Paper Weight Conversion: GSM, Ream
- Lion Paper Team
- Sep 17
- 5 min read
Paper GSM (g/m²) = mass per square meter (ISO 536).
Ream weight (kg) = length(m) × width(m) × GSM × 500 ÷ 1000.
K-coefficient (per mother sheet): K = length(m) × width(m) × 500 ÷ 1000.
889×1194 mm K=0.531; 787×1092 mm K=0.430; A0 K=0.500.
Quick Content Reach:
Why paper weight and Paper GSM matter to buyers
If you purchase notebooks and stationery at scale, paper weight determines print quality, opacity, stiffness, postage, procurement cost, and even retail returns. Paper GSM (grams per square meter) lets you compare stocks across regions, while ream weight (kg/ream) connects directly to logistics and pricing. In B2B stationery, mastering these conversions reduces over-spec, avoids see-through pages, and keeps invoices predictable.
Definitions and standards (clear and evidence-based)
Paper GSM (g/m²). Grammage per ISO 536 is mass per square meter; it’s the industry baseline for comparing “80 gsm copy paper,” “157 gsm art paper,” or “250 gsm board.”
Paper vs board. In commercial stationery, ≤200 gsm is typically called paper; >200 gsm is often board. This threshold is convention, not law—spec to performance, not just a number.
Ream & sheet counts. Unless stated otherwise, a ream is 500 sheets of the mother size. (Some mills differ; always confirm in your PO.)
Basis size (US). North America often quotes lb (basis weight) tied to a category basis size (e.g., Text 25×38 in, Cover 20×26 in). I’ll give a fast conversion later.
Core conversion formulas you can trust
1) Ream weight from Paper GSM (any size)
2) The K-coefficient shortcut (per mother size)
Common K values (evidence: pure geometry + unit math):
889×1194 mm→ 0.889×1.194×500/1000 ≈ 0.531
787×1092 mm → 0.787×1.092×500/1000 ≈ 0.430
A0 (exactly 1 m²) → 1×500/1000 = 0.500
A1 0.250, A2 0.125 … (each step halves area → halves K)
Paper Reams per Metric Ton (500-sheet ream)
What the numbers mean: values below are reams per metric ton for each mother sheet size.Formula: Reams/ton = Constant ÷ GSM.Constants by size: 787×1092 → 2327, 889×1194 → 1884, 880×1230 → 1847.7, 850×1168 → 2014.5.(If you need kg per ream, use: kg/ream = 1000 ÷ (reams/ton).)
Footnote (calculation):Reams/ton = (1000 kg/ton) ÷ (kg/ream), where kg/ream = length(m) × width(m) × GSM × 500 ÷ 1000.Equivalently, using the constants above: Reams/ton = Constant ÷ GSM.
Fast reference: when to use which method
Area formula: any unusual sheet (custom trims, Korean/JIS sizes, board).
K-coefficient: rapid quoting on standard mother sizes.
Per-ton costing: when suppliers quote $/t, convert once to $/kg and keep it fixed across jobs.
Worked examples (step-by-step, fully reconciled)
Example 1 — 157 gsm art paper, 16-up, 5,000 finished sheets
Inputs: K=0.531, GSM=157, ton price=¥7,500, waste=10%
Cost = 0.531×157×7500 ÷ 500 ÷ 16 × 5000 × 1.10 ≈ ¥430 ≈ $61.4
Example 2 — 80 gsm uncoated, 16-up, 8,000 finished sheets
Inputs: K=0.531, GSM=80, ton price=¥6,500, waste=10%
Cost = 0.531×80×6500 ÷ 500 ÷ 16 × 8000 × 1.10 ≈ ¥304 ≈ $43.4
Evidence: both examples are internally consistent by physics units (m, g, kg) and yield identical results via two independent routes.
Paper GSM to US basis weight (Text/Cover)
Because Paper GSM is metric and lb depends on category basis size, we convert with the basis area:
Basis areas (m²):
Text 25×38 in = 0.635×0.965 m ≈ 0.6128 m²
Cover 20×26 in = 0.508×0.660 m ≈ 0.3353 m²
Quick lookups (rounded):
50 lb Text ≈ 74 gsm; 80 lb Text ≈ 118 gsm
65 lb Cover ≈ 176 gsm; 80 lb Cover ≈ 216 gsm
Choosing paper weight by use-case (practical buyer guide)
Notebook inner pages: 70–100 gsm uncoated. For heavy markers, go 100–120 gsm to reduce show-through.
Premium journals: 100–120 gsm wood-free or tinted cream 90–100 gsm for fountain-pen friendliness.
Covers & dividers: 200–350 gsm (board). Lamination or varnish prevents scuff on dark solids.
Calendars/Planners: 100–150 gsm inner; 250–300 gsm cover; wire-O needs crease-resistant stock.
Gift wraps/cards: 80–120 gsm wraps; 250–350 gsm cards.
Evidence: The ranges above are based on opacity (TAPPI T425 style logic), stiffness, and common retail specs we manufacture for US/EU brands.
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How paper weight impacts cost, freight, and retail quality
Opacity & show-through. Higher Paper GSM improves legibility with gel pens and stamps; buyers report fewer returns.
Binding limits. For perfect binding, spine swell grows with GSM and page count—confirm max block thickness early.
Freight math. Courier charges by greater of volumetric or actual weight. Accurate ream kg avoids surprise surcharges.
Waste planning. 3–10% is typical; die-cut windows, tabs, or foil often push above 6%. Track waste by job type and negotiate.
Paper GSM, coatings, and feel (buyer-level nuance)
Two stocks at the same Paper weight can feel different.
Coated vs uncoated. Coated art papers feel denser, smoother, and print sharper but crack at folds—score before folding.
Bulk (caliper). “High-bulk” papers look thicker at the same GSM—good for premium notebooks without raising weight.
Moisture & environment. Humidity alters caliper slightly; re-calibrate cutting and die-punch in the wet season.
If you need a paper weight sanity check or a quote using your exact Paper GSM, send us your spec today.
Email: Leoxia@lion-paper.com | WhatsApp: +86 137 5075 6354
Or visit https://www.lionpaper-istyle.com and leave an inquiry—we’ll reply with the best suggestions to your custom project!
FAQs:
Q1: What is Paper GSM, exactly?
A: Paper GSM is grams per square meter under ISO 536; it standardizes paper weight globally.
Q2: How do I get ream weight from Paper GSM?
A: Ream kg = length(m) × width(m) × GSM × 500 ÷ 1000, or simply K × GSM for a known mother size.
Q3: What are typical K values?
A: 889×1194 mm: 0.531; 787×1092 mm: 0.430; A0: 0.500; A1: 0.250; A2: 0.125.
Q4: Is paper vs board defined only by GSM?
A: Many use 200 gsm as a rule of thumb, but stiffness and end-use matter. Specify performance, not just GSM.
Q5: How do I convert Paper GSM to US lb?
A: Use the basis-area formula; Text and Cover have different basis sizes, so lb values aren’t directly comparable.
Are you looking for a reliable manufacturer? Reach out to Lion Paper for a free quote and consultation. Let’s collaborate on creating custom writing paper products that will set your brand apart from the competition!
About Lion Paper
Company Name: Lion Paper Products
Office Address: 20th floor, Chuangyedasha Building, No. 135, Jinsui Road, Jiaxing City, Zhejiang Province, China
Factory Address: No.135, Xuri Road, Jiaxing City, Zhejiang, China
Email: Leoxia@lion-paper.com
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