Sources and scope for this page: APA / PS 1 — Structural Plywood (PS 1-20), covering the sanded panel thickness series and its tolerances; and PS 2-18, covering the span ratings referenced in the thickness discussion (one more source is tabulated below). No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. Full sources, values and limits on this page.
Page last changed 2026-10-19 · based on PS 1-20
Quick Answer
Plywood Panel Thicknesses
| Nominal thickness[2] | Series[2] | Actual thickness[2] | Actual thickness mm[1] | Note[2] |
|---|---|---|---|---|
| 1/4 # | Sanded, imperial | 7/32 | 5.6 | 3-ply; the thinnest common sanded panel |
| 3/8 # | Sanded, imperial | 11/32 | 8.7 | Sold as 3/8; the actual thickness is often equal to the nominal fraction step above |
| 1/2 # | Sanded, imperial | 15/32 | 11.9 | The most common general-purpose panel thickness |
| 5/8 # | Sanded, imperial | 19/32 | 15.1 | Underlayment and sheathing |
| 3/4 # | Sanded, imperial | 23/32 | 18.3 | The standard cabinet and furniture panel |
| 1 # | Sanded, imperial | 1-3/32 | 27.8 | Heavy shelving and structural panel |
| 3 mm # | Metric | 3 mm | 3.0 | Actual — metric plywood is sold at true thickness |
| 6 mm # | Metric | 6 mm | 6.0 | Actual — nearest to 1/4 in imperial |
| 9 mm # | Metric | 9 mm | 9.0 | Actual — nearest to 3/8 in imperial |
| 12 mm # | Metric | 12 mm | 12.0 | Actual — nearest to 1/2 in imperial |
| 15 mm # | Metric | 15 mm | 15.0 | Actual — nearest to 5/8 in imperial |
| 18 mm # | Metric | 18 mm | 18.0 | Actual — nearest to 3/4 in imperial |
| 21 mm # | Metric | 21 mm | 21.0 | Actual |
| 25 mm # | Metric | 25 mm | 25.0 | Actual — nearest to 1 in imperial |
Actual imperial thicknesses are the sanded-panel values, undersize from nominal by roughly 1/32 in because both faces are sanded after the panel is pressed. Tolerance is real and varies: APA-rated panels carry published thickness tolerances, and an individual panel can sit anywhere inside them. Metric plywood is sold at true thickness with its own tolerance, so mixing an 18 mm and a 23/32 in panel in the same assembly leaves a 0.26 mm step. Unfinished, unsanded and utility grades are often closer to nominal, since they are not sanded.
Why Sanded Plywood Is Undersize
A plywood panel is pressed oversize and then sanded on both faces to clean up the surface and bring it to a uniform thickness. Sanding removes roughly 1/32 in (0.8 mm) from the total, so the finished panel measures about 1/32 in under the nominal figure it is sold by. That is why the nominal sizes form a series of 32nds — 7/32, 11/32, 15/32, 19/32, 23/32 — and why the actual thickness is always the next 1/32 step up from the published nominal size.
The result is that nominal and actual do not line up in the way a buyer might expect. A panel sold as 3/4 in measures 23/32 in. A panel sold as 1/2 in measures 15/32 in. The pattern holds down the whole imperial series, and the discrepancy — 0.8 mm — is large enough to matter in a dado, a rebate or a stacked lamination.
Unsanded grades are the exception. Sheathing, utility and some structural panels are not sanded after pressing, so their actual thickness is much closer to nominal. This is why a builder can find that 1/2 in sheathing fits an opening that 1/2 in sanded plywood does not.
Imperial vs Metric Panels
Metric plywood — manufactured to European and Asian standards and increasingly common worldwide — is sold at its actual thickness. An 18 mm panel measures 18 mm; a 12 mm panel measures 12 mm. There is no nominal-versus-actual gap, and the tolerance is applied to the true figure rather than subtracted from a nominal one.
Because the two series were designed independently, they do not coincide. An 18 mm panel is 18.00 mm; a nominal 3/4 in sanded panel is 23/32 in = 18.26 mm. The metric panel is 0.26 mm thinner. For a single panel in a frame that is negligible. For a stack of ten laminations the difference is 2.6 mm, and for a dado cut to receive a specific panel it is the difference between a snug fit and a loose one.
