Length, Mass and Time Measurements

The SI is the measurement system adopted by most of the world for scientific and commercial purposes. It defines base units for each type of quantity: metre (m) for length, kilogram (kg) for mass, and second (s) for time.

The great advantage of the metric system is that conversions are always powers of 10 — you simply move the decimal point. This contrasts with imperial systems (inches, pounds, feet) where conversions are irregular (1 foot = 12 inches, 1 mile = 5,280 feet).

SI prefixes apply to all units. The most common in everyday life are: milli- (1/1,000), centi- (1/100), deci- (1/10), the base unit itself, deca- (×10), hecto- (×100), and kilo- (×1,000). For very large lengths we use the kilometre; for very small ones, the millimetre.

Length units form a staircase of powers of 10. Starting from the metre: 1 km = 1,000 m; 1 hm = 100 m; 1 dam = 10 m; 1 m = 1 m; 1 dm = 0.1 m; 1 cm = 0.01 m; 1 mm = 0.001 m.

In everyday life, we mainly use mm, cm, m, and km. A school ruler measures in centimetres and millimetres. A person's height is measured in metres (1.72 m) or centimetres (172 cm). Distances between cities are in kilometres. Material texture may be specified in millimetres.

To convert, use the rule: moving up the staircase (to a larger unit), divide by 10 for each step; moving down (to a smaller unit), multiply by 10. Example: convert 3.5 km to metres → we go down 3 steps → multiply by 10³ = 1,000 → 3,500 m.

The base unit of mass in the SI is the kilogram (kg), not the gram. Historically, the gram was defined first, and the prefix 'kilo' was added to create a more practical unit. Conversions follow the same decimal pattern: 1 kg = 1,000 g; 1 tonne = 1,000 kg = 1,000,000 g.

In everyday life, food is weighed in grams or kilograms. An apple weighs about 150 g. An adult person weighs 60–90 kg. A car weighs 1–2 tonnes. Industrial loads can reach hundreds of tonnes.

A common mistake is confusing mass with weight. In physics, weight is a force (measured in Newtons) that depends on local gravity; mass is constant. In everyday language, we informally use 'weight' to mean mass — 'I weigh 70 kg' — and this is understood in common speech, but technically it means 'I have a mass of 70 kg'.

Time units do not follow the decimal system, which makes their conversions different. The second (s) is the base unit. 1 minute = 60 seconds; 1 hour = 60 minutes = 3,600 seconds; 1 day = 24 hours = 86,400 seconds.

Moving up the scale: 1 week = 7 days; 1 month ≈ 30 or 31 days (February has 28 or 29); 1 year = 365 days (366 in a leap year); 1 decade = 10 years; 1 century = 100 years; 1 millennium = 1,000 years.

Time conversions require attention to the non-decimal base. To convert 2.5 hours to minutes: 2.5 × 60 = 150 minutes. To convert 150 minutes to hours: 150 ÷ 60 = 2.5 h. To convert 5,400 seconds to hours: 5,400 ÷ 3,600 = 1.5 h = 1 hour and 30 minutes.

The most common mistake is multiplying when you should divide (or vice versa). A good check: the larger unit should have the smaller number. 1,500 m = 1.5 km — the number decreased because km is larger than m. If your result was 1,500,000 km, you multiplied incorrectly.

Another mistake is mixing systems: calculating area in m² but using a length in cm without converting. Always convert everything to the same unit before calculating. In tests and practical situations, checking whether the unit of the result 'makes sense' (a distance in cm when the problem asks for km is a sign of error) is a powerful verification strategy.