Length-based tape
Best for childrenA Broselow-type tape converts length to a weight band and prints the doses. Lay the red end at the top of the head, read the color at the heel. Use it whenever it is available.
Step 1: get the weight in kilograms
Every weight-based dose in the SMOs is written per kilogram, and every patient tells you their weight in pounds. The whole chain of med math starts with one division, so it has to be automatic. One kilogram is 2.2 pounds. To go from pounds to kilograms, divide by 2.2. To go the other way, multiply by 2.2.
kg = lb ÷ 2.2·lb = kg × 2.2
Try it
81.8 kg
Doing it in your head
You will not have a calculator at 3 a.m. with a seizing child. Use the half-minus-ten-percent trick: it is accurate to within 1%.
- Halve the pounds. 180 lb → 90.
- Take 10% of that away. 10% of 90 is 9. 90 − 9 = 81 kg.
- Check against the exact answer: 180 ÷ 2.2 = 81.8 kg. Close enough for any dose in the book.
Why it works: half of the pounds is dividing by 2; the real divisor is 2.2, which is 10% bigger, so the answer should be about 10% smaller.
Rounding rule: round the weight to the nearest whole kilogram before you dose. Nobody draws up 0.818 mL; they draw up 0.8. The one exception is neonates and small infants, where a kilogram is a quarter of the patient. Keep one decimal under 10 kg.
When nobody knows the weight
Age formula, 1 to 10 years
kg ≈ (age × 2) + 8A 4-year-old is about 16 kg; an 8-year-old about 24 kg. This is the APLS formula; it underestimates heavier modern children a little, which is the safer direction.
Infants
Newborn ≈ 3.5 kgBirth weight doubles by about 5 months (≈7 kg) and triples by 1 year (≈10 kg). A 6-month-old is roughly 7–8 kg.
Adults
Ask, then lookPatients and families usually know. If not, estimate against yourself and your partner. For IV drugs with maximums (ketamine, lidocaine, amiodarone), an adult estimate that is off by 10 kg rarely changes the dose you give.
Never dose a child from a guessed weight when a parent, a tape or a scale is available. A 5 kg error in a 15 kg child is a 33% dosing error.
Common weights, memorized
| lb | kg (exact) | kg (rounded) | Who |
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Step 2: calculate the dose
A weight-based order says how much drug per kilogram. Multiply by the patient's kilograms to get the dose the patient receives. Then check it against the maximum: the SMOs cap most pediatric doses at the adult dose, and some adult doses at a fixed number.
dose = mg/kg × kg·then: if dose > max, give the max
Try it
Units: the place people get hurt
Doses are written in grams, milligrams, micrograms, or milliequivalents. The concentration on the vial may be in a different unit from the order. Convert the order to the vial's unit before you do anything else.
1 g
= 1,000 mgMagnesium 2 g = 2,000 mg. TXA 1 g = 1,000 mg.
1 mg
= 1,000 mcgFentanyl is in micrograms. 0.1 mg = 100 mcg. The epi "100 mcg" push-dose is 0.1 mg.
mEq
Chemical units, not weightSodium bicarb 8.4% is 1 mEq per mL. Treat mEq like mg in the math: 1 mEq/kg × 20 kg = 20 mEq = 20 mL.
Moving the decimal
Big unit → small unit: move right 32 g → 2,000 mg (right 3). 250 mcg → 0.25 mg (left 3). Write the zeros out; do not do this in your head on a child.
Write a leading zero (0.5 mg, never .5 mg) and never a trailing zero (5 mg, never 5.0 mg). A missed decimal point is a tenfold error, and tenfold errors are what kill children in med math.
