The short answer

A 5 mm skin cancer with a 5 mm safety margin becomes a 15 mm wound — the margin is added on both sides. A round wound cannot usually be stitched into a neat line, so it is tapered into an eye shape about three times as long as it is wide: roughly 45 mm. Because the sides of that eye shape are curved, the closed line ends up around 48 mm. In this example the scar is close to ten times the width of the spot you could see — and almost none of that extra length is cancer.

Key points
  • The scar is the size of the safe excision, not the size of the cancer — a 5 mm spot can leave a line of about 48 mm, close to ten times its width.
  • The safety margin is added on every side, so a 5 mm cancer with a 5 mm margin is a 15 mm wound, not a 10 mm one.
  • Australian recommendations are 2–3 mm for low-risk BCC and over 5 mm for high-risk BCC; 4 mm for low-risk SCC and 6 mm for high-risk SCC.
  • A round wound is not stitched closed directly — pulled into a line it bunches into raised puckers called dog ears at each end.
  • Removing a tapered wedge at each end turns the circle into an eye-shaped (fusiform) excision, traditionally about three times as long as it is wide.
All 10 key points
  • For a 15 mm wound that means roughly 45 mm of planned excision — the geometry, not the cancer, is what makes it long.
  • The closed line is a little longer again because it follows the curved edge of the eye shape, not the straight tip-to-tip measurement.
  • A long scar does not mean the cancer was large, advanced or aggressive — the pathology report, not the scar, records the tumour size and margin clearance.
  • A slightly longer, well-planned line usually heals flatter than a short closure under tension, which is why shortest is not the goal.
  • Not every operation follows this pattern: cancer type, body site, skin looseness and the choice of flap, graft or Mohs surgery all change the final scar.

The question comes up at nearly every post-operative check: the spot was tiny — why is the scar the length of my finger?

It is a fair question, and the answer is reassuring. The length of a skin cancer scar is mostly geometry, not disease. Two decisions drive it: how much normal-looking skin has to come out around the cancer to give it the best chance of a clear margin, and what shape the wound has to be before it will close flat. Neither is a judgement about how bad the cancer was.

Here is what that looks like, drawn to scale.

A 5 mm skin cancer compared, at the same scale, with the 48 mm scar it can leave Four shapes drawn at the same scale. First, a filled circle 5 millimetres across — the visible cancer. Second, a circle 15 millimetres across containing that same small spot — the wound once a 5 millimetre safety margin is added on every side. Third, an eye-shaped outline 45 millimetres long and 15 millimetres wide — the tapered excision needed to close the round wound flat. Fourth, a straight line about 48 millimetres long — the finished scar. 1 — the cancer you can see5 mm 2 — plus a 5 mm margin all round15 mm wound 3 — tapered to an eye shape45 mm long 4 — closed as a lineabout 48 mm
Figure 1. Drawn to scale. A 5 mm skin cancer, the 15 mm wound left once a 5 mm safety margin is added on every side, the 45 mm eye-shaped excision needed to close that wound flat, and the finished line of about 48 mm.

The three things that make the line longer than the cancer

  1. The margin is added on every side. It doubles once, not just at one edge.
  2. A circle will not close as a line. Tapered ends have to be cut away so the skin lies flat.
  3. The curved edges straighten as they close. The finished line follows the curve, which is longer than the straight distance between the tips.

Each of those steps is deliberate, and each of them adds length.

How a 5 mm spot becomes a line close to 5 cm

A worked example, not a promise of exact dimensions. These panels are schematic — Figure 1 shows the true relative sizes.

  1. Step 1

    The visible cancer

    5 mm

    The spot you could see or feel — in this example, 5 mm across.

  2. Step 2

    Add the safety margin

    15 mm across

    A 5 mm margin on every side makes the wound 15 mm wide, not 10 mm.

  3. Step 3

    Taper the ends

    about 45 mm

    A classic eye-shaped excision is about three times as long as it is wide.

  4. Step 4

    Close it as a line

    about 48 mm

    The curved wound edges straighten as they are stitched together.

