Unit Conversions And Orders Of Magnitude

Many questions in Biology require you to carry out unit conversions, this study note covers the most important ones!

Length Conversions:

Kilometres (km), Meters (m), Millimetres (mm), Micrometres (µm), and Nanometres (nm)

Each step up or down the scale represents a factor of 1000.

• 1 km = 1000 m
• 1 m = 1000 mm
• 1 mm = 1000 µm
• 1 µm = 1000 nm
  

Conversion Strategy:

• To convert to a smaller unit, multiply by 1000 for each step down. 
• To convert to a larger unit, divide by 1000 for each step up.

Examples:

Convert 5 mm to µm
5 mm x 1000 = 5000 µm. 

Convert 5000 µm to mm
5000 µm / 1000 = 5 mm.

Time Conversions:

Day, Hours, Minutes, and Seconds

• 1 day = 24 hours
• 1 hour = 60 minutes
• 1 minute = 60 seconds
• Therefore, 1 hour = 60 x 60 = 3600 seconds.
  

Conversion Strategy:

• To convert from hours to seconds, multiply by 60 and then 60 again, so multiply by 3600.
• To convert from seconds to hours, divide by 60 and then 60 again, so divide by 3600.

Examples:

Convert 2.5 minutes to seconds.
2.5 x 60 = 150 seconds.

Convert 1.75 hours to minutes.
1.75 x 60 = 105 minutes.

Convert 7 days into minutes.
7 x 24 = 168 hours x 60 = 10,080 minutes.

Mass conversions:

Kilograms (kg), grams (g), milligrams (mg), micrograms (µg) and nanograms (ng)

• 1 kg = 1000 g
• 1 g = 1000 mg
• 1 mg = 1000 µg
• 1 µg = 1000 ng
  

Conversion strategy:

• Bigger unit → smaller unit: multiply by 1000
• Smaller unit → bigger unit: divide by 1000

Examples:

Convert 0.5 g to mg
0.5 g x 1000 = 500 mg 

Convert 250 mg to g
250 mg ÷ 1000 = 0.25 g 

Convert 5 g to µg
5 g x1000 = 5000 mg x 1000 = 5,000,000 µg

Magnification:
When calculating magnification, ensure both image size and actual size are in the same unit before dividing.

• Units in Answers:
  Always check if your answer needs to be in a specific unit (e.g., µm, mm). 
• Use common sense:
  After converting, check if your answer is larger or smaller than the original value, as expected.

Practice Questions