Medical

Sock Compression Testing: What OEM Buyers Should Ask

Published: 2026-06-29By ZheSock TeamReading time: 6 min

Buying compression socks by sample feel is a weak way to source a medical product. Two lots can look the same, weigh within 1 to 2 grams per pair, and still miss the pressure target by 3 to 5 mmHg at the ankle. For OEM buyers, sock compression testing must link one pressure claim to one size chart, one yarn spec, one machine setup, and one approved production route. If a supplier cannot tie those items together, the mmHg claim is marketing, not control.

Table of Contents

1. What does sock compression testing actually measure?
2. Which reports and test records should OEM buyers request?
3. How do yarn, gauge, knit structure, and weight change compression?
4. What tolerances, sampling plans, and QC steps are realistic before bulk shipment?
5. How should buyers compare supplier quotes for tested compression socks?
6. What should you ask during factory audits and development calls?

What does sock compression testing actually measure?

Sock compression testing measures interface pressure in mmHg at set points on a leg form, usually the ankle, lower calf, and upper calf. For most medical and travel programs, the target is graduated compression. Pressure should be highest at the ankle, then drop as the sock moves up the leg. Common retail claims are 15 to 20 mmHg and 20 to 30 mmHg. Those numbers only mean something when they match a specific size chart, including ankle and calf ranges for S, M, L, and XL.

Ask the supplier for the exact test points and pass ranges by size. Do not accept a spec that only says 20 to 30 mmHg. A usable spec looks like this: size M, ankle 23 to 27 mmHg, lower calf 16 to 21 mmHg, upper calf 12 to 17 mmHg, tested on finished socks after boarding and packing simulation. If the factory gives one average number for the whole style, that is not enough for a medical line.

Compression results also depend on the production setup. Ask for the cylinder diameter, needle count, and yarn count used on the approved sample. Typical medical styles may run on 168N, 200N, or 240N circular knitting machines. A switch from 200N to 168N during peak season can change fit and pressure, even when the artwork, yarn blend, and pair weight look close.

Which reports and test records should OEM buyers request?

Request one report set for development and one for bulk. The development report should match the final yarn blend, final size chart, and final knitting structure. The bulk report should come from the actual production lot, not from a sample made 60 days earlier. At minimum, ask for sample ID, test date, size tested, pressure points, sample count, pass range, actual readings, and whether the sock was tested before washing or after washing.

For a new compression program, a practical pilot plan is 5 pairs per size. If you have 4 sizes, that means 20 pairs tested from the pilot lot. For bulk production, recheck 3 pairs per size every 2,000 to 3,000 pairs knitted, then pull 5 pairs per size from final packed stock. On a 20,000 pair order across 4 sizes, that usually means 44 to 60 pairs held for pressure checks, wear review, measurement, and reserve samples. That sounds heavy. It is still cheaper than a recall.

Ask these questions in writing:

Was the test done on finished socks after boarding and heat setting?
Was the test repeated after 1 wash and after 5 washes?
How many pairs per size were tested, 3, 5, or 10?
What was the ankle pass band in mmHg for each size?
Does the report list the same yarn blend, SKU, and packaging version shown on the purchase order?

If the supplier uses in-house pressure equipment, ask when it was last calibrated and how often readings are checked against a reference method. In-house testing is useful for process control. It does not replace lot-specific evidence when you are buying a medical compression product.

How do yarn, gauge, knit structure, and weight change compression?

Compression is built during knitting. It is not added later. The biggest variables are elastane percentage, covered yarn specification, nylon denier, cotton percentage, plating tension, welt structure, leg rib design, and heat setting. A common non-medical blend such as 75 percent cotton, 20 percent polyester, 5 percent elastane is not a realistic benchmark for a true 20 to 30 mmHg sock. Medical compression socks usually rely more on nylon and elastane to hold pressure after repeated wear and washing.

Ask for the full sock spec, not just fiber percentages. A usable production line should list details such as 55 percent nylon, 30 percent combed cotton, 15 percent elastane, 200N machine, 4.0 inch cylinder, size M pair weight 58 to 62 grams, foot knit gauge matching the approved sample, and boarding temperature and dwell time. If the supplier cannot give pair weight tolerance, machine needle count, and yarn count, process control is weak.

Small changes matter. A 2 gram drop on a size M pair can point to lower yarn feed or lower elastane use. A switch from a 200N machine to a 168N machine can change stretch recovery and local pressure. A change in heat setting can relax the leg section and cut mmHg, even if the sock still passes flat measurement. Ask the factory to keep knitting tension and machine setup records by shift, machine number, and operator for every bulk lot.

What tolerances, sampling plans, and QC steps are realistic before bulk shipment?

Compression socks need more than a final random check. A practical control plan starts with a pre-production sample, then a pilot run, then inline checks, then final inspection. For a first order, many buyers approve 20 to 40 pilot pairs before bulk starts. That pilot lot should confirm fit, pressure, appearance, pair weight, toe closure, and packaging. After approval, bulk production for custom medical socks often takes 30 to 45 days, depending on yarn booking, machine load, and packaging complexity. Development usually takes 7 to 10 days for color confirmation and 10 to 14 days for sample knitting and revision.