5 mm spot 15 mm wound 45 mm excision about 48 mm scar

Figure 2. The four stages of a straight-line (fusiform) skin cancer excision.

Step 1 — the visible cancer is only the starting size

When a skin cancer is removed, the surgeon does not cut along the edge you can see. A rim of normal-looking skin — the clinical margin — is taken all the way around it. That rim covers microscopic extensions of tumour that cannot be reliably seen with the naked eye, and it gives the pathologist a border of normal tissue to examine so they can report whether the cancer was completely removed.(1)

The step people find counterintuitive is the arithmetic. A 5 mm margin does not turn a 5 mm spot into a 10 mm wound. It adds 5 mm on the left and 5 mm on the right:

Visible spot5 mm
Safety margin5 mm on each side
Round wound5 + 5 + 5 = 15 mm across

How big is the margin really?

The 5 mm used above is a round number chosen to make the arithmetic easy. Real margins are individualised — the diagnosis, subtype, size, body site and risk features all change them. As a guide to what Australian practice looks like:

CancerTypical clinical margin
Basal cell carcinoma — low risk2–3 mm(1)
Basal cell carcinoma — high riskmore than 5 mm(1)
Squamous cell carcinoma — low riskabout 4 mm(1,2)
Squamous cell carcinoma — high riskabout 6 mm(1,2)
Melanoma in situabout 5 mm
Thin invasive melanoma (up to 1 mm)about 10 mm

The target these margins are chosen to hit is an incomplete-excision rate under 5% — that is, fewer than one in twenty excisions showing tumour at the edge of the specimen.(1) Margins for basal cell carcinoma have been debated for decades, and the published work behind these figures goes back to the 1980s.(3) Melanoma margins are set by the Breslow thickness measured on the diagnostic biopsy, which is why melanoma is usually a two-stage process: a diagnostic excision first, then a wider re-excision around the scar.

Step 2 — why a round wound is not simply stitched closed

Imagine pulling the mouth of a round drawstring bag into a straight line. The middle closes, but the spare material has nowhere to go and bunches up at both ends. Skin behaves the same way. Pull a circular wound into a line and you get two raised puckers, known in the trade as dog ears.

The fix is to remove a tapered wedge of normal skin at each end, converting the circle into a shape like an eye, a lemon or a rugby ball. The medical term is a fusiform — or elliptical — excision. It feels backwards, because it means removing extra healthy skin, but those tapered ends are exactly what allow the wound to sit flat instead of bulging.

The traditional planning guide is that a fusiform excision should be roughly three times as long as it is wide.(4) So a 15 mm-wide wound is planned as an excision of about 45 mm. That single ratio accounts for most of the scar length that surprises people.

Step 3 — why the closed line is longer again

The 45 mm figure is the straight, tip-to-tip length of the planned eye shape. But the sides of that eye are curved, and a curve is always longer than the straight line between its ends — the same way a curved ribbon gets longer when you lay it flat.

When the two curved edges are brought together with stitches, they straighten to form the final line. Research measuring real fusiform excisions found no significant difference between the arc length designed before the operation and the length of the incision afterwards — in other words, the finished line follows the curve, not the tip-to-tip measurement.(5) For a 45 mm by 15 mm design, that curve measures a little over 48 mm.

Occasionally a small extra trim is needed at one or both ends to remove residual puckering once the closure is under way. That is another reason the finished line can be marginally longer than the outline first drawn on the skin.

What scar length does — and does not — tell you

A long scar does not mean the cancer was long, advanced or unexpectedly aggressive. A very small, entirely low-risk cancer can leave a long linear scar purely because of the margin and the closure geometry described above.

What does tell you about the cancer is the pathology report. It records the measured tumour size, the subtype, how deep it went, and whether the examined edges were clear. Scar length answers none of those questions. If nobody has walked you through your report, ask at your follow-up — it is your result, and it is the document that actually matters.

