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Kraft paper GSM (grams per square metre) measures the basis weight of paper — heavier GSM means thicker, stronger paper. Common kraft paper ranges from 30 GSM (lightweight tissue and food wrap) through 80 GSM (standard mailing wrap) to 300+ GSM (heavy industrial liner and structural board). The right GSM depends on application: food contact uses 40–80 GSM, mailing and gift wrap 80–120 GSM, void-fill 70–100 GSM, corrugating medium 100–180 GSM, and outer liner 150–300 GSM.
Kraft Paper GSM Reference Chart
| GSM Range | Equivalent (lb/ream) | Typical Application | Strength Indicator | Common Use |
|---|---|---|---|---|
| 30–50 GSM | 20–34 lb | Tissue, food interleaf, light wrap | Mullen 8–18 psi | Bakery interleaf, tea bags, lightweight wrap |
| 50–70 GSM | 34–47 lb | Bag paper, light gift wrap | Mullen 18–28 psi | Sandwich wrap, light retail bags, butcher paper |
| 70–90 GSM | 47–61 lb | Mailing wrap, void fill, light bags | Mullen 28–40 psi | E-commerce packing, kraft mailers, light shopping bags |
| 90–120 GSM | 61–82 lb | Premium gift wrap, heavy bags | Mullen 40–55 psi | Retail kraft bags, premium void fill, gift wrap |
| 120–150 GSM | 82–102 lb | Light corrugating medium, heavy wrap | Mullen 55–75 psi | Inner fluting, heavy duty wrap, light cartons |
| 150–200 GSM | 102–137 lb | Standard corrugating, light liner | Mullen 75–110 psi, ECT 26–32 | Corrugating medium, inner liner, light boxes |
| 200–250 GSM | 137–171 lb | Heavy testliner, outer liner | Mullen 110–150 psi, ECT 32–40 | B-flute liner, retail boxes, light cartons |
| 250–300 GSM | 171–205 lb | Heavy outer liner, kraftliner | Mullen 150–200+ psi, ECT 40–55 | Heavy duty boxes, single-wall liner, structural |
What GSM Actually Measures
GSM (grams per square metre) is exactly what it sounds like: cut a perfect 1m × 1m square of paper, weigh it, and that weight in grams is the GSM. An 80 GSM paper means 80 grams per square metre, which translates to roughly 0.10–0.12 mm thickness for typical kraft. The same paper at 200 GSM runs 0.25–0.30 mm thick.
Two papers at the same GSM can have different thickness depending on bulk — measured as caliper divided by basis weight. A bulky uncalendered kraft at 100 GSM runs around 0.15 mm; the same 100 GSM after calendering (passing through pressing rollers) compresses to 0.11 mm. For applications where stiffness matters more than mass, bulk matters as much as GSM.
North American spec sheets often list basis weight in pounds per ream — but the ream size varies by paper grade. A ’40 lb’ kraft (40 pounds for 500 sheets at 24 × 36 inches) equals roughly 65 GSM. The conversion isn’t intuitive, which is why the global trend is toward GSM as the primary spec.
How GSM Maps to Strength
Higher GSM generally means higher strength, but the relationship isn’t perfectly linear and depends on fibre type and processing. Three strength tests dominate kraft paper specifications:
- Mullen Burst (TAPPI T 403): pressure that bursts a clamped sample. 80 GSM kraft typically tests 28–40 psi; 200 GSM tests 110–150 psi. Reported in psi in North America, kPa in Europe (1 psi = 6.895 kPa).
- Ring Crush (TAPPI T 822): compressive strength of a paper ring, predicting how the paper performs as corrugating medium or liner. Reported in lb/in or N/m.
- Edge Crush Test / ECT (TAPPI T 811): compressive strength of corrugated combined board, derived from liner and medium specifications. Reported in lb/in.
For pure kraft paper specs (uncombined, not corrugated), Mullen and ring crush are the primary measures. ECT applies to finished corrugated board where multiple papers are laminated to fluting.
Choosing GSM by Application
Food-Contact Wraps (40–80 GSM)
Sandwich wrap, bakery interleaf, butcher paper, and grease-resistant food sheets typically run 40–80 GSM. The priorities are food-contact safety (FDA 21 CFR 176.170 in the US, EU 1935/2004 in Europe), grease resistance (often achieved via PE coating or proprietary grease-resistant coatings), and printability for branding. Strength matters less because the wrap holds light items briefly.
Retail Bags and Mailing Wrap (70–120 GSM)
Standard kraft shopping bags run 80–110 GSM; heavy-duty retail bags reach 120 GSM. Mailing and shipping wrap typically lands in 70–100 GSM territory — strong enough to protect contents, light enough to keep shipping cost down. E-commerce applications increasingly favour 80 GSM with internal void-fill paper rather than heavier outer wrap. Mullen burst above 35 psi is a reasonable threshold for shipping reliability.
Void Fill and Protective Wrap (60–100 GSM)
Void-fill paper for e-commerce shipping uses 60–80 GSM honeycomb-perforated kraft that expands when crumpled. Protective wrap for furniture and large items runs heavier at 80–100 GSM. Lower GSM gives more volume per unit weight (better packing economics), but too low and the paper tears under cushioning load.
