Two sheets, same number, different feel

Order 300gsm card from two suppliers and you can end up with two noticeably different products. One feels crisp and rigid, the other softer and thicker. Both are correctly labelled. The number on the spec sheet describes weight, and weight is only one of the properties that decides how a sheet behaves on press, through a folder and in someone's hand.

For anyone specifying print regularly, understanding the difference between weight, thickness and bulk removes a whole class of surprises. It explains why a business card feels flimsy at a weight that sounded generous, why a folded leaflet cracks, and why a booklet ends up thicker than the shelf space allowed for it.

Weight: what gsm actually measures

Grammage, written as gsm, is the mass of one square metre of the sheet. It says nothing directly about how thick that square metre is. A dense, heavily compressed sheet and a light, airy sheet can weigh exactly the same while differing visibly in thickness.

This matters commercially because paper is bought by weight. A bulky sheet gives you more thickness per kilogram, which can make a publication feel substantial without increasing the paper cost. A dense sheet gives you a smooth, tight surface that reproduces fine detail well, but you pay in weight for every millimetre of thickness.

Postal costs are calculated on weight too. A mailing that feels impressively solid can quietly push a piece into the next postage band, and on a large mailing that difference outweighs any saving on stock.

Thickness: caliper and microns

Caliper is the measured thickness of a single sheet, expressed in microns or in thousandths of an inch. It is measured with a micrometer under a defined pressure, because paper compresses.

Caliper is the number that predicts physical behaviour. It tells you whether a card will stand up in a display, whether a cover will curl, how thick a bound book will be and whether a sheet will feed reliably through a particular machine. When a specification says a card must feel substantial, caliper is the honest way to write that requirement down.

Bulk ties the two together

Bulk is the relationship between the two: thickness for a given weight. A high-bulk sheet is thick for its weight, a low-bulk sheet is thin and dense. Publishers use high-bulk papers to give a book a satisfying spine without a punishing shipping weight, and low-bulk papers where page count is high and thickness needs controlling.

What changes bulk

• Fibre and furnish. Different pulps and fillers produce different densities from the start.
• Calendering. Passing the web between polished rollers compresses and smooths it. More calendering means a smoother, denser, thinner sheet at the same weight.
• Coating. A coated sheet carries a mineral layer on the surface. That layer adds weight and smoothness while adding little thickness, so coated stocks are generally lower bulk than uncoated at the same gsm.
• Construction. Boards built from layers behave differently from single-ply sheets, and duplex constructions can be engineered for stiffness rather than raw thickness.

The practical consequence is a rule worth remembering: at the same gsm, uncoated usually feels thicker and coated usually looks sharper. Which one is right depends on whether the piece has to impress by feel or by image quality.

Stiffness is a third property again

Weight and thickness both influence stiffness, but neither predicts it exactly. Stiffness is the resistance to bending, and it depends on thickness disproportionately, meaning a small increase in caliper produces a large increase in rigidity. It also depends on direction, because paper is stiffer across the grain than along it.

This is why a card can feel floppy despite a respectable weight, and why the same board cut two ways behaves differently. Grain direction is a whole subject on its own and is covered in paper grain direction.

Reading a specification properly

Opacity deserves a mention because it causes complaints that get blamed on printing. A lightweight sheet printed on both sides will show the reverse image through, and no adjustment on press fixes it. If show-through is unacceptable, the answer is a heavier or more opaque stock, decided before the job is quoted.

Where the difference shows up in practice

Business cards

Here caliper is what people actually judge. A card at a given weight in a bulky uncoated stock feels more substantial than the same weight in a dense coated one. If a client wants a card that feels premium, specify thickness rather than weight, or look at layered constructions where two or three sheets are bonded.

Folded leaflets

Thicker stock cracks along the fold unless it is scored first, and how badly depends on caliper, coating and grain. The mechanics are set out in scoring, creasing and perforation.

Booklets

Bulk determines the finished thickness, which in turn affects binding choice and, for saddle-stitched work, how much the inner pages creep outwards. A high-bulk text stock in a thick booklet can push it past what stitching handles comfortably. Binding options are compared in binding methods compared.

Large runs

On long runs the weight difference between two similar stocks becomes a real number on the delivery note and on the pallet count. It is worth checking before committing, alongside the other planning covered in planning a large print run.

Moisture, storage and why paper changes over time

Paper is hygroscopic, meaning it takes up and gives off moisture with the air around it. That single property explains a long list of production problems that get blamed on other things.

A pallet stored in a cold van and brought straight into a warm pressroom picks up moisture at the edges before the middle. The sheet expands unevenly and the edges ripple, which causes misregistration on a multi-colour job. The same effect in reverse produces tight edges and a wavy centre. Printers acclimatise stock in the pressroom for a reason, and rushing that step costs more time than it saves.

Finished work behaves the same way. A laminated sheet has a moisture barrier on the printed side and none on the reverse, so it curls as the two sides reach different moisture levels. Laminating both sides usually solves it. An uncoated leaflet left in a damp storeroom goes limp within weeks and never fully recovers.

The practical guidance is unglamorous and effective. Keep stock and finished work wrapped until it is needed, store it flat and off the floor, and avoid storerooms that swing between hot and cold. If a job is going to sit for months before distribution, mention that when specifying, because it can change the sensible choice of stock and finish.

How to specify without ambiguity

1. State the property that matters. If feel is the point, give a caliper. If postage is the constraint, give a maximum weight.
2. Name the stock, not just the number. A named paper carries a full set of properties. A bare gsm figure carries one.
3. Ask for a dummy. An unprinted mock-up in the actual stock, at the actual page count, answers questions no specification can.
4. Handle a sample before deciding. Two minutes with the real sheet beats any amount of comparison on a datasheet.
5. Say what it has to do. Stand upright, survive a mailing, lie flat, take a lamination. A printer can pick a suitable stock from a clear brief far better than from a number.
6. Check availability early. A specialist stock with a long lead time is a scheduling problem discovered too late.

Lamination changes the equation as well, adding both thickness and stiffness while altering the surface. That interaction is described in lamination compared.

How we handle stock decisions

When a job comes to us with a weight and nothing else, we ask what the piece has to do. A 350gsm card for a display stand and a 350gsm card for a mailing are different problems, and the right stock for one is the wrong stock for the other. Where thickness or stiffness is the real requirement, we will say so and quote against that rather than against a number that may not deliver what you expect.

For anything unusual we can supply a dummy in the proposed stock before the run, which is the only reliable way to judge a finished thickness. You can see the range under general printing services and across printing services. What we check on delivery is set out in quality control on a print run. To talk through a specification, use our contact page, and there is more about us on the about page.

The short version

Grammage measures weight per square metre and drives paper cost and postage. Caliper measures thickness and drives feel, spine width and machine handling. Bulk is thickness per unit weight, and it is why two sheets at the same gsm can differ visibly. Stiffness follows thickness disproportionately and depends on grain direction. Specify the property that actually matters for the job, name the stock rather than only the number, and ask for a dummy whenever finished thickness is part of the brief.