Probability Spinners
This interactive tool helps you explore probability through spinning wheels. Start with a set of fair spinners to see random outcomes, then move on to a single spinner you can bias by resizing its sectors. Collect results as a tally chart, total the frequencies, and work out the experimental probability of each outcome one step at a time.
It is built for the classroom: the working is drawn out by hand with the pen, so you can reveal as much or as little as you like and let students predict each step.
How to Use
Tab Navigation
There are two activities:
- Spinners: one to four fair spinners for generating random numbers.
- Bias & probability: a single spinner you can make unfair, with a results table and experimental probability.
🎡 Independent Spinners
The Spinners tab shows up to four fair spinners, each divided into equal sectors. Every sector is equally likely, so this is a quick way to generate random numbers or model simple chance events.
🧭 How to Use the Tool
Choose how many wheels to show with Total (1–4). Set the number of equal sections on each wheel with the Sides sliders (3–12) — for example, a 6-sided spinner behaves like rolling a dice.
Click any wheel to spin it on its own, or press Spin All to spin every wheel together with a slight stagger. When a spin finishes, the result pops up in the centre of the wheel.
Because every sector is the same size, each outcome has the same theoretical probability — a good starting point before exploring what happens when the chances are not equal.
⚖️ Bias & Experimental Probability
The Bias & probability tab uses a single spinner whose sectors can be different sizes, so some outcomes become more likely than others. This is the heart of the activity: collect data, build a tally, and compare what actually happens with what should happen.
Changing the spinner
Set the number of sectors with Sides (2–8); each sector is labelled A, B, C … . Drag the white handles on the spinner’s edge to resize the sectors and introduce bias — a bigger sector is more likely to be landed on.
Use Show to overlay the theoretical probability of each sector as a percentage, a decimal (to 3 d.p.), or the angle in degrees (with an arc). This is the value the experiment should get close to.
Collecting results
Press Spin 1x for a single spin, or Spin 10x for ten in a row. Each result is recorded as a tally mark in the matching row of the results table, drawn by the pen and grouped in fives. The more biased the spinner, the more lopsided the tally becomes.
Toggle Show Results to show or hide the table, and use Reset Data to clear all the results and start again.
Frequencies and probability
Press Update Frequencies to write the current count for each result into the Frequency column.
To work out the probabilities, set View to Probabilities. The spinner hides to make room, the frequencies are filled in, and a button appears. Pressing Show P(A) works out the first experimental probability longhand:
P(A) = frequency ÷ total, shown as a fraction and a decimal (3 d.p.).
The total is also written out as the sum of the frequencies, so students can see exactly where the denominator comes from. Press the button again for the next outcome (P(B), P(C) …); once every result is shown it reads Clear and resets.
The big idea
Compare the experimental probability from your data with the theoretical probability shown by Show. With only a few spins they can be quite different, but as you collect more and more results, the experimental probability gets closer to the theoretical value — the more trials, the more reliable the estimate.