A canker sore runs through five stages: a day or two of tingling before anything shows, two to three days of the ulcer opening and hurting most, a few days at full width, several days filling in from the edges, then closed tissue by day 10 to 14.
Most pages about this show you photographs of other people's mouths, which tell you very little: a photo has no scale, no date, and no way of saying whether the sore in it was getting better or worse that morning. What follows is drawn instead, all five stages at one millimetre scale, so the sizes can be compared to each other and to yours.
These are the stages of a canker sore specifically. A cold sore runs through a different sequence — blisters first, then a crust — and the two are drawn side by side here if you are not yet sure which one you are looking at.
Every diagram uses the same millimetre scale, so the sore really is widest at the peak and really is smaller by day eight.
No open ulcer · pain 2/10
Nothing, or a small pale bump. Often no mark at all yet.
A tingle, a prickle, or a spot that feels tight when you move your lip.
About 4mm across · pain 8/10
The crater opens: a white or yellow floor inside a bright red ring, widening daily.
The worst of it. Salt, citrus and toothpaste all find it.
About 7mm across · pain 5/10
At its widest, and holding still. The ring is still red but no longer spreading.
Easing, which is the confusing part — it hurts less while it is at its biggest.
About 3mm across · pain 2/10
Filling in from the edges. The white centre shrinks first, the ring fades to pink.
Noticeable against a tooth, not much else.
No open ulcer
Closed. A faint pale patch for a few days, then nothing — minor sores do not scar.
Gone.
The stage that is felt rather than seen. A tingle, a prickle, or a spot on your lip that feels tight when you move it. There may be nothing visible at all, or a small pale bump a few hours before it opens.
Almost nobody logs this one, and it is the most useful one to log. If you get sores often, the tingle is the only advance warning you will ever get, and knowing that yours reliably arrives two days ahead turns a surprise into a pattern.
The crater opens. A shallow round ulcer with a white or yellow floor inside a bright red ring, and it widens every day of this stage.
This is also when it hurts most, which matters more than it sounds: the pain peaks before the size does. Salt, citrus, toothpaste and the corner of a crisp all find it.
At its widest, and holding still. The ring is still red but no longer spreading, and the pain has already started to ease.
This is the confusing stage, and it is where most people decide something is wrong. The sore looks worse than it did on day two — it is genuinely bigger — while hurting less, and nothing changes for two or three days running. That plateau is the normal middle of a normal sore, not a stall.
Healing, visibly. A canker sore closes from the edges inward, so the white centre shrinks while the overall spot is still there. A sore that is mostly pink with a small white middle is further along than one that is mostly white, even at the same width.
Closed. A faint pale patch may sit there for a few days afterwards. Minor canker sores do not scar; if something is still marked in that spot weeks later, it is worth showing to a dentist, because it is not the sore.
| Stage | Days | Typical width | What changes |
|---|---|---|---|
| Prodrome | −2 to 0 | no ulcer | you feel it |
| Ulceration | 1 to 3 | 2–5mm | opens, widens, hurts |
| Peak | 4 to 6 | 4–8mm | nothing — it holds |
| Granulation | 7 to 10 | 2–5mm | fills in from the edges |
| Healed | 10 to 14 | closed | pale patch, then gone |
These are the ranges for a minor aphthous ulcer, the ordinary kind under about 10mm. Larger ones stretch every stage and can run for weeks. If you want the arithmetic for a particular sore, the duration estimator does it by size.
No, and this is the most common misreading of the stages. The white or yellow floor is fibrin — the same stuff as a scab, minus the drying out — and it is there from the moment the ulcer opens. It is not new skin, and its appearance is not a milestone.
What it does is get smaller. The useful question is not "is it white" but "is there less white than there was", which is a comparison, and comparisons need two observations.
The stages are easy to tell apart on a diagram and hard to tell apart in a mirror, because what separates stage 3 from stage 4 is a change in width of a millimetre or two — and nobody can hold a millimetre in their head from Tuesday.
The shortcut is that you do not actually need to name the stage. Two measurements two days apart answer the only question the stage was standing in for: is it getting smaller. Widening means stage 2, static means stage 3, narrowing means stage 4. The number tells you the stage, not the other way around.
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.