What is a shortfill, and why is it underfilled?
A shortfill is a bottle of nicotine-free e-liquid deliberately sold with space left in it. It exists because of the TPD: a bottle containing nicotine can be no bigger than 10ml, so anything larger has to ship at 0mg with headroom for you to add the nicotine yourself. A "100ml shortfill" is therefore 100ml of liquid in a 120ml bottle — 20ml of air that two 10ml nic shots are meant to fill.
That headroom is the number the calculator above cares about most, and it is the one every other calculator ignores. Plenty of combinations are arithmetically fine and physically impossible: 100ml to 6mg on 18mg shots needs 50ml of nicotine, which will not go into a bottle with 20ml of room.
How to work out your nic shots
Nicotine is a quantity, not a percentage. A 10ml shot at 18mg/ml carries 180mg of nicotine, and that amount is then spread through however much liquid ends up in the bottle. Two sums, depending on which way round you are working.
You know the strength you want
shot volume = (target × shortfill volume) ÷ (shot strength − target)
A 100ml shortfill to 6mg using 18mg shots: (6 × 100) ÷ (18 − 6) = 50ml, which is five 10ml shots. Divide by 10 and you have your answer in shots, because shots are sold in tens and nothing else.
You know what you are holding
final strength = (shot volume × shot strength) ÷ (shortfill + shot volume)
A 50ml shortfill plus one 18mg shot is 180mg spread through 60ml, which is 3mg/ml. That is the number nearly everyone is aiming for, and it is why 50ml bottles are sold in 60ml containers.
Freebase or nic salt — which shot to use
Shots come as both, and they are not interchangeable in feel. Freebase nicotine gives the sharper throat hit and is what most shortfill mixers use, because shortfills are usually high-VG liquids meant for airier kits. We stock it at 18mg in 70VG and 100VG — match the VG to the shortfill so you do not thin out a max-VG liquid with a 50/50 shot.
Nic salt shots are smoother at the same strength, which sounds better but works against you here: the smoothness is most useful at high strengths in a tight mouth-to-lung kit, and that is not what a 100ml shortfill is for. Salt shots are worth it in one case — a small shortfill you intend to vape in a pod kit.
Rule of thumb: shortfill in an airier kit, use a freebase 18mg shot and mix to 3mg. If you want 10mg or 20mg, you want a 10ml nic salt bottle instead, not a shortfill.
MTL vs DTL: which strength suits you
The strength you should mix to is decided by the kit, not by how much you used to smoke. The same nicotine in a tight kit and an airy one are different experiences, because the airy one moves several times as much liquid past your lungs per puff.
- MTL — tight, cigarette-like. Above 1.0Ω, low power, small clouds. Shortfills are rarely the right format here; a 10ml nic salt at 10mg or 20mg is. If you do mix one, 6mg is the ceiling worth trying.
- RDL — the middle. 0.6Ω to 1.0Ω. 3mg to 6mg, and the most common home for a 50ml shortfill.
- DTL — airy, warm, lots of vapour. Below 0.6Ω. 3mg, and 1.5mg is not unusual. A 6mg shortfill through a 0.2Ω coil is genuinely unpleasant, and it is the single most common way people put themselves off a new kit.
If you overshoot, it is fixable: top the bottle up with the same 0mg liquid and the strength falls in proportion. Put the larger final volume into the calculator above to see where it lands. Undershooting is the more common mistake, and it shows up as vaping more often rather than as anything you can taste.