Rules for writing SI units and symbols correctly
SI symbols follow fixed rules. Leave a space after the number, add no plural s or full stop, and use capitals only for units named after people.
The writing rules, right and wrong
| Rule | Right | Wrong |
|---|---|---|
| Space between the number and the unit symbol | 100 mg | 100mg |
| A symbol never takes a plural s | 10 kg | 10 kgs |
| No full stop after a symbol, except at the end of a sentence | 10 mg of salt | 10 mg. of salt |
| A zero before the decimal point for values below 1 | 0.928 g | .928 g |
| Capital symbol for a unit named after a person | 30.5 J, 5 N | 30.5 j, 5 n |
| Full unit names in small letters | five newtons, ten pascals | five Newtons, ten Pascals |
| A space between symbols in a combined unit | 5 m s⁻¹ | 5 ms⁻¹ (reads as 'per millisecond') |
| Not more than one solidus (/) | J/(K mol) or J K⁻¹ mol⁻¹ (your NIOS lesson prints J/K mol) | J/K/mol |
Look-alike symbols that trap learners
One wrong capital can change a value a billion times: 1 mm is 10⁻³ m, but 1 Mm is 10⁶ m.
Writing unit names in words
- Write unit names in small letters, even when they come from a person's name: newton, pascal, joule, kelvin. The one exception is Celsius in 'degree Celsius'.
- In words, the current SI guide lets a unit name take a normal plural: 30.5 joules, 5 kilograms. Your NIOS lesson writes it without s (30.5 joule). The symbol never changes: 30.5 J.
- For powers, say squared or cubed after the name: metre per second squared.
- Area and volume are the exception: the word comes first, as in square metre and cubic centimetre.
- Write kelvin as 300 K, with no degree sign. Celsius keeps its sign: 27 °C.
A note on °C
Your NIOS lesson writes °C with no space after the number (25°C), the same as an angle (30°). The current international SI guide puts a space before °C (25 °C). It keeps 'no space' only for degrees, minutes and seconds of angle. Haya writes 25 °C. If an exam question asks for the rule from your lesson, give the lesson's version.
Worked example: correct the mistakes
Rewrite correctly: (a) 250 Gms of sugar (b) a speed of 12 ms⁻¹ (c) .5 Kg (d) 40 Joules.
- (a) The symbol for gram is a small g with no s: 250 g.
- (b) Without a space, ms means millisecond. Write 12 m s⁻¹.
- (c) Put a zero before the point, and kilo is a small k: 0.5 kg.
- (d) A unit name is written in small letters and its symbol is J: 40 J. In words, 40 joules (your NIOS lesson writes 40 joule).
250 g; 12 m s⁻¹; 0.5 kg; 40 J
Packets sold in India must print the net quantity in standard units, such as 500 g or 1 L. Reading '500 mg' on a medicine label as '500 g' would be a dangerous thousand-fold mistake.
1 marks: “True or false: '5 kgs' is the correct way to write five kilograms.” False. Symbols never take a plural s; write 5 kg.
2 marks: “What is the difference between 'm s' and 'ms'? Why does the space matter?” m s (with a space) is metre second; ms (no space) is millisecond. Without the space, the first m is read as the prefix milli, which changes the meaning.
2 marks: “Correct these and give the rule each breaks: (i) 20 Kg. (ii) 7 n” (i) 20 kg: kilo is a small k, and no full stop follows a symbol. (ii) 7 N: the newton is named after a person, so its symbol is a capital N.
Space, no s, no dot, zero first, capital for a person.