Understanding Lab Measurement: mL, IU, mg, µg, and Conversions

Common Units You’ll See

• mL (milliliter) — A unit of volume. 1 mL = 0.001 L. Typical syringe and vial volumes are expressed in mL.
• µL (microliter) — A thousandth of a milliliter. 1 µL = 0.001 mL. Used for very small volumes (pipettes).
• mg (milligram) — A unit of mass. 1 mg = 0.001 g.
• µg (microgram) — A thousandth of a milligram. 1 µg = 0.001 mg = 0.000001 g.
• IU (International Unit) — A unit that measures biological effect, not mass or volume. The mass that makes up 1 IU depends on the substance; always check the product label.

Basic Conversion Rules

Here are the core numeric relationships to memorize or keep handy:

• Volume: 1 mL = 1000 µL
• Mass: 1 mg = 1000 µg
• Mass ↔ Volume (concentration): mass (mg) ÷ concentration (mg/mL) = volume (mL)

Example Conversions (step-by-step)

1) Convert micrograms to milligrams

Example: 500 µg → ? mg

Step-by-step:
500 µg ÷ 1000 = 0.5 mg.
So, 500 µg = 0.5 mg.

2) Find volume from mass and concentration

Example: You have a solution labeled 10 mg/mL. You need 2 mg. What volume should you draw?

Step-by-step:
Formula: volume (mL) = mass (mg) ÷ concentration (mg/mL).
Plug values: volume = 2 mg ÷ 10 mg/mL = 0.2 mL.
Convert to µL if needed: 0.2 mL × 1000 = 200 µL.
So, draw 0.2 mL (200 µL).

3) When IU appears on a label

Because an IU measures biological activity, you cannot convert IU to mg or mL without additional information from the manufacturer (for example, the label might say “1 mL contains 1000 IU” — that gives you the conversion you need). Always use the product-specific concentration or the manufacturer’s datasheet.

Quick Reference Table

Practical Tips for the Bench

• Always write units clearly (mL vs µL, mg vs µg). A misplaced µ can cause a 1000× error.
• When in doubt, calculate slowly and show your work. Double-check arithmetic for small volumes.
• Use appropriately sized syringes or pipettes for accuracy (e.g., use a 1 mL syringe for 0.2 mL rather than a 10 mL syringe).
• For IU-based products, refer to the product label or datasheet for the IU ↔ mass/volume relationship.