Ah, the chilli pepper. The cause of both giddy excitement and palpable fear, depending on who you ask. For some, the thrill of ultimate heat surpasses anything that a rollercoaster or extreme sport might provide. For others, the mere mention of the word spicy sends shivers uncontrollably down the spine.

If, like me, you fall somewhere in the middle, then you’ve probably found yourself wondering what chilli is best for your personal tastes. Going with the mild ones you already know feels safe but boring, while making the mistake of going one tier too hot will leave you questioning whether your mouth will ever feel normal again. That’s where the Scoville scale comes in so handy, and in this guide, we’ll be explaining the now-famous measure of spiciness, as well as its origins.

Jump to:

• What is the Scoville scale?
• The origins of the Scoville scale
• Notable chillies and their SHU ratings
• How to grow hotter (and milder) chillies

What is the Scoville scale?

At its simplest, the Scoville scale is a measure of heat. Not sweating-in-a-sauna kind of heat, but rather the scorches-your-tongue-and-makes-you-cry kind of heat. Measured in Scoville Heat Units (SHU), the scale takes into account the levels of capsaicinoids in a chilli pepper. These chemicals form the active component within chillies, giving them their characteristic punchy pungency. It’s also one of the differences between sweet bell peppers and chilli peppers; the former has an SHU rating of 0, while the latter, as we’ll go on to see, can have SHU figures well over two million.

The origins of the Scoville scale

The Scoville scale was created in 1912 by American pharmacist, Wilbur Scoville. While we now associate the scale with chilli peppers in a culinary context, Scoville’s original intentions were to help in the production of Heet liniment, a topical cream with painkilling properties that contained capsaicin (the primary capsaicinoid and which has analgesic qualities when applied directly on the skin).

To help with production and ensure dosing was accurate when extracting and using the capsaicin, Scoville went about creating a means of measuring concentrations of the chemical found within various chilli peppers, which is a smart ol’ thing to do when you think about it, if you took the same quantity of jalapeño pepper – which has a relatively low SHU – as you did Pepper X, the world’s hottest pepper as of 2023, and then put them into your cream, you’re going to get markedly different results.

Scoville organoleptic test (pre-1980s)

What he came up with was a test which, to my mind, sounds like some sort of exam that a keyboardist would sit, but then that’s why he was a scientist and I’m not. Known as an organoleptic test, quantities of a dried chilli pepper would be taken and dissolved in alcohol, before then being added to sugar water. A panel of five tasters then consume the solution, reducing the concentration until three or more of the tasters can’t detect any heat anymore. This concentration is then used to determine the chilli’s SHU rating.

Now, clearly, there’s a pretty glaring weakness with this method – it relies on subjective judgement. One person’s tastebuds are different from another's, one person might be bunged up with a nasty cold one day, one person might hold a grudge against the other four tasters, the list goes on… Fortunately, these days, we have a more reliable and precise means of measuring SHU.

High-performance liquid chromatography (1980s onwards)

Since the 1980s, we’ve used a scientific method called high-performance liquid chromatography, or HPLC, to determine a chilli’s SHU rating. The science of this method is far beyond my little pea brain can comprehend but, in essence, it boils down to passing a sample through a high-pressure column, which ultimately leads to the solution’s components being separated and detected, from which (using some fancy calculations) the chilli’s SHU can be calculated.

Notable chillies and their SHU ratings

So, that’s the scale and where it came from, now let’s take a look at some popular chillies and how they rank, going from mildest to hottest.

‘Trinidad Perfume’ (50-500 SHU)

Kicking us off is the ‘Trinidad Perfume’ chilli pepper, whose flavour is distinctly like the habanero (which we’ll talk about later on) but without the associated heat. This makes it ideal for creating milder versions of Caribbean dishes like jerk chicken and Mexican recipes such as xni-pec salsa.

Jalapeño (4,000-8,500 SHU)

A classic of supermarket shelves, jalapeños have a bit more heat than very mild chillies, like ‘Trinidad Perfume’, while still themselves being (relatively) on the gentler end of the spice spectrum. For many people, though, these little peppers still pack plenty of heat. If you look at the Scoville scale, it falls somewhere between medium and hot. We’re particular fans of pickled jalapeños and jalapeño poppers (breaded jalapeños filled with cheese and sometimes meat).

