The Metric System vs Imperial System — Complete Weight Guide
Understand the difference between the metric and imperial systems for weight. Learn metric units (kg, g) vs imperial units (lb, oz, stone) with a complete conversion reference and history.
The metric and imperial systems are the two dominant measurement frameworks used worldwide. Understanding both — and knowing how to convert between them — is increasingly important in a globalised world. Here’s the complete guide.
A Brief History
The Metric System
The metric system was developed in France in the late 18th century during the French Revolution, with the goal of creating a universal, decimal-based system of measurement. The modern form — the International System of Units (SI) — was established in 1960 and has been adopted by virtually every country on Earth.
The Imperial System
The imperial system evolved from traditional English measures going back to medieval times. It was formalised in the British Weights and Measures Act of 1824. The US adopted a version of this system (US customary units) independently, which differs slightly from the British imperial system (particularly for volume).
Metric Weight Units
The metric system uses a consistent decimal structure based on the kilogram (kg) as the base unit of mass:
| Unit | Symbol | Value in Grams | Value in kg |
|---|---|---|---|
| Metric Ton | t | 1,000,000 g | 1,000 kg |
| Kilogram | kg | 1,000 g | 1 kg (base) |
| Hectogram | hg | 100 g | 0.1 kg |
| Decagram | dag | 10 g | 0.01 kg |
| Gram | g | 1 g | 0.001 kg |
| Decigram | dg | 0.1 g | 0.0001 kg |
| Centigram | cg | 0.01 g | 0.00001 kg |
| Milligram | mg | 0.001 g | 0.000001 kg |
| Microgram | µg | 0.000001 g | 0.000000001 kg |
Imperial/US Customary Weight Units
The imperial system uses non-decimal relationships between its weight units:
| Unit | Symbol | Relation | In kg |
|---|---|---|---|
| Long Ton | LT | 2,240 lb | 1,016.05 kg |
| Short Ton (US) | T | 2,000 lb | 907.185 kg |
| Stone | st | 14 lb | 6.35029 kg |
| Pound | lb | 16 oz | 0.45359237 kg |
| Ounce | oz | 1/16 lb | 0.028350 kg |
| Dram | dr | 1/16 oz | 0.001772 kg |
| Grain | gr | 1/7000 lb | 0.000065 kg |
Metric vs Imperial: Side-by-Side Comparison
| Property | Metric | Imperial |
|---|---|---|
| Base unit of mass | Kilogram (kg) | Pound (lb) |
| Structure | Decimal (×10) | Non-decimal (×16, ×14, ×2240) |
| International adoption | 195+ countries | USA primarily |
| Scientific use | Universal | Rare (except USA) |
| Defined by | Planck constant (quantum) | Treaty (0.45359237 kg = 1 lb) |
| Ease of conversion | Very easy (×10 steps) | Complex (non-decimal) |
Key Conversion Factors
| Metric Unit | Imperial Equivalent | Factor |
|---|---|---|
| 1 kg | 2.2046 lbs | kg × 2.2046 |
| 1 g | 0.03527 oz | g × 0.03527 |
| 1 metric ton | 2,204.62 lbs | t × 2204.62 |
| 1 lb | 0.45359 kg | lb × 0.45359 |
| 1 oz | 28.3495 g | oz × 28.350 |
| 1 stone | 6.35029 kg | st × 6.350 |
Which Countries Use Which System?
Metric (SI): France, Germany, Italy, Spain, China, Japan, India, Brazil, Australia, Russia, South Korea, Netherlands, Sweden, Norway, Denmark… (195+ countries)
Imperial/US Customary: United States (primary everyday use)
Mixed (official metric, informal imperial): United Kingdom (miles for roads, pints for beer/milk, stones for body weight)
Note: Even in the USA, the metric system is used in science, medicine, the military, and manufacturing.
Why Does the USA Still Use Imperial?
The USA was among the first countries to formally adopt the metric system (Congress approved it in 1866), but unlike most nations, it never made metric mandatory for everyday use. Cultural inertia, the cost of transitioning infrastructure, and consumer preference have kept imperial units dominant in American daily life despite multiple government initiatives to switch.
Metric System in Science
All scientific fields worldwide — including in the USA — use the metric/SI system exclusively. Units like kilograms, meters, seconds, and liters are universal in:
- Physics and chemistry
- Biology and medicine
- Engineering
- Space exploration (NASA uses metric)
- International trade and shipping
Frequently Asked Questions
What is the difference between metric and imperial?
The metric system (SI) uses decimal-based units (kg, g, km, m) while the imperial system uses non-decimal relationships (lb, oz, miles, feet). The metric system is used by 195+ countries; imperial is primarily used in the USA.
Which is more accurate — metric or imperial?
Neither is inherently more accurate. Both can measure with equal precision. However, metric is simpler to use for calculations because all conversions are powers of 10.
Why is 1 kg not exactly 2 pounds?
The pound was not originally derived from the kilogram. They evolved as separate systems and were only linked by international agreement in 1959, when 1 lb was defined as exactly 0.45359237 kg — making the kg-to-lb factor 2.2046…, not a round number.
What is mass vs weight in the metric system?
In the metric system, mass is measured in kilograms (kg) while weight (as a force) is measured in Newtons (N). In everyday language, “weight” and “mass” are used interchangeably, but scientifically they are different. See our What Is Mass? page for a full explanation.