For finished goods inspection, ask the supplier which AQL level they use. A common starting point is AQL 2.5 for major defects and AQL 4.0 for minor defects. For compression socks sold into medical or pharmacy channels, some buyers tighten major defects to AQL 1.5. Define defects in advance. A major defect may include a wrong size label, ankle pressure outside the pass band, pair mismatch, wrong fiber content statement, or a visible knitting fault in the leg. Minor defects may include light shade variation within the approved tolerance or slightly uneven folding.

A realistic inline plan looks like this:

Check 5 pairs per size from the first machine setup before bulk release.
Recheck measurements and appearance every 500 to 1,000 pairs per machine.
Pull 3 to 5 pairs per size every 2,000 to 3,000 pairs for compression testing.
Record machine number, operator, date, and carton range for each checked batch.
Hold failed lots for segregation, retest, and final disposition. Do not mix them back into passing stock.

Also ask what happens if the lot fails. Some factories reboard and retest. Some downgrade to non-medical channels. Some scrap. Get that answer before the purchase order is released.

How should buyers compare supplier quotes for tested compression socks?

Price comparisons mean very little if one quote includes process control and the other does not. Ask every supplier to separate sample charges, test charges, packaging, barcode labeling, carton marks, and final inspection. Also ask whether the quote includes one sample revision or two. Compression sock development often needs at least one revision, especially when the size chart changes after the first fit test.

For custom private label compression socks, a trial program may start at 100 pairs for sample-based development, but bulk MOQs usually start at 3,000 to 5,000 pairs per style for workable pricing. Split that across 4 sizes and 3 colors, and the quantity per variant drops fast. Control gets harder. As a broad market range, simple 15 to 20 mmHg styles can land around USD 1.20 to 1.80 per pair at 5,000 pairs. More technical 20 to 30 mmHg programs with higher elastane use, denser knitting, and retail packaging often run around USD 2.20 to 4.50 per pair. Add more if you require lot-level testing, premium yarn inputs, or complex pack-outs.

When reviewing a quote, compare these items line by line:

Pressure claim tested by each size, or only by one style average.
Machine setup named in the spec, such as 168N, 200N, or 240N.
Number of development rounds included in the price.
Whether wash testing is included after 1 wash or 5 washes.
Packing method by exact SKU count or mixed size ratio.
Current compliance documents, such as OEKO-TEX, BSCI, Sedex, ISO 9001, GOTS, GRS, or CE, only when relevant to the product and materials used.

Cheap quotes usually remove something. Most often it is testing, elastane content, or process discipline.

What should you ask during factory audits and development calls?

Ask who controls compression on the floor. Get a job title and a name, not our QC team. Ownership matters. You also need to know whether the same knitting line runs standard sport socks and medical compression socks on the same day. If it does, ask how long the changeover takes, what setup sheet is used, and who signs off on the first bulk pairs.

During an audit or video call, ask for proof of traceability. A serious supplier should be able to trace one finished carton back to machine number, knitting date, operator, yarn lot, boarding batch, and packing date. Ask to see one actual production record. If records are only verbal, walk away.

Good audit questions are simple:

How many compression machines run this structure today, 10, 20, or 50?
What is the calibration interval for pressure equipment, 30 days, 90 days, or another fixed cycle?
What pair weight tolerance is allowed by size, plus or minus 1 gram or plus or minus 2 grams?
What boarding settings are locked for this style?
How are failed pressure samples isolated from passing stock?

Also verify materials and documents. If the supplier shows OEKO-TEX documentation, ask whether the actual nylon, cotton, or elastane booked for your order falls under that valid scope. If they claim ISO 9001, ask how that system appears in the daily records for your style. Background claims are easy. Lot records are harder. Ask for the harder proof.

Frequently Asked Questions

How many samples should I ask for before approving a compression sock?

For a new style, ask for 3 to 5 pairs per size for wear and fit review, plus 5 pairs per size for sock compression testing if those pairs will not be used for wear trials. With 4 sizes, that usually means 32 to 40 pairs for first approval. If the sock is revised, repeat the same plan on the corrected sample set.

Can a supplier claim 20 to 30 mmHg without third-party testing?

They can print the claim, but you should not rely on it without evidence tied to your own style and size chart. Ask for a recent lot-specific report or a pilot-lot report with actual mmHg readings by size and by test point. A generic report from another program, another yarn blend, or another season is not strong enough for a medical product.

Does washing change compression performance?

Yes. Washing can change pressure, recovery, and leg length. Ask for readings on finished socks, then after at least 1 wash. For higher-risk programs, ask for checks after 5 washes too. If pressure drops too much after normal care, the original mmHg claim will not hold in real use.

What MOQ is realistic for tested custom compression socks?

For development, some factories can start around 100 pairs for a sample-based program or pilot run. For bulk pricing, 3,000 to 5,000 pairs per style is more typical. If you split that across several sizes, colors, and pack formats, the true MOQ per SKU can drop too low for stable medical sock production.

What should be in the tech pack for compression sock sourcing?

Include the target mmHg range by size, pressure points to be measured, ankle and calf circumference chart, leg length, yarn composition, yarn count if available, machine needle count, cylinder size, approved pair weight by size, toe closure method, boarding requirements, wash instructions, packaging details, barcode list, AQL target, and required test timing such as finished goods, after 1 wash, and after 5 washes.

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