100ml e-liquid and nic shots, the maths step by step

Every 100ml e-liquid on a British shelf says 0mg on the label. None arrives with nicotine in it, and that is not a marketing choice. The bottle itself holds the answer, provided you read two figures that are easily confused, the volume of liquid and the capacity of the bottle. Between the two sits the space set aside for nicotine shots, and a simple sum that the maker has already done for you, even if the shop page rounds the result.

Why nicotine only goes in after you buy

A 100ml e-liquid is never sold with nicotine, because the Tobacco and Related Products Regulations 2016 cap any nicotine refill bottle at 10ml and 20mg/ml. The large bottle is therefore sold at 0mg, as a shortfill, in a bottle bigger than its contents, ready for 10ml nicotine shots.

With two 18mg/ml shots, a 100ml shortfill reaches exactly 3.0mg/ml. With two 20mg/ml shots it reaches 3.3mg/ml. Those two results sum up the format, and the rest of this article shows where they come from.

What the bottle tells you before you open it

A shortfill reads like an assembly plan. Before you even open it, the label and the shop listing say how much room is left and for how many shots.

Two volumes that are easy to confuse

Liquid volume and bottle capacity are two separate figures. A 100ml shortfill holds 100ml of nicotine-free liquid in a 120ml bottle, as the UK retailers Vape UK and Vape Club describe the format on their 100ml pages.

The same logic applies to 50ml shortfills. The French maker Alfaliquid, for instance, packs its 50ml shake and vape range in 70ml bottles filled to 50ml. Reading only the big number on the label means ignoring half the plan.

The empty space, a reserve sized for two shots

The gap at the top is not an under-fill. A 120ml bottle holding 100ml leaves 20ml, exactly two 10ml shots. A 70ml bottle holding 50ml leaves the same room.

The maker sets the upper limit of the mix that way. The same French maker states that its 50ml bottles are designed to take two boosters at most, and the bottle simply has no room for more.

Working out 100ml e-liquid strength, line by line

The final strength of any mix is always found the same way. Add up the milligrams of nicotine each bottle brings, then divide by the total volume. The nicotine-free base counts as zero milligrams, but its whole volume counts.

For a 100ml e-liquid topped up with two 10ml shots at 20mg/ml, the sum runs as follows:

One shot brings 10ml × 20mg/ml = 200mg of nicotine.
Two shots bring 200mg × 2 = 400mg.
The base brings 100ml × 0mg/ml = 0mg.
The total volume reaches 100ml + 20ml = 120ml.
The final strength is 400mg ÷ 120ml = 3.33mg/ml, rounded to 3.3.

With 18mg/ml shots, the same steps give 360mg for 120ml, exactly 3.0mg/ml. With a single 20mg/ml shot, you get 200mg for 110ml, or 1.8mg/ml. The bottle mixer on our home page applies exactly this formula and shows the same figures.

How many shots for the strength you want

The sum also works backwards, starting from the strength you are aiming for. That is the real question in front of the bottle, and the answer depends on its capacity as much as on arithmetic.

The reverse formula

For a nicotine-free base of V ml, shots of b ml at c mg/ml and a target strength T, the number of shots equals (T × V) ÷ (b × (c − T)). Round the result up to the next whole number, since a shot is not split in practice.

To reach 3mg/ml in a 100ml shortfill with 18mg/ml shots, you get 300 ÷ 150 = 2 shots, for exactly 3.0mg/ml. To reach 6mg/ml with 20mg/ml shots, you get 600 ÷ 140 = 4.3 shots, a volume the 120ml bottle cannot hold.

The table of possible mixes

The table covers the common shortfill sizes with 10ml shots at the two strengths found on UK shelves. It stops at two shots, the limit set by the bottle capacity.

