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Metric System, Household Systems, Converting Within & Between

Systems

Calhoun Community College

A physician writes a prescription for Colchicine and accidentally orders “10.0 mg,” when he should have ordered “1.0 mg.”

That’s a tiny decimal error, a mistake even the best doctor could make.

But it can be catastrophic for the patient.

The higher dose could cause Colchicine poisoning, similar to arsenic poisoning: burning in the mouth and throat, excruciating abdominal pain. Internal organs would melt away and death would likely occur

within 24 to 72 hours.

The Shocking Truth About Medication Errors (http://www.forbes.com/sites/leahbinder/2013/09/03/the-shocking-truth-about-medication-errors/)

Preferred for prescribing and administering medication

It is a decimal system based on multiples of 10

Primary units of measurement is:

Gram for weight

Liter for volume

Meter for length

unit base the times1000or thousandone

unit base theof10

1or 0.1or tenth one

unit base theof 100

1or 0.01or hundredth one

unit base theof 1000

1or 0.001or h thousandtone

unit base theof 1,000,000

1or 0.000001or millionth one

Weight g = gram (base unit)mg = milligram

mcg (µg) = microgramkg = kilogram

Volume L = liter (base unit)mL = millilitercc = cubic centimeter (not used anymore)

Length m = meter (base unit)cm = centimetermm = millimeter

The unit or abbreviation ALWAYS follows the amount,

put a space between them (lowercase except for L for

liters).

◦ Example: 5 g NOT g 5

Decimals are used to designate fractional metric units.

◦ Example: 1.5 mL NOT 1 ½ mL

Omit unnecessary zeros.

◦ Example: 1.5 g NOT 1.50 g

Use a zero to emphasize the decimal point for

fractional metric units of less than 1 (National

Patient Safety Goals). Example: 0.5 mg NOT .5 mg

◦ This is critical… It will prevent confusion &

potential dosage error. Think about what would

happened if overlooked the decimal point and

misinterpreted the medication order as 5 mg

instead of 0.5 mg. The dosage would be 10 times

too much. It could cause DEATH!!!!!!!!!!!

When in doubt, double check, DO NOT GUESS.

Ask the writer for clarification

Unit Abbreviation Equivalents

kilogram kg 1 kg = 1000 g

gram g

NOT Gm or gm

1 g = 1000 mg

milligram mg 1 mg = 1000 mcg

microgram mcg 0.001 mg = 1 mcg

liter L

NOT ℓ

1 L = 1000 mL

milliliter mL

NOT mℓ

0.001 L = 1 mL = 1 cc

centimeter

DO NOT USE LIST

meter m 1 m = 100 cm = 1000 mm

centimeter cm 1 cm = 10 mm

millimeter mm 0.1 cm = 1 mm

1 inch = 2.5 cm

grain gr gr i = 60 (65) mg

ounce oz 1 ounce = 30 mL

pint pt 16 oz = 500 mL

quart qt pt ii = 32 oz = 1000 mL = 1 L

Dram* 1 dram = 4 mL

a) .3g

b) 0.3 Gm

c) 0.3 g

d) .3 Gm

e) 0.30 g

a) .3g

b) 0.3 Gm

c) 0.3 g

d) .3 Gm

e) 0.30 g

a) 1 1/3 ml

b) 1.33 mL

c) 1.33 ML

d) 1 1/3 ML

e) 1.330 mL

a) 1 1/3 ml

b) 1.33 mL

c) 1.33 ML

d) 1 1/3 ML

e) 1.330 mL

a) 5 KG

b) 5.0 kg

c) kg 5

d) 5 kg

e) 5 kG

a) 5 KG

b) 5.0 kg

c) kg 5

d) 5 kg

e) 5 kG

a) Lasix 20.0 mg

b) Lasix 20 mg

c) Lasix 20 MG

d) Lasix mg 20

e) Lasix 20 Mg

a) Lasix 20.0 mg

b) Lasix 20 mg

c) Lasix 20 MG

d) Lasix mg 20

e) Lasix 20 Mg

drop gtt Used for IV calculations

Teaspoon* t (or tsp) 1 t = 5 mL

Tablespoon* T (or tbs) 1 T = 3 t = 15 mL

ounce (fluid) oz 1 oz = 30 mL

ounce (weight) oz 1 lb = 16 oz

Cup* cup 1 cup = 8 oz = 240 mL

60 mggr i

gr ¼ 15 mg

gr ss30 mg

45 mg gr ¾

To convert from a larger unit of measurement to a smaller one; MULTIPLY or move the decimal point to the right.◦ Example: 3 L = ? mL◦ THINK: Larger to Smaller: (×)◦ Equivalent: 1L = 1000mL Conversion factor 1000 3 L = ___mL 3 × 1000 = 3000 mL or move decimal 3 places to right

3L = 3.000 = 3000 mL

Convert 0.3 g to mg

(think 1 g = 1000 mg, conversion factor is 1000)

Think: Larger to smaller, MULTIPLY

Multiple by 1000: ◦ 0.3 g = 0.3 x 1000 = 300 mg

or move decimal 3 places to right ◦ 0.3 g = 0.300 = 300 mg

To convert from a smaller unit of measurement to a larger one,DIVIDE or move the decimal point to the left.◦ Example: 400 mg = ? g◦ THINK: Smaller to Larger: ()◦ Equivalent: 1 g = 1000 mg (conversion factor = 1000)

400 mg = 400 1000 = 0.4 g Or move the decimal 3 places to the left 400 = 0.400 = 0.4 g

Convert 5000 mL to L

(think 1 L = 1000 mL, conversion factor is 1000)

Think: Smaller to Larger, DIVIDE

Divide by 1000: ◦ 5000 mL = 5000 ÷ 1000 = 5 L

or move decimal 3 places to left ◦ 5000 mL = 5.000 = 5 L

You can use this diagram when converting dosages within the metric system.

MEMORIZE this too:

1 kg = 2.2 lb

kg g mg mcg

Move decimal point 3 places to the left for each step.

Move decimal point 3 places to the right for each step.

Can be used within the same system or between systems

Solving for X 8 mg = ___ g

1. State the known equivalent first, then add the incomplete ration to the other side, labeling all terms in the proportion

1000 mg: 1 g = 8 mg : x g

1. Solve for X (multiply inner terms, then outer terms)

◦ 1 X 8 = 1000 X x

◦ 8 = 1000 x

◦ 1 x 8=1000x X 1

◦ 1000 1000

◦ 8 = 1000 x

◦ 1000 1000

◦ X = 8/1000 = 0.008 g

◦ X = 0.008 g

Simpler to multiple or divide…

Solving for X 8 mg = ___ g

◦ Convert 8 mg = ___ g

◦ Approximate equivalent: 1g = 1000 mg

◦ Conversion factor is 1000

◦ Think: Smaller to larger, therefore divide (or move the decimal 3 places to the left)

8 mg = 8 ÷ 1000 = 0.008 g

To convert from a larger unit to a smaller unit of measure, you multiply by the conversion factor and To convert from a smaller to a larger unit of measure, you must divide by the conversion factor.

Example: Convert 45 mL to tApproximate Equivalent 1 t = 5 mL

Conversion factor = 5

THINK: Smaller to Larger so DIVIDE 45 / 5 = 9 t

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