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Suture Sizes
Suture size is a measure of the strand's diameter, not its strength or its length. Two scales are in use: the United States Pharmacopeia (USP) scale, where sizes run from 7 down through 0 and then into 2-0, 3-0 and smaller — so more zeros means a thinner strand — and the metric scale, which is simply the diameter in tenths of a millimetre.
Why it matters
The USP scale reads backwards to anyone meeting it for the first time: 6-0 is thinner than 2, and the number gets larger as the strand gets finer. The metric scale does not have that problem, because it is just the diameter. Knowing that they are two descriptions of the same physical quantity — and that USP also fixes a minimum tensile strength at each size — is what makes an ordering table readable.
Size is diameter
A suture’s size is a statement about how thick the strand is. It is not its length, and it is not directly its strength — though the two are linked, because the standard that fixes the diameter also fixes a minimum strength to go with it.
Two scales describe the same measurement. The United States Pharmacopeia (USP) classification is the one used most widely internationally. The metric scale, used by the European Pharmacopoeia and quoted alongside it, is simply the diameter in tenths of a millimetre: metric 3 is 0.3 mm.
The scale that reads backwards
The USP scale is the part that catches people out. Above zero it behaves normally — 1, 2, 3 — each thicker than the last. At zero it turns around and starts counting zeros: 2-0, 3-0, 4-0, sometimes written 00, 000, 0000.
Each additional zero is a finer strand. A 6-0 is thinner than a 3-0 is thinner than a 0 is thinner than a 2. The metric scale has no such quirk, which is why the two are usually printed together.
Collagen and synthetic are not interchangeable at the same number
The chart below has two pairs of columns, and that is not a formatting choice. USP assigns collagen and synthetic sutures different diameter ranges at the same nominal size. A collagen 3-0 falls between 0.300 and 0.349 mm; a synthetic 3-0 falls between 0.200 and 0.249 mm. Both are correctly labelled 3-0.
What else the size fixes
USP does not only bound the diameter. At each size it also sets a minimum tensile strength the strand must meet. So a size is a description of a strand that is simultaneously within a diameter range and above a strength floor — which is why the scale is a classification rather than just a measurement.
U.S.P. suture sizes chart
Diameter ranges for collagen and synthetic sutures. All figures in millimetres.
| U.S.P. Size | COLLAGEN SUTURES | SYNTHETIC SUTURES | ||
|---|---|---|---|---|
| Metric Size | Diameter Range | Metric Size | Diameter Range | |
| # 7 | — | — | 9 | 0.900–0.999 |
| # 6 | — | — | 8 | 0.800–0.899 |
| # 5 | — | — | 7 | 0.700–0.799 |
| # 4 | 8 | 0.800–0.899 | 6 | 0.600–0.699 |
| # 3 | 7 | 0.700–0.799 | 6 | 0.600–0.699 |
| # 2 | 6 | 0.600–0.699 | 5 | 0.500–0.599 |
| # 1 | 5 | 0.500–0.599 | 4 | 0.400–0.499 |
| # 0 | 4 | 0.400–0.499 | 3.5 | 0.350–0.399 |
| # 2-0 | 3.5 | 0.350–0.399 | 3 | 0.300–0.349 |
| # 3-0 | 3 | 0.300–0.349 | 2 | 0.200–0.249 |
| # 4-0 | 2 | 0.200–0.249 | 1.5 | 0.150–0.199 |
| # 5-0 | 1.5 | 0.150–0.199 | 1 | 0.100–0.149 |
| # 6-0 | 1 | 0.100–0.149 | 0.7 | 0.070–0.099 |
| # 7-0 | 0.7 | 0.070–0.099 | 0.5 | 0.050–0.069 |
| # 8-0 | 0.5 | 0.050–0.069 | 0.4 | 0.040–0.049 |
| # 9-0 | 0.4 | 0.040–0.049 | 0.3 | 0.030–0.039 |
| # 10-0 | — | — | 0.2 | 0.020–0.029 |
Finding the sizes for a product
The chart above is the standard. Which of those sizes a particular product is actually offered in is a different question, and it is answered on that product’s own page: every ordering table lists each code with its suture size and the needle supplied with it.
Key terminology
- USP
- United States Pharmacopeia. Its suture classification is the one most widely used internationally. It sets, for each size, a diameter range and a minimum tensile strength the strand must meet.
- Metric size (EP gauge)
- The diameter in tenths of a millimetre. Metric 3 is 0.3 mm. Used by the European Pharmacopoeia and quoted alongside USP.
- The zeros
- Below USP 0 the scale continues 2-0, 3-0, 4-0 and so on — sometimes written 00, 000. Each additional zero is a thinner strand.
- Collagen vs synthetic
- The two have separate USP columns. The same USP size maps to a different diameter range depending on which it is — a collagen 2-0 and a synthetic 2-0 are not the same diameter.
Technical characteristics
- What size measures
- The diameter of the strand
- USP scale direction
- 7 is the thickest; more zeros means thinner (2-0, 3-0, 4-0…)
- Metric scale
- Diameter in tenths of a millimetre — metric 3 is 0.3 mm
- Also fixed by USP
- A minimum tensile strength at each size, alongside the diameter range
- Two material columns
- Collagen and synthetic sutures have separate diameter ranges at the same USP size
- Sizes available on a given product
- Stated in that product's ordering table and instructions for use
What clinicians weigh when selecting
The scale runs backwards
More zeros is thinner. A 6-0 is finer than a 3-0, which is finer than a 0, which is finer than a 2. This is the single most common misreading of an ordering table.
Collagen and synthetic differ at the same number
The chart has two pairs of columns for that reason. A collagen 3-0 and a synthetic 3-0 are both "3-0" and are not the same diameter.
Diameter is not the only thing the size fixes
USP also sets a minimum tensile strength per size. The size therefore describes a strand that is both within a diameter range and above a strength floor.
Size is not the needle
Suture size and needle length are separate specifications. An ordering table gives both per code, and they vary independently.
Frequently asked questions
Why is 6-0 thinner than 2?
Because the USP scale runs downward through 0 and then into the zeros. Above 0 the numbers count up in thickness (1, 2, 3…); below it they count up in zeros (2-0, 3-0, 4-0…), and each additional zero is a finer strand.
What is the difference between USP size and metric size?
They measure the same thing on different scales. Metric is the diameter in tenths of a millimetre — metric 3 is 0.3 mm. USP is a classification with a diameter range and a minimum tensile strength at each step.
Why do collagen and synthetic sutures have different charts?
Because USP assigns them different diameter ranges at the same nominal size. A collagen 3-0 sits at 0.300–0.349 mm; a synthetic 3-0 sits at 0.200–0.249 mm.
Does a larger size mean a stronger suture?
USP sets a higher minimum tensile strength for each larger size, so a larger size is required to meet a higher floor. What that means for a given product is stated in its instructions for use.
What sizes does a particular product come in?
That is product-specific. Each product page carries an ordering table listing every code with its suture size and needle, and the instructions for use state the specification each size meets.
Instructions for use
The sizes offered for a given product, and the specification each one meets, are stated in that product's ordering table and in the instructions for use supplied with it. Availability varies by market.
Medical & regulatory notice
This article is general reference information about suture sizing for healthcare professionals. It is not medical advice, not a recommendation for any procedure or patient, and not a substitute for the instructions for use supplied with the product or for clinical judgement. Product availability and approved indications vary by market. Nothing here should be read as a claim of clinical benefit or of superiority over another material.