Serrano (10,000-25,000 SHU)

The second most utilised chilli in Mexico after jalapeños, serrano chillies are a step up on the heat front, and would be characterised by many chilli connoisseurs as being nestled in between medium-hot and hot. You know, it gets the tastebuds tingling without turning the tongue into a furious lavafield. Whip up a batch of serrano hot sauce if you’ve got more peppers than you know what to do with. Alternatively, you can try your hand at a traditional Mexican pozole verde (green hominy and chicken soup).

Cayenne (30,000-50,000 SHU)

These red vials of heat are as bright and attractive as they come, adding intense colour to any sunny bed or border. Dry your chillies and crush them, either with a grinder or, if you want to be old school about it, with a pestle and mortar. Use it to add heat to your tacos, fajitas and enchiladas.

Fun fact: If you look at the Scoville scale’s official website, jalapeños, serranos and cayennes are all still characterised as being ‘mild hot’. Yikes. The maddest part? There’s still over two million SHUs to ratchet up to…

‘Apache’ (75,000 SHU)

Now we’re starting to get to the eye-watering end of the spectrum – in my eyes, at least! I’m sure there are spice-heads out there scoffing at the notion that 75,000 SHU is ‘hot’, but there we go. ‘Apache’ is an RHS-award-winning, dwarf variety whose small fruits deliver some seriously punchy heat (for most people’s palates, anyway). Well-suited to container growing, ‘Apache’ is perfect for the patio, greenhouse or even a sunny windowsill.

Scotch Bonnet (150,000-300,000 SHU)

Phwoar, those beads of sweat are really beginning to fall. Scotch bonnet chillies derive their name from their similarity in appearance (if you squint your eyes and really look for it) to the traditional Scottish cap, the tam o' shanter, inspired by a poem penned by the great Robert Burns (an appropriate surname, given the topic). An objectively hot chilli, the scotch bonnet is used extensively throughout Caribbean cuisine, as well as in Maldivan and West African cooking.

Habanero (250,000 SHU)

Pint of milk, anyone? For the braver souls out there wanting to start their journey towards spicier destinations, the habanero is a must-try. At a quarter-of-a-million SHUs, this chilli is hot and then some. There are even spicier variations, too, like the chocolate habanero, which comes in at a whopping 425,000-577,000 SHUs. Now, I’ve heard of chilli chocolates but surely that takes the crown… While many chillies hit you instantly with their heat, the habanero’s spicy profile takes shape over a few minutes and up to an hour.

Trinidad Scorpion (1,000,000-1,463,700 SHU)

Not for the faint of heart, the Trinidad scorpion, also known as Trinidad scorpion ‘Butch T’, is a pepper whose stratospheric pungency means that wearing gloves when handling the fruits is recommended. At one point the world’s hottest chilli according to the Guinness World Book of Records, this chilli may have lost claim to that title, but it’s certainly not lost any of that searing heat. With their pointed, scorpion-like ends, these peppers most definitely have a sting in their tale.

Pepper X (2,693,000 SHU)

And last but certainly not least is the current world’s hot chilli pepper, Pepper X. Grown by the doyen of the chilli world, Smokin’ Ed Currie, who was also the mastermind behind the previous record holder, the Carolina Reaper, Pepper X comes in at over one-quarter-of-a-billion SHUs. Yep, you read that right. Billion with a B. We’re going to take a pass on this one, Ed, and leave you to your fiery shenanigans, but thanks all the same.

How to grow hotter chillies

If you want to grow hotter chillies, you can either purchase a hotter variety, or you can maximise the pungency of the chillies you’re already growing. There are a couple things to consider when thinking about how to do this. Keeping your chillies in warmer, sunnier conditions is thought to lead to greater pungency, which is why a greenhouse is such a good location to grow your plant(s).

Picking your chillies when they’re fully ripe also gives you chillies with maximised heat output. Somewhat counter-intuitively, placing your chilli plant under stress conditions can also help improve spiciness. While you may get a reduced overall yield, snapping off growth, choosing not to feed and underwatering your chilli plant can lead to hotter chillies.

How to dial down the heat of your chillies

If you love chilli flavour but not so much the heat, then there are a few options available to you. One of the easiest methods is to pick your chillies while underripe and still immature. As chillies ripen they become hotter, so picking them prematurely can mitigate against some of that heat. Keep your plant(s) fed and well-watered, and consider growing them outside, rather than in a warm greenhouse.

Final thoughts

How brave are you feeling? We’ve got mild chillies, we’ve got super-hot chillies, we’ve got everything in between. Take your pick. We won’t judge… Whilst you’re here, check out our guides to growing chillies and peppers, as well as our article on growing vegetables in pots.