Short answer
Probability describes how likely an event is to happen. We measure it on a scale from 0 (impossible) to 1 (certain). Words such as unlikely, even chance, and likely correspond to specific regions on this scale, and you can place everyday events on it as fractions, decimals, or percentages.
At a glance
- Key stage
- Key Stage 3
- Subject
- Probability
- Type
- Explainer
- For
- Students
- Read time
- 4 min
- Last updated
- 21 October 2026
Where this fits
- Key Stage 3Years 7–9This article
- GCSEYears 10–11
What are the key probability words?
Five words mark out positions on the probability scale. Memorise them from left (least likely) to right (most likely):
| Word | Probability | Example event |
|---|---|---|
| Impossible | 0 | Rolling a 7 on a standard dice |
| Unlikely | Between 0 and ½ | Rolling a 6 on a fair dice (prob = 1/6) |
| Even chance | Exactly ½ | Flipping a fair coin and getting heads |
| Likely | Between ½ and 1 | Drawing a red card from a standard deck (prob = 1/2... equals even; drawing a club gives 1/4 — unlikely; drawing "not a 6" from a dice gives 5/6 — likely) |
| Certain | 1 | Rolling a number less than 7 on a standard dice |
Even chance (also called an evens chance) means the event and its complement are equally likely.
How do you place a probability on the scale?
The probability scale is a number line from 0 to 1. To place an event:
- Convert the probability to a decimal if it is a fraction (e.g. ⅓ = 0.333…).
- Mark the position on the line.
- Use the vocabulary: anything below 0.5 is unlikely; above 0.5 is likely; exactly 0.5 is even chance.
Examples to place on the scale:
| Event | Probability | Position word |
|---|---|---|
| A baby is born on a Sunday | 1/7 ≈ 0.14 | Unlikely |
| It will rain at some point in November in London | ≈ 0.9 | Likely |
| A spinner with 3 equal sections landing on red | 1/3 ≈ 0.33 | Unlikely (below ½) |
| Picking a vowel from the letters A, E, I, O, U, B | 5/6 ≈ 0.83 | Likely |
Why must probabilities always be between 0 and 1?
A probability less than 0 or greater than 1 is impossible — it has no meaning. When you calculate a probability and get an answer outside this range, you have made an error.
- 0 means the event cannot happen.
- 1 means the event must happen.
- 0.5 means the event is equally as likely to happen as not.
Probabilities can be written as fractions (e.g. ¾), decimals (0.75), or percentages (75%) — all three are equivalent and acceptable in KS3.
What does "the complement" mean in probability language?
The complement of an event is everything that is not that event. If the probability of an event A is p, then the probability of A not happening = 1 − p.
| Event A | P(A) | Complement (not A) | P(not A) |
|---|---|---|---|
| Roll a 3 on a fair dice | 1/6 | Not roll a 3 | 5/6 |
| Rain tomorrow | 0.3 | No rain tomorrow | 0.7 |
| Pick a red sweet | 3/10 | Not pick red | 7/10 |
The complement rule is a powerful shortcut: it is often easier to find the probability of an event NOT happening and subtract from 1.
How do you write a probability as a fraction?
For a fair experiment where all outcomes are equally likely:
P(event) = number of favourable outcomes ÷ total number of outcomes
- A bag contains 3 red and 5 blue counters. P(red) = 3/8.
- A fair dice: P(even number) = 3/6 = 1/2.
- A pack of 52 cards: P(king) = 4/52 = 1/13.
Always simplify the fraction. Check: is the answer between 0 and 1? ✓
How does probability language link to real life?
Weather forecasts, medical test results, and safety assessments all use probability. A doctor saying "there is a 90% chance this treatment will work" means P(success) = 0.9 — high on the scale, towards certain. A weather forecast of 20% chance of rain places the event in the "unlikely" region. Understanding the scale helps you evaluate these statements critically.
Frequently asked questions
Can a probability be a percentage greater than 100%?
No. A probability of 100% means the event is certain (probability = 1). Any value above 100% (or above 1 as a decimal) is meaningless in probability and indicates an error in calculation.
Is "probable" the same as "likely"?
In everyday speech, "probable" and "likely" are synonyms. In GCSE maths vocabulary, the standard terms are the five listed above (impossible, unlikely, even chance, likely, certain). Using "probable" in an answer is acceptable, but examiners look for these five precise words.
How do I know if two events have the same probability?
Calculate the probability of each event as a fraction in its simplest form, or as a decimal. If the values are equal, the events have the same probability. For example, P(head on a coin) = 1/2 and P(odd number on a fair dice) = 3/6 = 1/2 — these are equal.
What does it mean when a question says "fair" or "unbiased"?
A fair or unbiased coin, dice, or spinner is one where all outcomes are equally likely. A fair six-sided dice has probability 1/6 for each face. If a question does not say "fair," you may need to use relative frequency (experimental probability) instead of the theoretical formula.
Professor Pi explains probability from first principles — add the AI Tutors connector at aitutors.me.
Key terms
- Impossible
- Unlikely
- Even chance
- Likely
- Certain
- evens
- cannot happen
- must happen