11-13-2025, 01:30 PM
SI Units & Conversions — Complete Reference Sheet
The SI system (International System of Units) is the standard for all science and engineering.
This sheet gives you a clear, quick-reference guide for the units, prefixes, and conversions you’ll need in GCSE, A-Level, and beyond.
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1. The Seven Base SI Units
These are the fundamental units all others come from:
• Length → meter (m)
• Mass → kilogram (kg)
• Time → second (s)
• Electric current → ampere (A)
• Temperature → kelvin (K)
• Amount of substance → mole (mol)
• Luminous intensity → candela (cd)
Everything else is a derived unit.
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2. Common Derived Units (Used in Science)
• Force → newton (N)
N = kg·m/s[sup]2[/sup]
• Energy → joule (J)
J = N·m = kg·m[sup]2[/sup]/s[sup]2[/sup]
• Power → watt (W)
W = J/s
• Pressure → pascal (Pa)
Pa = N/m[sup]2[/sup]
• Charge → coulomb ©
C = A·s
• Voltage → volt (V)
V = W/A
• Frequency → hertz (Hz)
Hz = 1/s
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3. SI Prefixes — Essential Memorisation
Large Values
• kilo (k) = 10[sup]3[/sup] = 1,000
• mega (M) = 10[sup]6[/sup] = 1,000,000
• giga (G) = 10[sup]9[/sup]
• tera (T) = 10[sup]12[/sup]
Small Values
• milli (m) = 10[sup]−3[/sup]
• micro (μ) = 10[sup]−6[/sup]
• nano (n) = 10[sup]−9[/sup]
• pico (p) = 10[sup]−12[/sup]
These appear constantly in physics questions.
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4. Converting Between Units
Rule:
Move the decimal point according to the power of 10.
Examples:
• 5 km → m
km → m = ×1000
5 km = 5000 m
• 0.02 m → cm
m → cm = ×100
0.02 m = 2 cm
• 4000 μm → mm
μm → mm = ÷1000
4000 μm = 4 mm
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5. Mass, Volume, and Density Conversions
Mass:
• 1 kg = 1000 g
• 1 g = 1000 mg
Volume:
• 1 L = 1000 mL
• 1 mL = 1 cm³
Density Formula:
ρ = m / V
• density (kg/m³)
• mass (kg)
• volume (m³)
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6. Time Conversions (Always Needed)
• 1 min = 60 s
• 1 hour = 3600 s
• 1 day = 24 hours
• 1 year ≈ 3.15 × 10[sup]7[/sup] s (useful in astrophysics)
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7. Temperature Conversions
Celsius → Kelvin:
K = °C + 273
Examples:
• 25°C = 298 K
• −10°C = 263 K
Kelvin → Celsius:
°C = K − 273
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8. Speed, Distance, and Time
v = d / t
• m/s = meters ÷ seconds
Conversions:
• km/h → m/s = ÷ 3.6
• m/s → km/h = × 3.6
Examples:
• 72 km/h = 20 m/s
• 30 m/s = 108 km/h
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9. Common Mistakes
❌ Forgetting to convert time to seconds
✔ Always convert before calculating
❌ Mixing units (m with cm, kg with g)
✔ Keep units consistent
❌ Using °C in gas equations
✔ Always convert to Kelvin
❌ Confusing m (meters) with m (milli-)
✔ Context tells you the difference
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Summary
This sheet covers all essential SI units:
• Base units
• Derived units
• Prefixes
• Conversions
• Temperature
• Speed & time
Memorise the prefixes and practice conversions — they'll appear constantly in maths, physics, chemistry, computing, and engineering.
The SI system (International System of Units) is the standard for all science and engineering.
This sheet gives you a clear, quick-reference guide for the units, prefixes, and conversions you’ll need in GCSE, A-Level, and beyond.
-----------------------------------------------------------------------
1. The Seven Base SI Units
These are the fundamental units all others come from:
• Length → meter (m)
• Mass → kilogram (kg)
• Time → second (s)
• Electric current → ampere (A)
• Temperature → kelvin (K)
• Amount of substance → mole (mol)
• Luminous intensity → candela (cd)
Everything else is a derived unit.
-----------------------------------------------------------------------
2. Common Derived Units (Used in Science)
• Force → newton (N)
N = kg·m/s[sup]2[/sup]
• Energy → joule (J)
J = N·m = kg·m[sup]2[/sup]/s[sup]2[/sup]
• Power → watt (W)
W = J/s
• Pressure → pascal (Pa)
Pa = N/m[sup]2[/sup]
• Charge → coulomb ©
C = A·s
• Voltage → volt (V)
V = W/A
• Frequency → hertz (Hz)
Hz = 1/s
-----------------------------------------------------------------------
3. SI Prefixes — Essential Memorisation
Large Values
• kilo (k) = 10[sup]3[/sup] = 1,000
• mega (M) = 10[sup]6[/sup] = 1,000,000
• giga (G) = 10[sup]9[/sup]
• tera (T) = 10[sup]12[/sup]
Small Values
• milli (m) = 10[sup]−3[/sup]
• micro (μ) = 10[sup]−6[/sup]
• nano (n) = 10[sup]−9[/sup]
• pico (p) = 10[sup]−12[/sup]
These appear constantly in physics questions.
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4. Converting Between Units
Rule:
Move the decimal point according to the power of 10.
Examples:
• 5 km → m
km → m = ×1000
5 km = 5000 m
• 0.02 m → cm
m → cm = ×100
0.02 m = 2 cm
• 4000 μm → mm
μm → mm = ÷1000
4000 μm = 4 mm
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5. Mass, Volume, and Density Conversions
Mass:
• 1 kg = 1000 g
• 1 g = 1000 mg
Volume:
• 1 L = 1000 mL
• 1 mL = 1 cm³
Density Formula:
ρ = m / V
• density (kg/m³)
• mass (kg)
• volume (m³)
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6. Time Conversions (Always Needed)
• 1 min = 60 s
• 1 hour = 3600 s
• 1 day = 24 hours
• 1 year ≈ 3.15 × 10[sup]7[/sup] s (useful in astrophysics)
-----------------------------------------------------------------------
7. Temperature Conversions
Celsius → Kelvin:
K = °C + 273
Examples:
• 25°C = 298 K
• −10°C = 263 K
Kelvin → Celsius:
°C = K − 273
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8. Speed, Distance, and Time
v = d / t
• m/s = meters ÷ seconds
Conversions:
• km/h → m/s = ÷ 3.6
• m/s → km/h = × 3.6
Examples:
• 72 km/h = 20 m/s
• 30 m/s = 108 km/h
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9. Common Mistakes
❌ Forgetting to convert time to seconds
✔ Always convert before calculating
❌ Mixing units (m with cm, kg with g)
✔ Keep units consistent
❌ Using °C in gas equations
✔ Always convert to Kelvin
❌ Confusing m (meters) with m (milli-)
✔ Context tells you the difference
-----------------------------------------------------------------------
Summary
This sheet covers all essential SI units:
• Base units
• Derived units
• Prefixes
• Conversions
• Temperature
• Speed & time
Memorise the prefixes and practice conversions — they'll appear constantly in maths, physics, chemistry, computing, and engineering.