GSM to MM Calculator

GSM Measures Weight, Millimeters Measure Thickness

GSM stands for grams per square meter, and it describes weight, not thickness. Cut a sheet to one meter by one meter, put it on a lab balance, and the reading in grams is the grammage. A 500 sheet ream of 8.5 by 11 inch paper at 75 gsm weighs about 5 pounds, because the ream covers 30.2 square meters of paper.

Thickness is a separate property, measured in millimeters or microns. Two sheets can carry the same grammage and differ in caliper by a third, because one was pressed harder at the wet end, or loaded with more calcium carbonate, or calendered at a higher nip pressure. That is why a GSM to mm conversion needs a second number: the density of the sheet, or its bulk.

The definitions come from standards, not from marketing. ISO 536:2019 defines grammage as the mass of a unit area of paper or board, reported in grams per square meter to three significant figures. Caliper is defined by TAPPI T 411 om-21, which presses a single sheet between two platens at 50 kPa, about 7.3 psi. ISO 534:2011 covers thickness, density and specific volume, and it separates single sheet thickness from the bulking thickness measured on a pack of sheets. Pressure is part of the result: a soft sheet squashes under the gauge, so a caliper quoted without a method is half a number.

US shops often quote mils or points instead of millimeters. One mil is one thousandth of an inch, which is 0.0254 mm, and one paper point is the same size. A 12 point cover stock and a 12 mil cover stock are the same sheet, 0.3048 mm thick.

The Two Formulas and How to Swap Them

There are two ways to write the conversion, and they describe the same sheet of paper.

Weight route: thickness in millimeters = GSM / (1000 x density), with density in grams per cubic centimeter.

Bulk route: thickness in microns = GSM x bulk, with bulk in cubic centimeters per gram.

Bulk and density are reciprocals, so bulk = 1 / density. Coated paper with a bulk of 0.75 has an apparent density of 1.33 g/cm3. Run both routes on an 80 gsm sheet: 80 / (1000 x 1.333) = 0.060 mm, and 80 x 0.75 = 60 microns. The answers match because the two inputs describe one physical property.

Go backward when you have a caliper and need a weight. Bulk = 1000 x thickness in mm / GSM. A board that measures 0.27 mm at 300 gsm has a bulk of 1000 x 0.27 / 300 = 0.90, and its density is 1.11 g/cm3.

The 1000 in the first formula is unit work, not a fudge factor. Density in grams per cubic centimeter is a million grams per cubic meter, and one cubic meter spread one millimeter deep covers a thousand square meters. Divide one by the other and the 1000 falls out.

Typical Bulk Ranges by Paper Grade

Bulk groups by grade because furnish and finish drive it. Coated sheets are dense, since mineral pigment and calendering press the fiber together. Newsprint and bulky book papers stay open and light. The table gives working ranges, and the last column shows what each range produces at a fixed 80 gsm.

Treat those ranges as a starting point. Bulk moves with pulp mix, filler load, coating weight, calendering pressure and moisture content. Sheets with recycled fiber usually run bulkier than virgin fiber at the same grammage, and a sheet pulled from the outside of a roll reads thicker than one from the core.

The pattern matches published mill data: uncoated offset near 1.3, gloss coated from 0.7 to 0.85, newsprint near 1.6, high-bulk book grades from 1.5 to 2.0. If a sheet lands outside its grade range, check the test pressure before you decide the paper is unusual.

What Government Paper Specs Show

The US Government Publishing Office publishes Government Paper Specification Standards, and the September 2019 edition (Volume 13) lists basis weight and thickness side by side for every grade it buys. That pairing lets you compute bulk from a real specification instead of a rule of thumb.

Basis weight is the weight of 500 sheets in a fixed base size, not the weight of one square meter. Text papers use 25 by 38 inches, which is 950 square inches, or 0.6129 square meters per sheet. Five hundred sheets cover 306.45 square meters, so 453.59 / 306.45 = 1.48 grams per square meter for every pound of basis weight.

A 60 pound uncoated text sheet is therefore 60 x 1.48 = 88.8 gsm. The GPO lists its caliper at 4.7 mils, or 0.119 mm, and 119 / 88.8 = 1.34. That sits in the middle of the uncoated range, which suggests the typical table above is not fiction.

