GSM to Ounces: A Conversion Sheet for Pad Buyers

“Your 300 gsm is our 9 ounce, and our 9 ounce is not your 300 gsm.” A purchasing manager sent that after receiving two samples that felt different despite matching numbers on paper. She was right on both counts, and the reason is that grammage, thickness and absorbency are three different measurements that the industry quotes interchangeably.

Most pad enquiries cross at least one unit boundary, and a surprising number of specification arguments are really conversion errors. Getting the arithmetic straight takes ten minutes and prevents a container of the wrong product.

The basic conversions

The conversion is straightforward: one ounce per square yard equals 33.9 grams per square metre. The trouble starts because not everyone is quoting the same thing. Some suppliers quote the total weight of a multilayer construction, some quote only the absorbent core, and some quote a nominal figure that rounds to the nearest common value. A “300 gsm” pad from one factory can be 280 gsm of core plus 40 gsm of film, while another means 300 gsm total including film, which leaves a noticeably thinner core.

Grammage is not thickness, and neither is capacity

Two pads at 300 gsm can differ by a millimetre in loft depending on how the fibre is laid and whether it is needled, air-laid or thermobonded. Loft affects how the pad sits in a tray and how it behaves under load, and it is invisible in a grammage figure. Ask for thickness in millimetres at a stated pressure, usually 0.5 kPa, because thickness measured with no load tells you very little.

Capacity is a third thing entirely, driven by the SAP loading rather than the total weight. A 250 gsm pad with a high SAP ratio can hold more fluid than a 400 gsm pad with mostly pulp. This is why buying on grammage alone is unreliable, and why a capacity figure in millilitres per square decimetre, at a stated temperature and fluid, is a better primary specification.

Where the errors actually bite

The classic failure is a buyer switching from a European to an Asian supplier and specifying “300 gsm” without stating whether that is core or total. The delivered pad is ten percent lighter in the core, absorbs less, and passes a visual inspection. The problem surfaces three weeks later as saturated trays.

The second is tray fit. Pads are often quoted in inches by US suppliers and millimetres elsewhere, and a 10 by 14 inch pad is 254 by 356 millimetres, not 250 by 350. On a tray with a tight base, a four millimetre difference at each edge is enough to create the wicking path that stains cartons.

The third is roll width. If you cut pads in-house, the roll is specified in millimetres or inches and the yield calculation depends on the trim. A 500 millimetre roll yields fewer 254 millimetre pads than expected once blade clearance is counted.

You can check a delivered pad in about two minutes with a kitchen scale and a straight edge. Cut a hundred by hundred millimetre square, weigh it in grams, and multiply by one hundred to get grammage. Then measure thickness with a caliper under a light load, and compare both against the drawing. If the weight is right and the thickness is low, the pad has been calendered harder than the sample you approved, and it will sit flatter and feel denser in the tray. If the thickness is right and the weight is low, the core is light.

Put four numbers on the purchase order and the ambiguity disappears: grammage basis (core or total), thickness at 0.5 kPa, capacity in millilitres per square decimetre at 4 degrees, and dimensions in millimetres with a tolerance. The pad specification data is set out on that basis, which makes side-by-side comparison possible without conversion guesswork.

Convert once, write it down, and put the basis on the drawing. It saves an argument that no one ever wins.