Shortfills look confusing the first time you meet one. You pick up a 100ml bottle, notice it is not full, and wonder whether you have been short-changed. You have not. The gap is there on purpose, and filling it correctly takes about two minutes.

This guide walks through the whole process step by step, including the sums, so you end up with the strength you actually wanted rather than a guess. If you want the background on why shortfills exist at all, our explainer on what shortfills are covers it, and our older piece comparing shortfills and 10ml bottles is worth a look if you are still deciding which format suits you.

Why shortfills need a nic shot

UK regulations cap any bottle containing nicotine at 10ml. That is why every nicotine e-liquid you see on the shelf is in a small bottle. It also means a manufacturer cannot legally sell you a large bottle with nicotine already in it.

The workaround is simple and entirely above board. Larger bottles are sold nicotine-free, deliberately underfilled to leave headroom, and you add the nicotine yourself using a separate 10ml nicotine shot. A shot is usually 18mg or 20mg of unflavoured nicotine in a plain base, sold in its own compliant 10ml bottle.

Step 1: Check what you have bought

Look at the two numbers on the bottle. A shortfill described as "100ml in a 120ml bottle" holds 100ml of nicotine-free liquid with 20ml of space. One described as "50ml in a 60ml bottle" holds 50ml with 10ml of space.

The headroom tells you how many nic shots it takes. A 20ml gap needs two 10ml shots. A 10ml gap needs one. If the bottle does not state it clearly, the gap you can see is your answer.

Step 2: Work out the strength you will end up with

This is the part people skip, and it is the reason a mix sometimes feels weaker than expected. The sum is easier than it looks.

1. Multiply the shot strength by the number of shots. Two 18mg shots give you 36 units of nicotine.
2. Divide that by the total finished volume in millilitres. For 100ml of shortfill plus 20ml of shots, the total is 120ml.
3. 36 divided by 120 gives 0.3, so multiply by 10 to read it as a strength. The finished liquid is 3mg/ml.

The usual results are worth memorising. A 50ml shortfill in a 60ml bottle with one 18mg shot finishes at 3mg. A 100ml shortfill in a 120ml bottle with two 18mg shots also finishes at 3mg. Using 20mg shots instead nudges both to a little over 3mg. If you want 6mg you need a smaller shortfill relative to the shots, which is why most shortfills are aimed at people who vape at low strengths.

That is the honest limitation of the format. Shortfills are built for direct-to-lung vaping, where 3mg is normal. If you need 10mg or 20mg, a 10ml bottle is the format for you, and no mixing is involved.

Step 3: Add the shot

Take the cap off the shortfill and set it somewhere it will not roll away. Open the nic shot. Squeeze it steadily into the shortfill rather than tipping it, which gives you more control and less mess. Empty the shot completely, since the sum above assumes all 10ml goes in.

If you are adding two shots, add them one after the other before you put the cap back on. Wipe the neck of the bottle and screw the cap down firmly.

Step 4: Shake it properly

Nicotine shots are usually thinner than the shortfill they are going into, so they will sit in a layer rather than mixing themselves. Shake the bottle hard for a good thirty seconds until it looks uniform and slightly frothy. Leave it to settle for a few minutes and the froth will disappear.

A gentle swirl is not enough. If you skip this, the first few fills will be weaker than the last few, and you will wonder why the flavour drifts as the bottle empties.

Step 5: Let it steep, if it needs it

Some liquids taste right immediately. Others, particularly heavier dessert and custard flavours, improve after a few days sitting somewhere cool and dark with the cap on. This is called steeping, and it simply gives the flavour concentrates time to settle into the base.

If a freshly mixed shortfill tastes a bit flat, give it three or four days before you write it off. Shake it once a day if you remember. Fruit and menthol blends generally need this least.

Handling and storage

Nicotine shots are nicotine in a fairly concentrated form, so treat them with the same care you would any household chemical. Keep them well away from children and pets, close the cap properly, and wash your hands if you spill any on your skin. Store the finished bottle upright, somewhere cool and out of direct sunlight.

The whole process in order

• Check the headroom to see how many shots you need.
• Do the sum so you know the finished strength before you start.
• Add the shot or shots completely, one at a time.
• Cap it and shake hard for thirty seconds.
• Steep for a few days if the flavour is a heavy one.
• Store it upright, cool and dark, away from children.

That is the whole of it. Once you have mixed one, the rest take a couple of minutes without thinking. Our shortfill range covers the flavours, and we are rated 4.8 out of 5 by over 4,800 UK customers, so if you would rather have someone check your first mix, our advisers are happy to talk it through with no pressure.

Worth noting for planning your spend: a vaping duty of £2.20 per 10ml applies to e-liquid from 1 October 2026, and it is charged on the nicotine-containing liquid, so the way you buy will make a difference to running costs.