Weight-based orders from the SMOs
These are the orders the quiz draws from. Doses come from the Region 8 SMOs and your department's vial concentrations; they are here to practice the math, not to replace the protocol in your hand.
| Drug | Order | Per kg | Max | Vial |
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Step 3: turn the dose into a volume
You cannot push milligrams. You push milliliters. The vial tells you how many milligrams are in each milliliter: that is the concentration. Divide the dose you want by the concentration and you get the volume to draw up. Most people learn it as "desired over have, times the volume it comes in"; it is the same thing.
mL = dose ÷ concentration (mg/mL)·or: (desired ÷ have) × vial mL
Try it
Reading the concentration off the vial
Vials label their strength three ways. All three become mg/mL.
mg per mL
Read it straightKetamine 50 mg/mL. Versed 5 mg/mL. Amiodarone 150 mg in 3 mL = 50 mg/mL (divide total mg by total mL).
Ratio
1:1,000 = 1 mg/mLThe second number is how many mL hold 1 g. 1:1,000 → 1 g in 1,000 mL → 1 mg/mL. 1:10,000 → 1 g in 10,000 mL → 0.1 mg/mL. Ten times weaker.
Percent
% × 10 = mg/mLPercent is grams per 100 mL. Lidocaine 2% = 2 g/100 mL = 20 mg/mL. D10 = 10 g/100 mL = 100 mg/mL = 25 g in 250 mL. Bicarb 8.4% = 84 mg/mL, which happens to be 1 mEq/mL.
Total in the vial
DivideA prefilled syringe of epi 1:10,000 says "1 mg in 10 mL". 1 ÷ 10 = 0.1 mg/mL. Atropine "1 mg/10 mL" is the same, 0.1 mg/mL.
Sanity check every volume against the syringe in your hand. If the math says 15 mL of fentanyl, you divided the wrong way: the right answer is almost never bigger than the vial.
The whole chain, worked
A 6-year-old is seizing. No IV yet. The order is Versed 0.2 mg/kg IN, max 2.5 mg. Mom says he weighs about 50 pounds. Vial: 5 mg/mL.
- Weight. 50 ÷ 2.2 = 22.7 → 23 kg. (Head math: half is 25, less 10% is 22.5.)
- Dose. 0.2 mg/kg × 23 kg = 4.6 mg. Max is 2.5 mg, so the dose is 2.5 mg.
- Volume. 2.5 mg ÷ 5 mg/mL = 0.5 mL.
- Syringe. 0.5 mL belongs in a 1 mL or 3 mL syringe where you can see it, not a 10 mL. Split it between the nostrils with the atomizer.
- Say it out loud. "Versed 2.5 milligrams, 0.5 mL, intranasal." Your partner checks the vial and the syringe. Two sets of eyes on every pediatric dose.
Step 4: read the syringe
The math is worthless if the plunger stops at the wrong line. Three habits: pick a syringe where the dose sits in the middle of the scale, know what each small line is worth before you pull, and read at the right part of the plunger.
Where to read
The flat top edge of the black rubber tipNot the point of the dome, not the bottom skirt. The line where the plunger's leading edge meets the barrel is the volume.
Which syringe
Smallest that holds the dose0.3 mL in a 10 mL syringe is one and a half lines; in a 1 mL syringe it is 30 lines. Precision comes from the syringe, not the person.
What a line is worth
Count the lines between labelsBetween "1" and "2" on a 3 mL syringe there are 10 spaces, so each line is 0.1 mL. On a 10 mL syringe there are 5, so each is 0.2 mL. Count every time; it is the same hand motion with a different value.
Air and the hub
Expel air before readingPull a little extra, flick, push the air out to the line. The fluid in the needle hub does not count; it was never on the scale.
Explore each syringe
Plunger at 0.0 mL
Drag the plunger, or use ← → (shift for bigger steps).
Practice: read it
The plunger is set. What volume is drawn up?
mL
Practice: draw it up
Drag the plunger to — mL, then check.
Plunger at 0.0 mL
Ask me about
Enter to check · N next. Full problems are graded on the final mL; the steps are shown after you answer.
Mastery
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