The pairing across the two series is: 3 mm with 1/8 in, 6 mm with 1/4 in, 9 mm with 3/8 in, 12 mm with 1/2 in, 15 mm with 5/8 in, 18 mm with 3/4 in, and 25 mm with 1 in. In every case the metric panel is slightly thinner than the imperial nominal size, and slightly thicker than the imperial actual size — 18 mm sits between 23/32 in (18.26 mm) and the 17.5 mm that would be 1/32 under it.
For anything load-bearing, plywood thickness is a structural input, not a convenience. Panel span ratings and shear values are published against a specific thickness and grade, so substituting an 18 mm panel for a rated 23/32 in panel changes the structural capacity by about 1.5% in bending — small, but enough that a specification should name the actual thickness rather than "3/4 inch plywood".
Frequently Asked Questions
- What is the actual thickness of 3/4 inch plywood?
- 23/32 in, or 18.26 mm — about 18.3 mm. Sanded imperial plywood is undersize from its nominal thickness by roughly 1/32 in because both faces are sanded after pressing. The same rule applies down the series: 1/2 in plywood is 15/32 in (11.9 mm) and 5/8 in plywood is 19/32 in (15.1 mm). Unsanded sheathing grades are closer to nominal.
- Is 18 mm plywood the same as 3/4 inch plywood?
- Close but not identical. An 18 mm panel is exactly 18.00 mm; a nominal 3/4 in sanded panel is 23/32 in, or 18.26 mm. The metric panel is 0.26 mm thinner. That is immaterial for a single panel and significant for a stack of laminations or for a dado cut to a specific thickness. Metric plywood is sold at true thickness; imperial plywood is sold at nominal size and delivered undersize.
- Why is metric plywood sold at its actual thickness?
- Because the metric panel standards specify the actual thickness as the controlled dimension, with a tolerance applied to it. The imperial system kept a nominal thickness as the product identifier after sanding standards reduced the delivered size. The practical difference is that an 18 mm panel really is 18 mm, while a 3/4 in panel is 23/32 in — so any substitution between the two series needs the actual figures, not the labels.
- Does plywood thickness matter structurally?
- Yes. Plywood span ratings and shear capacities are published against a specific thickness and grade, and bending stiffness scales with the cube of thickness — so the 1.4% difference between an 18 mm panel and a 23/32 in panel changes stiffness by about 4%. More importantly, the rating tables are grade-specific: a panel of the same thickness but a different grade or exposure classification may have a different span rating. Specify actual thickness and grade, not a nominal size.
Related
Value Sources
References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.
| # | Source | Type | Revision / method |
|---|---|---|---|
| [1] | Metric thickness converted from the actual imperial thickness | derived | Actual imperial thickness × 25.4, rounded to one decimal place. Metric panels are listed at their true nominal thickness, which is the actual figure — no conversion is applied to those rows. |
| [2] | HPVA / APA published panel thickness standards and manufacturer tolerance tables | reference | compilations as published 2024–2026 |
Data Sources
| Standard | Revision | What it covers on this page |
|---|---|---|
| APA / PS 1 — Structural Plywood | PS 1-20 | the sanded panel thickness series and its tolerances |
| PS 2 — Performance Standard for Wood Structural Panels | PS 2-18 | the span ratings referenced in the thickness discussion |
| EN 636 — Plywood, specifications | EN 636:2012+A1:2015 | the metric panel thickness series and its tolerances |
Verification scope
No page-specific independent source review is recorded here. The listed references and formulas explain the basis of the compilation; automated consistency checks do not verify the original sources or establish product conformity.
Derived values — the following values on this page are calculated, not taken directly from the standard:
| Value | How it is derived |
|---|---|
| Actual thickness in millimetres | Imperial actual thickness × 25.4, rounded to one decimal place. Metric rows carry their true nominal thickness with no conversion. Checked at build time against the published metric equivalents (23/32 in = 18.3 mm, 15/32 in = 11.9 mm). |
Actual thicknesses are typical sanded-panel values; individual panels vary within the published thickness tolerance, and unsanded sheathing and utility grades are closer to nominal. Metric panels are sold at true thickness with their own tolerance. For structural work, use the thickness published in the span rating table you are designing from rather than the nominal size on the panel edge.
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