Shortfill	Shots	Shot strength	Final volume	Final strength
50ml	1	18mg/ml	60ml	3.0mg/ml
50ml	1	20mg/ml	60ml	3.3mg/ml
50ml	2	18mg/ml	70ml	5.1mg/ml
50ml	2	20mg/ml	70ml	5.7mg/ml
100ml	1	18mg/ml	110ml	1.6mg/ml
100ml	1	20mg/ml	110ml	1.8mg/ml
100ml	2	18mg/ml	120ml	3.0mg/ml
100ml	2	20mg/ml	120ml	3.3mg/ml

The round number on the page and the exact one

Shop pages often round the result. The first retailer quoted above describes its 100ml range as giving 120ml of “approximately 3-4mg/ml” with two shots, when the sum gives 3.0mg/ml with 18mg shots and 3.3mg/ml with 20mg shots. The French maker announces 3mg/ml and 6mg/ml for its 50ml range with one and two boosters, against 3.3 and 5.7mg/ml by calculation.

The gap is small, but it is real, and it runs both ways. The shot strength explains most of it, so read the shot label as carefully as the shortfill label before comparing two large bottles.

The nic shot is a bottle to read too

A nicotine shot is a nicotine bottle in its own right. It carries the same compulsory wording as a ready-to-vape 10ml, and its own label shapes the final mix well beyond its strength alone.

Its strength and ratio

UK shops sell 10ml shots at 18mg/ml and at 20mg/ml, and that figure is the one that enters the sum. Shots also come in several ratios, 50/50, 70VG or 100VG for example, and that ratio shifts the ratio of the finished mix.

Working in volume, a 100ml shortfill at 20/80 topped up with two 50/50 shots holds 30ml of PG out of 120ml, a 25/75 mix. Makers such as the French brand quoted earlier recommend a shot with the same ratio as the liquid, so that the dilution is not thrown off.

What UK rules cover

The 10ml and 20mg/ml limits apply to products sold to the end user, even if the buyer intends to modify them, the MHRA reminds retailers. Nicotine-free e-liquids sit outside those regulations and fall under general product safety rules instead.

FAQ

These questions come up just before the first shot goes in. The answers rely on what makers and retailers publish and on the same sum as above, with no guesswork.

Do nic salt shots change the maths?

No. A 20mg/ml shot brings 200mg of nicotine in 10ml, whether it is salts or freebase. The final strength is worked out the same way, only the form of nicotine changes.

Should you shake the bottle after adding shots?

Yes, firmly. Until the mix is even, the strength is not the same throughout the bottle. Some makers also advise a resting time, stated on the shortfill’s product page.

Can a shortfill be topped up without nicotine?

Yes, with a nicotine-free base poured in place of the shot, to vape at 0mg/ml. The bottle is designed to be topped up either way, and the liquid is not meant to be vaped on its own.

Why not just buy 10ml bottles?

The 10ml is the only size sold with nicotine already in it, with an exact strength printed on the label. A shortfill gives volume, a 10ml gives a strength chosen in advance, with no sums, up to 20mg/ml.

How can you check the maths quickly?

Enter each bottle into the bottle mixer on our home page, for example 100ml at 0mg/ml, then two lines of 10ml at 20mg/ml. The tool shows 120ml at 3.3mg/ml, the result worked out above.

100ml e-liquid comes down to two shots and one division

A nicotine-free large bottle becomes a nicotine liquid once shots go in, within the room the bottle has left for them. Strength is found by dividing the nicotine added by the total volume, and the real capacity of the bottle sets how many shots fit.

Remember the reference figures, 3.0mg/ml with two 18mg shots and 3.3mg/ml with two 20mg shots. Topping up a 100ml e-liquid means reading two labels, the shortfill’s and the shot’s, before doing a division the bottle has already planned for. For the strength you start from, our guide to reading a vape liquid bottle covers every line on the label.

On the same shelf

Written for adult readers only. Vaping products usually contain nicotine, which is addictive, and nicotine vaping products are not for sale to under-18s in the UK. La Fiole Bleue describes bottles and labels, and nothing here is a buying recommendation.