There is also a cosmetic paradox worth knowing about: a somewhat longer, carefully planned line usually heals more neatly than a shorter closure fighting tension, or one that leaves bulky ends. Where the anatomy allows, the line is also oriented along a natural skin crease or the direction in which the skin closes most comfortably, so that it settles into the face or body rather than cutting across it.

Not every skin cancer operation follows this pattern

The 5 mm → 15 mm → 45 mm → 48 mm example explains a common straight-line closure. It is not a universal formula. The final repair depends on:

  • the type and risk level of the cancer, and the margin it requires;
  • the size of the visible lesion or the previous biopsy scar;
  • how much loose skin is available at that site, and which way it moves;
  • nearby structures such as the eyelid, nose, lip, ear or a joint;
  • whether the best repair is a straight closure, a flap, a skin graft, Mohs surgery, or healing without stitches;
  • whether the lesion is suitable for a non-surgical treatment instead — some superficial cancers and pre-cancers are, and they leave no surgical line at all.

Two cancers of identical visible size can therefore leave very different scars. The aim is never the shortest possible line. It is to remove the cancer safely, protect how the area works and moves, and choose the repair most likely to heal well.

Helping the scar settle

Most surgical scars look their worst in the first few weeks and then keep improving for roughly 12 to 18 months as the collagen inside them remodels. What genuinely helps:

  • Follow the wound-care instructions you were given for the first fortnight — that period sets the foundation for everything after it.
  • Keep the scar out of the sun. New scar tissue burns and pigments easily; covering it or using SPF 50+ once the skin has healed is the single most useful long-term habit.
  • Massage it gently once the wound is fully closed and your doctor says it is safe, which helps a firm scar soften.
  • Do not smoke. Nicotine constricts the small vessels the healing scar depends on.
  • Be patient with redness. Pink or purple is normal for months and is not a sign the scar will stay that colour.

Have the scar reviewed rather than waiting it out if it becomes progressively thicker, redder, itchier or raised beyond the original line after the first couple of months — that pattern can indicate a hypertrophic or keloid scar, and it is far easier to settle early. A range of scar treatments can also improve the texture and colour of an established scar once it has matured.

Before your operation, it is fair to ask

You are entitled to know what is planned before the local anaesthetic goes in. Reasonable questions:

  • How long will the line be, and can I see it drawn on my skin?
  • Which way will it run, and will it sit in a natural crease?
  • What margin are you taking, and why that size for this cancer?
  • Is a straight closure the plan, or a flap or graft?
  • When will I get the pathology result, and how will I hear about it?

Worried about a spot, or about a scar?

Our doctors assess and treat skin cancer at both the Ivanhoe and Diamond Creek clinics. A targeted spot check suits up to three specific lesions you are concerned about; a full skin check is the better option for whole-body screening or if you have a history of skin cancer. If you have already been treated, life after skin cancer covers surveillance and what happens next.

Fees and appointment types for every skin cancer service are listed on the skin check services and fees page.

