How the Scoville scale works
The Scoville scale gives chillies a number for their heat. Here is where the number comes from, why it is only ever "about", and what the scale cannot tell you.
The original test
In 1912 the American pharmacist Wilbur Scoville devised a way to rate how hot a chilli is. A sample of the chilli was soaked in alcohol to pull out the compounds that cause the burn. That extract was then diluted in sugar water, a little more each time, and a panel of tasters sipped it. The test stopped when most of the panel could no longer detect any heat.
The number of times the extract had to be diluted is the Scoville rating. If it took 5,000 dilutions before the heat vanished, the chilli was rated at 5,000 SHU. A bell pepper needs no dilution at all, so it rates zero.
Scoville described his test in a short paper, "Note on Capsicums" (Journal of the American Pharmaceutical Association, 1912, volume 1, page 453, as cited by New Mexico State University): doi.org/10.1002/jps.3080010520. For a plain-language guide to how chilli heat is measured, see Measuring Chile Pepper Heat by Paul Bosland and Stephanie Walker (New Mexico State University, 2010).
How it is measured now
Tasting is subjective, since it depends on each taster's palate and sensitivity, and a taster can only handle so many samples. So laboratories now usually use a machine method called high-performance liquid chromatography (HPLC). It measures the amount of capsaicin and related compounds, together called capsaicinoids, in the sample.
The machine result is then converted to Scoville units. A figure commonly cited is that one part per million of capsaicinoid is about 15 to 16 SHU. On that basis pure capsaicin comes out at around 16 million SHU, the top of the scale. Different laboratories use slightly different conversion factors, which is one reason published figures for the same pepper do not match exactly.
Why every figure is a range
You will rarely see one exact number for a pepper here, and that is deliberate. Heat varies with:
- Variety and breeding. Peppers sold under one name can differ in heat.
- Growing conditions. Heat, water stress, soil and sunlight all change how much capsaicin a plant makes.
- Ripeness. Heat can change as the pod matures.
- Which part you test. Most heat sits in the white pith and the membrane that holds the seeds, not in the seeds themselves.
- The sample and the method. Different samples, laboratories and conversion factors give different results.
New Mexico State University's chilli researchers describe heat as the result of a plant's genetics together with how it interacts with its environment. They say a mild variety will become hotter when it is stressed in the field, a hot variety grown in ideal conditions may be only moderately hot, and a plant bred for low heat will not become truly hot even when stressed (their guide). Record-setting figures, such as those for the hottest peppers, usually come from a small number of tested samples, so an average on a record certificate does not mean every pod of that variety will match it.
What the scale does not tell you
- How hot it feels to you. People differ in how they sense capsaicin, and tolerance builds with regular eating.
- How long the burn lasts or where you feel it. Some chillies hit the front of the mouth fast and fade; others build slowly in the throat.
- Heat from other things. Black pepper, ginger and mustard-family foods such as horseradish and wasabi taste hot for different chemical reasons and are not rated this way.
- Flavour. Two peppers with the same rating can taste very different.
How "times hotter" is worked out here
The compare tool takes the middle of each published range and divides one by the other. It also shows the gap you would get if you compared the top of one range with the bottom of the other, so you can see how wide the uncertainty is. If one item scores zero, no ratio is shown, because dividing by zero is meaningless. If two ranges overlap, the tool says so.
Our heat bands
The Scoville scale has no official named levels. The bands on the chart are our own grouping, chosen to make the chart easier to read:
| Band | Middle of range, in SHU |
|---|---|
| Mild | Under 2,500 |
| Medium | 2,500 to 25,000 |
| Hot | 25,000 to 100,000 |
| Very hot | 100,000 to 500,000 |
| Superhot | 500,000 to 1,500,000 |
| Extreme | Over 1,500,000 |
Handling chillies safely
This is general guidance, not medical advice.
- Wear disposable gloves when cutting very hot chillies, and wash your hands, the knife and the board afterwards.
- Keep your hands away from your eyes, nose and mouth, and remember contact lenses.
- If your skin burns, washing with soap and cool water is the usual first step. Water alone does little, because capsaicin does not dissolve well in it.
- For a burning mouth, dairy products such as milk or yoghurt are commonly suggested, and many people find them more helpful than water.
- Take extra care with extract-strength sauces and with items such as pepper spray, which are far stronger than any food. Keep all of these away from children and pets.
- If you get a severe reaction, trouble breathing, or pain that does not settle, get medical help.
- Do not take part in chilli eating contests or dares. See our health warning.