Most canker sores heal in 7 to 14 days. A small one under 5mm is usually gone in about a week; a larger one can take two weeks or more. Pain peaks in the first three days and fades well before the sore closes.
The reason this question gets asked so often is that a canker sore gives you almost no feedback. It hurts about the same on day two as it does on day five, you cannot see it properly without a phone torch and an awkward angle, and every day it is still there feels like evidence that something is wrong. So here is the whole course, laid out in days.
One thing to rule out first: all of this is about canker sores, the ulcers inside your mouth. If yours is on the lip border or the skin outside it, or it blistered before it opened, you are reading about the wrong sore and the timeline below will not fit.
For most people, seven to fourteen days, and the width at its worst is what decides where in that range you land.
| Width at its widest | Usually gone by | What it is |
|---|---|---|
| Under 5mm | Day 7–10 | A minor aphthous ulcer — the ordinary kind |
| 5–10mm | Day 10–14 | Still minor, just wider, so it takes longer to fill in |
| Over 10mm | Two weeks to six | A major ulcer. Uncommon, slower, and worth showing to a dentist |
Width matters more than pain here. A 3mm sore in a spot your tooth keeps catching can be agonising and still be gone by Friday; a broad, shallow one on the inside of a cheek can be merely annoying and take a fortnight.
Set the two things you can actually observe.
A small (under 5mm) sore typically runs 7–10 days, so from day 3 that is roughly 4–7 more days.
The ordinary kind. Most close inside a week and leave nothing behind.
This is a population range, not your sore. The only way to know what yours is doing is a second measurement.
Log this one and compare tomorrowDay 0–1, before you can see it. A patch of mouth tingles, prickles or burns. Nothing is visible yet. This is the stage where people say they can feel one coming, and they are usually right.
Day 1–3, it opens and grows. A small round crater appears with a white or yellow floor and a red ring around it. It gets wider over these days, and this is when it hurts most. A canker sore getting bigger in the first three days is normal. It is the single most common reason people decide something has gone wrong when nothing has, and when growth is and is not worth acting on is its own question.
Day 4–7, the standstill. The growing stops. The pain starts dropping away noticeably, often faster than the sore itself changes. The white centre is still there and the sore looks, to the naked eye, more or less identical from one day to the next. This is the stretch that feels endless, and it is also the stretch where measuring is the only thing that will tell you anything.
Day 7–14, it closes. The crater fills in from the edges, so the white centre shrinks before the sore vanishes. The red halo fades to pink, then to nothing. A minor ulcer heals without scarring — if you can still find the spot a month later by looking, that is worth mentioning to a dentist.
Less time than the sore does. Pain typically peaks in the first two to three days and eases from day four onward, while the sore itself is still clearly there for another week.
That gap trips people up in both directions. Feeling better on day five does not mean it has healed, and still being able to see it on day ten after the pain has gone does not mean it is stuck. Pain and size are two different measurements and they do not move together — which is exactly why Canker Core records them separately rather than asking you how it is going.
Three practical lines, none of which are a diagnosis:
Any of those is worth a dentist. Not because it is likely to be something serious, but because at that point the useful thing is an examination rather than another week of waiting. Two weeks and it is still there picks that up — what separates a slow sore from a stuck one, and what to bring to the appointment.
Everything above is a population range. It tells you what canker sores do in general, which is genuinely useful on day three and almost useless on day nine, when the only question you care about is whether yours is closing.
There is one reliable way to answer that, and it takes about fifteen seconds:
A sore that has gone from 6mm to 4mm is healing, whatever it feels like. A sore that reads 6mm both times has not moved, whatever you hoped. Memory is not up to this job — a sore you looked at on Tuesday and again on Thursday will look the same to you both times, because two millimetres is invisible to recollection and obvious to a ruler.
Canker Core is a log, not a diagnosis. Nothing here is medical advice. A sore that has not started shrinking after two weeks, keeps coming back in the same spot, or comes with a fever or a rash is worth showing to a dentist or doctor.
Mark the sore, log its width and pain, and compare tomorrow. About fifteen seconds.