Two rows reward a closer look. The 70 pound hi-bulk text used for return mailers carries a 7.0 mil minimum, a bulk of 1.72, and the GPO ties that number to a USPS caliper rule for mail. The 26.1 pound newsprint uses a 24 by 36 inch base, which converts to 42.5 gsm, and its 2.7 mil caliper gives a bulk of 1.62. Same arithmetic, different base sheet.

Worked Example: 80 GSM Coated and 300 GSM Board

Start with an 80 gsm gloss coated sheet at a bulk of 0.75. The thickness is 80 x 0.75 = 60 microns, or 0.060 mm. In US units, 60 / 25.4 = 2.36 mils, so the sheet measures 2.36 points. Its apparent density is 1 / 0.75 = 1.33 g/cm3, close to what a filled and calendered coated sheet reads on a lab balance.

Now a 300 gsm SBS board at a bulk of 0.90. The thickness is 300 x 0.90 = 270 microns, or 0.27 mm, and 270 / 25.4 = 10.6 points. Density is 1.11 g/cm3. A 10.6 point board is common retail carton stock: stiff enough to stand on a shelf, thin enough to fold on a score line.

Try the reverse direction. A converter quotes 12 point CCNB and you need a grammage. Twelve points is 12 x 0.0254 = 0.3048 mm, and at a bulk of 0.95 the weight is 304.8 / 0.95 = 321 gsm, the figure most 12 point recycled carton specs carry.

Notice how far bulk moves the answer at that thickness. The same 12 point sheet at a bulk of 1.05 weighs 290 gsm instead of 321, a 31 gsm spread on a single carton. Ask for grammage and caliper together, or you are pricing a guess.

Points, Mils and Basis Weight

Purchase orders mix three unit systems, so keep the bridges in one place. The table converts common grammages at three bulk values.

Basis weight is the odd one out because it depends on sheet size. Book paper uses 25 by 38 inches, bond uses 17 by 22, cover uses 20 by 26, and index uses 25.5 by 30.5. An 80 pound text sheet is 118 gsm, while an 80 pound cover sheet is 216 gsm, even though both carry the same number on the label.

For any base size, GSM = pounds x 1406 / (width in inches x length in inches). For 25 by 38 that is 1406 / 950 = 1.48, the factor used in the GPO example. For 20 by 26 the factor is 2.70, which is why cover weights look so much heavier than text weights with the same number.

How to Measure Caliper at Your Desk

A caliper gauge with a flat foot and a light spring is the right tool. A machine shop vernier with knife edges reads high on paper, because the edges bite into the sheet at the contact line.

For a single sheet, take five readings across the sheet and average them. For thin text papers, stack ten sheets, measure the stack, and divide by ten. That averages out surface roughness and lands within a micron or two of the single sheet value.

A full ream makes a good cross check. At 80 gsm and a bulk of 0.75 each sheet is 0.060 mm, so 500 x 0.060 = 30 mm of stack. Measure the ream with a ruler and you should see close to 30 mm, less the wrap.

Check the units on the gauge before you write anything down. Many digital gauges read thousandths of an inch, so 0.060 mm shows as 2.4. Multiply by 25.4 to get microns, or divide microns by 25.4 to get mils.

Why the Number You Get Is an Estimate

Three tolerances sit behind every conversion. Basis weight usually carries plus or minus 5 percent. Caliper carries its own band, and the GPO allows plus or minus 0.0005 inch on many grades. Bulk varies across the web and from roll to roll.

Stack them and a nominal 0.27 mm board can arrive between 0.26 and 0.28 mm. For a carton that has to fit a 0.30 mm slot, that band is fine. For a reprint that must match an earlier run, ask the mill for the caliper range on the lot rather than the catalog average.

Use the calculator for planning: spine width, mail piece thickness, stack height, shipping weight, or whether a sheet will pass through a printer rated for 0.30 mm. Use the mill data sheet and a gauge when you are accepting a delivery, because a purchase order is written against the sheet in the warehouse, not against a typical value.