How Do Chinese Factories Handle Metric vs Imperial Specifications?

Chinese factories work in millimetres. Every drawing, every CNC program, every tooling set and every quality record on the floor is metric, so an imperial specification is converted once, on paper, and then manufactured in metric — which means the conversion itself is the risk, not the machining. In practice this works cleanly on made-to-order items where you supply the dimension, and it fails on standard-module products — cabinets, doors, appliances, plumbing rough-ins — where an American or British nominal size does not exist as a Chinese standard size and gets rounded to the nearest metric module. The defence is simple and it is always the same: put the controlling dimension in millimetres on the drawing, state the tolerance, and require the factory to confirm it back before tooling. FBM Sourcing works as your sourcing agent and sourcing partner throughout this process.

Where Does the Metric–Imperial Gap Actually Cause Problems?

Almost never in a machine shop, and almost always in a module. These are the recurring cases on project packages.

• Kitchen and joinery modules. US cabinets are built on 3 in increments — 30 in, 33 in, 36 in wide — while Chinese production lines are set for 600 mm, 800 mm and 900 mm. 36 in is 914.4 mm, so a 900 mm cabinet delivered against a 36 in drawing leaves a 14.4 mm gap per unit, which becomes over 100 mm across a seven-unit run.
• Doors. A 3’0” × 7’0” leaf is 914 × 2,134 mm; the standard Chinese module is 900 × 2,100 mm. The difference matters at the frame, the hardware set-out and the fire-rated assembly, where the certified configuration is a specific size range.
• Sheet goods. 4’ × 8’ is 1,220 × 2,440 mm against a standard 1,220 × 2,440 mm export panel, but domestic Chinese stock is often 1,220 × 2,440 or 1,830 × 2,440 — confirm the panel size before a millwork layout is drawn around it.
• Plumbing rough-ins. A US toilet is set out at a 12 in rough-in (305 mm); the common metric set-out is 300 mm. Basin and shower waste positions have the same issue, and the tolerance is usually absorbed by the pan design rather than by the plumber.
• Fasteners and threads. Metric threads and imperial (UNC/UNF, BSP/NPT) threads are not interchangeable. Plumbing connections are the usual casualty: BSP and NPT are both “half inch” and do not seal against each other.
• Electrical. Voltage, frequency and plug type are a separate matter from dimensions, and are covered in voltage and electrical standards for lighting and appliances.

How Should an Imperial Drawing Be Issued to a Chinese Factory?

Dual dimensioning sounds helpful and causes most of the errors, because it invites the factory to choose. A clear hierarchy works better.

State one controlling unit

Put millimetres as the controlling dimension and imperial in brackets for reference, with a note on the title block that says which governs. Where a drawing is issued in inches only, ask for a metric-converted version back from the factory and check it — the conversion is then documented and the responsibility is visible.

Give tolerances, not just sizes

A dimension without a tolerance is an invitation to round. Typical workable values on project goods: ±1 mm on cabinet carcass and door leaf dimensions, ±2 mm on welded metalwork sections, ±1.5 mm on stone and sintered slab cuts, and ±3 mm on site-fixed frames where packing takes up the difference.

Convert critical interfaces exactly, then round the rest

Not every dimension needs 914.4 mm. The ones that do are the interfaces: anything meeting an existing building element, anything meeting another trade’s product, and anything governed by a certified assembly. Everything internal can be rounded to a sensible metric figure without consequence.

Fix it in the shop drawing, not in the order

The shop drawing is where the conversion becomes real. Approve dimensions against the site survey rather than against the design intent drawing — the process is described in how the shop drawing approval cycle works with Chinese factories, and the documents produced are listed in what installation drawings and assembly documents come with custom FF&E.

What About Standards, Not Just Dimensions?

Unit conversion is the easy half. The harder half is that an imperial market usually brings its own standards regime with it, and those are not converted — they are met or they are not.

• Gauge and grade: sheet metal specified as 16 gauge is nominally 1.5 mm, and 18 gauge nominally 1.2 mm. Order in millimetres and state the minimum, because gauge tables differ between standards.
• Lumber: a US 2×4 is 38 × 89 mm finished, not 50 × 100 mm. Timber packages ordered by nominal size arrive oversized or undersized depending on which convention the factory assumed.
• Fire and safety: ASTM E84 and the EN 13501 classes are different test methods with no conversion between them; a product tested to one does not automatically hold the other. The scope of each is in fire rating standards explained.
• Formaldehyde: CARB Phase 2 and TSCA Title VI apply to US-bound panel products regardless of the units on the drawing — see formaldehyde standards for China cabinetry.
• Accessibility: ADA dimensions are expressed in inches and are legal minimums, not preferences. Converting 34 in to 860 mm rather than 863.6 mm can put a counter out of compliance.

How FBM Sourcing Handles Imperial Specifications

FBM Sourcing manages the entire China procurement package for overseas construction projects. On a hotel, apartment, school, office or retail package we convert the schedule once, in writing, and issue the factory a metric drawing set with the controlling dimensions, tolerances and interface points stated explicitly; the converted set goes back to the design team for approval before tooling, so the conversion is checked by the people who drew the original rather than assumed on the factory floor. Certified assemblies — fire doors, glazing, accessible fittings — are matched to the tested configuration for the destination market, not to the nearest metric module. Our own FBM Sourcing QC team measures against the approved drawing on the factory floor, with a full inspection before shipment. All quotations, invoices and shipping documents are issued by FBM Sourcing — you deal with us, and we carry the responsibility. For products you have selected yourself, we charge a 5–8% commission (8% below USD 50,000, 5% above); for products we source for you, we quote a direct price. Sometimes, on a large project, the joinery alone fill full containers; sometimes the joinery share containers with the other product categories of the same project; and sometimes, in one batch, we combine a dozen or more product categories from several projects of the same client into a few containers — always full containers, and for us this is routine, well-practised work.

If your drawings are in inches and your package is going to a Chinese factory, send us the drawing set, the quantities, the destination port and the standards that apply. Our team will review it and set out a quotation — use the quote button above or WhatsApp +86 135 6007 5057.