Frequently asked questions

  • Why is my skin cancer scar so much bigger than the skin cancer was?
    Because the scar is not the size of the cancer — it is the size of the safe excision plus the shape needed to close it flat. A 5 mm cancer with a 5 mm margin on every side is already a 15 mm wound, and a round 15 mm wound is tapered into an eye shape roughly three times as long as it is wide before it can be stitched into a smooth line. That is about 45 mm of planned excision and a finished line of roughly 48 mm — close to ten times the width of the original spot.
  • Does a long scar mean the skin cancer was much larger or more dangerous than it looked?
    Usually not. Scar length is driven mainly by the safety margin and the geometry needed for a flat closure, so a very small, low-risk cancer can still leave a long line. The pathology report — not the scar — tells you the measured tumour size, its subtype and whether the examined edges were clear. If you have not been through your pathology result, ask for it at your follow-up.
  • Why can't a round skin cancer wound just be stitched closed?
    Pulling a circle into a straight line leaves spare skin bunched at each end, which forms raised puckers known as dog ears. Removing a tapered wedge at each end — turning the circle into an eye, lemon or rugby-ball shape — lets the edges meet evenly and lie flat. It removes more normal skin, but it is what produces a neat line instead of two lumps.
  • Is a skin cancer scar always three times the length of the wound?
    No. The 3:1 length-to-width ratio is a traditional planning guide, not a fixed rule. The actual design depends on the cancer type and margin, the body site, how much loose skin is available, the direction the skin closes most comfortably, and whether the repair is a straight closure, a flap, a graft or Mohs surgery. Some closures are shorter, some longer, and some deliberately curve to sit in a natural crease.
  • Why is the finished scar longer than the shape drawn on my skin?
    The pen line is measured tip to tip in a straight line, but each side of the eye shape is curved, and a curve is longer than the straight distance between its ends. When the two curved edges are stitched together they straighten out, so the closed line follows the curved edge length rather than the tip-to-tip measurement. Published measurements of fusiform excisions found no significant difference between the planned arc length and the final incision length. A small extra trim at one end to remove residual puckering can add a little more.
  • Can the surgeon make my skin cancer scar shorter?
    Sometimes, but a shorter incision is not automatically a better scar. Closing a wound under too much tension, or leaving the ends too blunt, tends to produce a widened, puckered or distorted result — and near the eyelid, nose, lip or ear it can pull a structure out of shape. The aim is the smallest safe excision and the repair most likely to heal well, which is often a slightly longer but flatter line.
  • Why is my treatment scar bigger than the biopsy scar?
    A biopsy is designed to make the diagnosis and often removes only part of the lesion, so it can be small. Definitive treatment then removes the known cancer or the biopsy site together with the full safety margin and closes the resulting wound, so the treatment scar is normally larger than the biopsy mark.
  • Will the scar stay this obvious?
    Most surgical scars are at their most noticeable in the first few weeks, then continue to soften, flatten and fade over roughly 12 to 18 months. Sun protection over the scar, gentle massage once the wound has fully healed, and not smoking all help. A scar that is getting thicker, redder or itchier after the first couple of months should be reviewed rather than waited out.
  • Would Mohs surgery have left a smaller scar?
    It can, for selected cancers in selected places. Mohs surgery removes tissue in stages and checks the margins under the microscope during the procedure, so less normal skin may be sacrificed — which is why it is often chosen for high-risk cancers on the face. It is not the right answer for every cancer or every site, it takes considerably longer, and the closure still has to be designed to sit flat. Whether it is appropriate for a particular lesion is a decision made with your doctor.
  • Does the surgeon know how long the scar will be before the operation?
    Usually yes — the planned shape is normally drawn on the skin before the local anaesthetic goes in, and you can ask to see it and ask how long it will be. The final length can change slightly once the wound is open and the skin tension is assessed, or if a little extra trimming is needed at the ends.

References

  1. Mutimer CA, Dicker AJ. Marking a surgical margin for excision of a keratinocyte cancer. Aust J Gen Pract. 2021;50(6):377-379.DOI: 10.31128/AJGP-12-19-5171
  2. Brodland DG, Zitelli JA. Surgical margins for excision of primary cutaneous squamous cell carcinoma. J Am Acad Dermatol. 1992;27(2 Pt 1):241-248.DOI: 10.1016/0190-9622(92)70178-I
  3. Wolf DJ, Zitelli JA. Surgical margins for basal cell carcinoma. Arch Dermatol. 1987;123(3):340-344.
  4. Zuber TJ. Fusiform excision. Am Fam Physician. 2003;67(7):1539-1544.
  5. Zou R, Lin F, Hao C, Zhou D, Liang J, Wang H. Assessment of mathematical model for elliptical excision: solving the doubt about vertex angle and predicting postoperative wound length. BMC Surg. 2023;23(1):328.DOI: 10.1186/s12893-023-02234-w

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Portrait of Dr Christopher Irwin

By Dr Christopher Irwin, MBChB, FRACGP, MMed (Skin Cancer), FACAM, MSCCA

Last reviewed 2026-08-23 · Editorial policy