Copy Paper Explained: GSM, Brightness and Opacity

GSM — what the weight number means

GSM is grams per square metre, measured under ISO 536. It is the mass of one square metre of the paper, and it is the closest thing to a single quality indicator that copy paper has — heavier paper is generally stiffer, more opaque and feels more substantial.

One caution about reading spec sheets: GSM is weight, not thickness. Those are different standards — thickness is measured under ISO 534 and quoted in microns. Two 80gsm papers can differ in thickness depending on how bulky the fibre is, and a 90gsm sheet from one brand can genuinely be thinner than an 80gsm sheet from another. If stiffness is what you're after, check the micron figure, not just the gsm.

Opacity — the number that actually matters for double-sided

Opacity is the percentage of light that doesn't pass through the sheet, measured under ISO 2471. At 95% opacity, 95% of the light is blocked. It is the spec that decides whether printing on the back shows through to the front, which makes it the one to check if your office prints duplex to save paper.

Opacity climbs with weight, and the published figures let you see exactly what the upgrade buys. Real numbers from manufacturer spec sheets:

So going from 80 to 100gsm buys you roughly two percentage points of opacity. Whether that is worth the price difference depends entirely on what you are printing — for internal duplex reports, 80gsm is fine; for a double-sided document going to a client, the 100gsm is noticeable in the hand as well as through the page.

Whiteness and brightness — two different things, neither what you think

These get used interchangeably in retail and they are genuinely different measurements on different standards, measuring different things. They cannot be converted into each other.

• ISO brightness (ISO 2470) measures reflectance of blue light only, at a single narrow wavelength around 457nm. It is a narrow technical measure, not a measure of how white the paper looks.
• CIE whiteness (ISO 11475) measures reflectance across the whole visible spectrum under standardised daylight. This is the one that corresponds to what your eye actually sees.

Australian office papers typically publish a CIE whiteness figure between about 150 and 170. If you're wondering how a percentage-looking number exceeds 100 — it isn't a percentage. Optical brightening agents absorb invisible UV light and re-emit it as visible blue, so measured whiteness can exceed the theoretical ceiling of a perfectly white non-fluorescent sheet.

What this means when you're buying

Two practical points. First, the human eye can't reliably distinguish brightness differences of a couple of points, so the gap between a 165 and a 170 is marketing rather than something you'll see on the desk. Second, high whiteness comes from those optical brighteners, which do nothing for the paper's actual performance and are excluded from some environmental certifications — so chasing the highest number can work directly against your sustainability position.

One more trap: US-market paper is sold on a different brightness scale entirely (the “92 / 96 / 100 brightness” numbers on American reams). Do not compare that figure with an ISO or CIE number — they are not the same scale.

What your printer will actually take

HP's own LaserJet media guidance gives a recommended range of 60–160 gsm, and warns that paper outside a printer's specified weight range can cause “misfeeds, misstacking, jams, poor print quality, or excessive mechanical wear”.

In practice:

• 70–100 gsm — runs in essentially any office laser or inkjet main tray.
• 120 gsm — within range for most machines, but worth checking your model.
• Above 160 gsm — use the bypass or multipurpose tray, which feeds the sheet on a straighter path, and check the model specifically.

The other half of this is the paper's own moisture. Copy paper is hygroscopic — a ream left open in a humid storeroom will curl and jam regardless of its weight. Keep reams wrapped until use and store them flat, not on edge.

Certification — FSC, PEFC and what tenders actually ask for

Two schemes dominate. FSC is a single global standard-setting body issuing its own certification. PEFC works differently — it endorses national standards rather than writing one global one; in Australia its member is Responsible Wood, whose forest standard is AS/NZS 4708.

The FSC labels mean specific things:

Being straight about what's required

You will read that FSC certification is “required” for Australian government tenders. That is an overstatement, and worth understanding properly if you're preparing a submission.

• The Commonwealth's Environmentally Sustainable Procurement Policy currently covers construction, ICT, textiles, furniture and equipment above set thresholds — not paper or office supplies.
• State guidance often specifies it. Victoria's paper procurement guide, for instance, advises that all paper sourced be FSC or PEFC certified and prefers high recycled content — but these are recommendations, not mandates.
• The genuinely binding law is the Illegal Logging Prohibition Act, which does cover paper and pulp. Certification doesn't exempt you, but it does route a consignment onto a simpler due-diligence pathway.

The honest summary: certification is frequently written into tender evaluation criteria and materially simplifies compliance, but it is not a universal legal requirement for buying copy paper. If a tender asks, the answer matters; if nobody's asking, buy on fitness for purpose.

Five things worth knowing

• A ream is 500 sheets. Standardised under ISO 4046 — anything else should be called a pack.
• Buy by the carton, not the ream. Copy paper does not perish and the per-ream price drops materially at carton and pallet quantities.
• Standardise on one weight across the office. Mixed weights in adjacent trays is how the wrong paper ends up in the client proposal.
• Lowest workable weight is the greener choice. Victorian government guidance explicitly directs buyers to use the lowest weight that achieves a fit-for-purpose result.
• Store flat and wrapped. Curl is a storage problem, not a paper-quality problem.