Risk is one of those words everybody uses and few people measure. “That seems risky” is a feeling. A score and a percentage are something you can compare, discuss and act on.
This risk calculator gives you two ways to do that. One rates a single risk by how likely it’s and how bad it would be. The other shows how a small chance adds up when you face it again and again, which is where most people’s instincts go wrong.
How to use the Risk Calculator#
- Choose what you want to work out. Pick a risk score, or the chance of at least one occurrence.
- For a risk score. Rate how likely it’s and how bad it would be, each from 1 to 5.
- For repeated chances. Enter the chance each time as a percentage, and how many times it will happen.
- Read the result. You get a risk level and score, or the overall probability.
The risk matrix#
A risk matrix is the standard way to rate a risk when you can’t put an exact probability on it. You score the likelihood from 1 (rare) to 5 (almost certain) and the impact from 1 (insignificant) to 5 (severe), then multiply.
A risk that’s possible (3) with a moderate impact (3) scores 9 out of 25, which falls in the medium band. A rare event (1) with a severe impact (5) scores 5, still medium. A likely event (4) with a major impact (4) scores 16, which is high.
What the score means#
The bands below are a common convention, and many organisations use similar ones. Yours may draw the lines elsewhere:
| Score | Level | Usual response |
|---|---|---|
| 1 to 4 | Low | Accept it and keep an eye on it |
| 5 to 9 | Medium | Plan controls and review regularly |
| 10 to 16 | High | Act soon to cut the likelihood or the impact |
| 17 to 25 | Critical | Act now. Avoid it, transfer it or cut it hard |
The numbers are ranks, not measurements, so a score of 20 isn’t “twice as risky” as 10. Use the matrix to sort and discuss risks, not to do arithmetic on them.
How small chances add up#
The second mode answers a different question. If something has a 1 percent chance of happening each time, how likely is it to happen at least once over 50 tries? It isn’t 50 percent. The right formula is:
For a 1 percent chance over 50 tries, that gives 39.5 percent. Here’s how it grows:
| Tries | Chance of at least one hit at 1% each time |
|---|---|
| 10 | 9.56% |
| 50 | 39.50% |
| 100 | 63.40% |
| 365 | 97.45% |
A 1 percent daily chance sounds tiny, but over a year it’s nearly certain. A 0.1 percent chance repeated 1,000 times comes to 63.2 percent. Repeated 70 times, a 1 percent risk has a 51 percent chance of happening at least once, and so does a 5 percent risk repeated 14 times. This is why a rare fault in a system that runs thousands of times a day will certainly show up, and why one-off checks are not enough for risks that repeat.
When the formula applies#
The repeated-chance formula assumes each try is independent, so one outcome doesn’t change the next. A 30 percent chance of rain on each of three days works out to 65.7 percent that at least one of them is wet, if the days are independent, which weather often isn’t. A related warning goes for the matrix. If two risks are linked, such as a power cut and a server failure, scoring them separately understates the pair.
Getting more from a risk score#
- Score the risk you have after your current controls, not before.
- Write down why you picked each number. The reasoning is worth more than the score.
- Ask two people to score it separately, then compare. Big gaps show where assumptions differ.
- Revisit it. Likelihood and impact both change over time.
This calculator is a thinking aid and not professional advice for safety, medical or financial decisions.
Frequently asked questions#
Multiply the likelihood by the impact, each rated from 1 to 5. A likelihood of 3 and an impact of 4 gives a score of 12, which is high.
It’s a grid with five levels of likelihood and five levels of impact. Each cell has a score from 1 to 25 that maps to a risk level.
Use 1 − (1 − p) ^ n, where p is the chance each time and n is the number of tries. For 1 percent over 50 tries it’s 39.5 percent.
Not exactly. The scores are rankings, so they show order and priority but not exact size.
Yes, as a way to think clearly about likelihood and consequences. It isn’t a substitute for professional advice on health, safety or money.
It is a grid that scores a risk as likelihood multiplied by impact, each on a scale such as 1 to 5. Scores are grouped into bands such as low, medium, high and critical, which guide what to do. It is a simple way to compare risks and decide which to deal with first, though it relies on judgement for the inputs.