Paper Basis Weight Calculator
Convert between GSM (metric) and US basis weight (lbs/ream) for any paper grade. Estimates caliper thickness and M-weight.
About this calculator
US basis weight is a confusing unit because it's not a weight-per-area measure directly — it's the weight of one ream (traditionally 500 sheets) cut to a grade-specific "basic size" that has nothing to do with the sheet size you actually buy. This calculator handles that by looking up the correct basic size in square inches for whichever grade you select — 17×22 for bond/writing/ledger, 25×38 for book/text/offset, 20×26 for cover, 22.5×28.5 for bristol, 25.5×30.5 for index, or 24×36 for tag — and converting your basis weight in pounds into GSM (grams per square meter) using the exact inches-per-square-meter constant (1550.0031) alongside the standard 453.592 g/lb conversion. