The 10 ml, 20 mg UK bottle, reviewed as a design constraint

Almost every distinctive feature of the British e-liquid shelf traces back to two figures in the Tobacco and Related Products Regulations 2016: a 20 mg/ml ceiling on nicotine concentration, and a 10 ml limit on refill-bottle size for nicotine-containing liquid. A third — a 2 ml cap on tank and pod capacity — shapes the hardware alongside them.

Those numbers are not new, and they are frequently discussed as an inconvenience. It is worth reviewing them properly, as a design constraint with genuine merits and genuine costs.

The constraints

What the constraints achieve

A predictable ceiling. A user cannot inadvertently buy a nicotine-containing liquid at an unexpectedly high strength, because the maximum is fixed and known. That is a genuine consumer-protection outcome, and it makes the shelf legible in a way less constrained markets are not.

Small containers limit exposure from a single accident. The rationale for a 10 ml cap is straightforwardly about the quantity of nicotine in a single bottle. Whatever one thinks of the size as a convenience matter, the reasoning is coherent.

Standardisation. Because every nicotine bottle is 10 ml, price comparison is trivial and packaging is consistent. Markets without a size cap are markedly harder to shop.

What they cost

Packaging waste. The most-cited criticism, and a fair one. Supplying a given volume of liquid in 10 ml units requires a great many more bottles, caps and cartons than larger containers would.

Inconvenience for higher-volume users, who buy in multiples and refill frequently.

The 2 ml tank cap is the constraint users notice most in daily use, particularly with sub-ohm hardware, and it sits awkwardly with devices designed for markets without it.

The shortfill workaround

The market’s response deserves describing plainly, because it is a direct consequence of the rules.

Shortfills are larger bottles of nicotine-free liquid, part-filled, sold alongside small nicotine shots that the user adds. Because the large bottle contains no nicotine, the 10 ml limit does not apply to it; because the shot is small, it complies. The user performs the mixing.

This is lawful and entirely ordinary in UK retail. It is also, unavoidably, a workaround: the packaging-waste objection is substantially reduced, and in exchange the final nicotine strength depends on the user adding the right shot to the right bottle. The arithmetic is simple, and errors in it are the user’s.

That trade — less waste, more user responsibility — is the clearest illustration available of what these constraints do and what they push into.

The shortfill arithmetic, briefly

Since the format exists because of these rules, the arithmetic deserves stating plainly.

A shortfill is a larger bottle deliberately part-filled, leaving headroom for nicotine shots. The user adds the shots, and the resulting strength depends on the ratio between what was already in the bottle and what was added. Retailers generally state the combination that produces a given strength, and the shots themselves are sold at a fixed concentration.

The consequence worth noting is that the user is now responsible for a step the manufacturer previously performed. Adding one shot instead of two, or adding shots to a bottle sized for a different combination, produces a strength other than the one intended. The error is easy to make and easy to avoid, and the packaging is generally clear about what goes with what.

We would simply observe that a regulation designed to bound the nicotine in a single container has produced a widely used format in which the final concentration is set at a kitchen table.

Pros and cons

Verdict

Our assessment is that these constraints have worn better than their critics expected. The strength ceiling in particular has given the UK market a predictability that other markets lack, and it has been in place long enough that products are designed for it rather than around it. The bottle-size limit is the weaker of the two, because the shortfill format demonstrates that the market will find a compliant path to larger containers regardless — and does so in a way that moves a small but real responsibility onto the person doing the mixing.

These products contain nicotine, which is addictive. They are sold to over-18s only in the UK, and this coverage is for adult vapers and adult smokers considering switching.