Corrugating Medium (100–180 GSM)
The fluted middle layer of corrugated board typically runs 110–150 GSM for standard B-flute and C-flute applications, up to 180 GSM for heavy double-wall containers. Recycled-content medium dominates this segment because the fluted layer doesn’t need cosmetic appearance. Ring crush and CMT (concora medium test) are the relevant strength specs.
Outer Liner (150–300 GSM)
The outer faces of corrugated boxes typically run 150–250 GSM for standard packaging, 250–300 GSM for heavy-duty industrial containers and structural shipping boxes. White-top kraft liner (kraft on the back, white on the print face) is common at 150–200 GSM for retail-display packaging. Outer liner Mullen burst typically exceeds 100 psi and ECT contribution drives the box compression strength rating.
Virgin vs Recycled Fibre at the Same GSM
Two papers at identical GSM can perform very differently depending on fibre type. Virgin kraft (made from fresh softwood pulp) has longer fibres — typically 2.5–3.5 mm — that interlock more strongly. Recycled kraft has shorter fibres averaging 1.0–1.8 mm because each recycling cycle shortens fibres.
At 200 GSM:
- Virgin kraft Mullen burst: 130–155 psi typical.
- Recycled kraft Mullen burst: 95–120 psi typical.
- Tensile strength MD (machine direction): virgin 5,500–7,000 N/m vs recycled 3,800–5,200 N/m.
That’s roughly a 25–35% strength penalty for recycled fibre at equivalent GSM. The trade-off is cost (recycled is 20–30% cheaper) and sustainability profile. For applications where strength is the binding constraint (heavy industrial bags, structural box outer liner), engineers often spec virgin. For corrugating medium and inner liner where strength is a function of the overall combined board, recycled wins on cost.
How GSM Affects Cost
Kraft paper pricing scales roughly linearly with GSM at constant fibre type and quality grade. Indicative wholesale pricing in mid-2025 (FOB mill, 5+ tonne lots):
- 60 GSM virgin kraft: $980–$1,180 / tonne.
- 100 GSM virgin kraft: $920–$1,100 / tonne (slightly cheaper per tonne because higher-volume grades have better mill economics).
- 150 GSM kraft testliner: $720–$880 / tonne for recycled, $920–$1,080 / tonne for virgin.
- 200 GSM heavy testliner: $700–$850 / tonne for recycled, $880–$1,050 / tonne for virgin.
- 300 GSM kraftliner outer: $920–$1,100 / tonne for virgin.
Per square metre cost is the GSM × per-tonne cost ÷ 1,000,000. So 100 GSM virgin at $1,000/tonne costs $0.10/m². A typical retail kraft bag at 110 GSM with 0.15 m² of paper has roughly $0.016 of paper material content.
Frequently Asked Questions
Is higher GSM always better?
No. Heavier paper costs more, ships heavier (driving freight cost), and stiffens — which can be a problem for applications like void fill where flexibility is desirable. Match GSM to the strength requirement; over-specifying drives cost without value.
How do I convert GSM to pounds?
It depends on the ream size. For kraft paper (24 × 36 inch ream basis): GSM × 0.00139 ≈ pounds-per-500-sheet ream. So 80 GSM ≈ 49 lb basis weight, 200 GSM ≈ 137 lb. The conversion isn’t universal across paper grades — bond paper uses a different ream size — which is why GSM has become the international standard.
What GSM should I use for an Amazon-style mailer?
Standard kraft mailers run 80–100 GSM with a tear-strip closure. Heavy items benefit from 110–120 GSM. Lighter than 80 GSM tears too easily during automated sortation.
Can I get the same strength at lower GSM with virgin fibre?
Often yes. A 90 GSM virgin kraft can match the strength of 110 GSM recycled kraft on Mullen burst. The cost trade-off depends on fibre price spread — when virgin pulp is expensive, recycled at higher GSM may still come out cheaper despite the weight.
Does GSM tell me about printability?
Indirectly. GSM correlates with stiffness, which affects how the paper feeds through printers and converters. But surface treatment (uncoated, MG/machine glazed, calendered) and brightness drive printability more than GSM. Specify both: ‘120 GSM MG natural kraft, brightness ISO 30%’ is more useful than just ‘120 GSM kraft’.
What GSM do you supply at Kangchuang?
Kangchuang produces industrial kraft from 40 GSM through 300 GSM, in virgin and recycled fibre lines, FSC chain-of-custody certified. Standard widths from 200 mm to 2,400 mm, in roll or sheet format. Typical lead time is 3–4 weeks door-to-door for international destinations including 5–10 days mill production and ocean transit.
Conclusion
Kraft paper GSM is the primary specification that drives strength, cost, and converting performance. Light grades (40–80 GSM) handle food contact and light wrap; mid-range (90–150 GSM) covers retail bags and mailing; heavy grades (150–300 GSM) carry corrugating medium and outer liner duty. Get the GSM wrong and you either fail in service or pay too much for over-specified paper.
Need help specifying the right GSM for your packaging application? Contact Kangchuang Papers for a sample pack covering 40–300 GSM industrial kraft grades, FSC certified, with lab test data on every grade.
