What rate? 0.05 × 100 × 60 ÷ 16 = 18.75, about 18.8 mL/hr.

Frequently asked questions

What is the formula for mcg/kg/min to mL/hr?

mL/hr = dose (mcg/kg/min) × weight (kg) × 60 ÷ concentration (mcg/mL). The 60 turns minutes into hours, and the concentration must be in mcg per mL.

How do you convert mL/hr back to mcg/kg/min?

Multiply the rate by the concentration in mcg/mL, then divide by 60 and by the weight in kg. Use it to check what an infusion is actually delivering.

Why is insulin in units/h the same as mL/hr?

Because the usual infusion is mixed at 1 unit per mL. If your bag is a different strength, divide the units per hour by the units per mL instead.

Do I use actual body weight for drip calculations?

Use the weight your order and protocol specify. Many use actual weight in kg; some drugs and protocols use an adjusted or dosing weight, and the pump must hold the same number.

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Sources

  1. Hospira. Dopamine hydrochloride in 5% dextrose injection, prescribing information (DailyMed). Link
  2. Baxter Healthcare. Dobutamine hydrochloride in 5% dextrose injection, prescribing information (DailyMed). Link
  3. WG Critical Care. Norepinephrine bitartrate in sodium chloride injection, prescribing information (DailyMed). Link
  4. B. Braun Medical. Heparin sodium in 5% dextrose injection, prescribing information (DailyMed). Link
  5. Baxter Healthcare. Myxredlin (insulin human in 0.9% sodium chloride), prescribing information (DailyMed). Link
  6. Garcia DA, Baglin TP, Weitz JI, Samama MM. Parenteral anticoagulants: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: ACCP Evidence-Based Clinical Practice Guidelines. Chest. 2012;141(2 Suppl):e24S–e43S. Link
  7. Umpierrez GE, Davis GM, ElSayed NA, et al. Hyperglycemic crises in adults with diabetes: a consensus report. Diabetes Care. 2024;47(8):1257–1275. Link