Before finalising a close-fitting part, record the exact panel product and its specification, then measure the stock you will use. Keep the purchasing label and those measurements as separate entries on the drawing. Converting a label into millimetres helps with arithmetic; it does not measure the sheet.
This guide provides a planning record and a worked dimensional example. The example is hypothetical, and no panel measurements or workshop tests are claimed.
Keep four numbers separate
| Number | What it answers | What to record |
|---|---|---|
| Nominal thickness or category | Which product description are you ordering? | The supplier's wording and units |
| Converted value | What does that written number equal in another unit? | The original value and conversion |
| Product specification | What thickness requirement does this exact product declare? | The manufacturer's document and any stated tolerance |
| Measured thickness | What readings did you obtain from the supplied stock? | Values, locations, instrument and date |
The Canadian Wood Council's 2025 softwood-plywood declaration, section 1.1 on printed page 4, describes nominal dimensions as an industry reporting convention and identifies manufacturing variation in actual thickness. That passage concerns Canadian softwood plywood. The document is an environmental declaration; it supplies no universal thickness allowance for a fitted part.
Keep a manufacturer's tolerance separate from your project's required fit. One describes a product requirement; the other comes from the actual design. If a quoted panel has no documented tolerance, ask the supplier for the relevant specification before depending on a particular range.
A precise conversion still needs the right input
NIST defines an inch as exactly 25.4 mm. Therefore:
3/4 inch = 0.75 × 25.4 mm = 19.05 mm.
That is an exact conversion of the written dimension. An 18 mm number is different by 1.05 mm. Neither number tells you the actual thickness of an unspecified plywood product.
A drawing headed “3/4-inch plywood” needs clarification before its fit-dependent dimensions become final. Establish whether it assumes an actual 19.05 mm thickness or uses the wording to identify a nominal product. Record the intended stock input explicitly.
See what two sides do to an opening
Consider a purely hypothetical opening between two side pieces within a 600 mm outside width. For this illustration, opening width equals outside width minus the two side thicknesses. Joinery, finishes and fit allowances are outside this arithmetic.
| Hypothetical input for each side | Calculation | Opening |
|---|---|---|
| 19.05 mm | 600 − 19.05 − 19.05 | 561.90 mm |
| 18.5 mm | 600 − 18.5 − 18.5 | 563.0 mm |
Changing each input by 0.55 mm changes the opening by 1.10 mm. These are invented example inputs, not measurements from our shop, a product specification or a recommended clearance.
The useful step is identifying every dimension that depends on stock thickness. When you replace an assumption with your own readings, revisit those dependent dimensions together. Rounding one number or adjusting one part leaves the other assumptions unresolved.
What a calibrated panel tells you
Columbia Forest Products explains that a calibrated panel typically uses a separately produced core sanded to tighter thickness limits before decorative veneers are applied. Its explanation also identifies natural and manufacturing causes of panel thickness variation.
Use that description to ask a better purchasing question: what tolerance is specified for the finished panel you are ordering? “Calibrated” is useful manufacturing information; the actual specification and supplied-stock readings still belong in your record. No numerical tolerance is inferred here from the term.
Make a stock record you can use again
For a Canadian quotation, keep the manufacturer, exact product/SKU, core and face description, nominal thickness with units, sheet dimensions and quantity together. Confirm any documented thickness tolerance and its scope. Retain the dated CAD quotation and delivery terms separately from your dimensional assumptions.
Once you have the intended stock:
- Follow the measuring instrument's instructions and check its zero as directed.
- Record several accessible measurement locations relevant to the intended parts, along with the instrument and date.
- Keep individual readings and locations when they differ; investigate a spread that affects the intended fit.
- Use precision your measurement method supports, then update the drawing's stock input and dependent dimensions.
- Resolve the required fit, finishes and other design allowances separately before treating a dimension as final.
Keep this record with the material reference on the drawing. It gives a replacement sheet or a later revision something concrete to compare with.
For solid timber and changing room conditions, continue with why dry wood still moves. Its moisture discussion addresses another reason to revisit a dimension; it does not supply a plywood thickness conversion table.
HOLD YOUR GROUND.
Sources & further reading
Source information was checked for this guide’s publication date. Follow the manufacturer’s current manual for your exact tool.