Growing in the first three days is what canker sores do: they open, widen, then stop by about day four. Widening after that is the one size change worth acting on. Pain is not a size measurement — only two widths, two days apart, are.
This is the day-two panic, and it is almost always a false alarm. A canker sore does not appear at full size and shrink from there. It opens small, widens for two or three days, stops, and only then starts closing — so the stretch where it is visibly getting worse is a normal part of the course rather than a departure from it.
The useful question is not whether it grew. It is when it grew.
In the first three days, yes. That is the single most common reason people decide something has gone wrong when nothing has.
The sequence is the same almost every time: a day of tingling before anything is visible, then a small crater with a white or yellow floor and a red ring, then two or three days of that crater widening as the surface breaks down around its edge. Full size lands around day three or four. After that it fills in from the rim, which means the white centre shrinks before the sore disappears — one more thing that looks like deterioration and is not.
The day-by-day course is the long version, and the five stages drawn is what each of those steps actually looks like.
Three cases, and they are narrow on purpose:
Everything else — that it hurts more, that it looks angrier, that the white part is bigger than yesterday — is the ordinary first half of an ordinary sore.
| 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 is the best single predictor of how long yours will take, which is why it is the number worth having and pain is not. A crater fills in from its rim, so a 10mm sore is a good deal more than twice the work of a 5mm one.
Because pain tracks where a sore is far more than how wide it is. A 3mm ulcer sitting exactly where your teeth close can be agonising; a broad shallow one on the inside of a cheek can be merely annoying and take twice as long.
Pain also peaks in the first two to three days and fades from day four onward, while the sore itself is clearly visible for another week. So early on, rising pain and increasing size happen together and feel like one event — and then they separate, and the sore keeps closing long after it has stopped hurting. They are two measurements that do not move together, which is why the app records them separately instead of asking how it is going.
Eyeballing a sore against a ruler is good to about a millimetre. That sets the whole method:
One millimetre of difference is noise. Two is real. And a single look — today, without yesterday's number — cannot tell you whether it is growing, stalled or closing, no matter how carefully you look at it.
Widest point, in millimetres, two days apart.
Narrowing — that is healing
Down 2mm. A sore that is measurably smaller two days later is closing, whatever it still feels like.
Keep the numbers insteadIf the numbers come back the same on two dates a week apart, the question has changed from growth to whether it is moving at all, which has its own thresholds and its own appointment.
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.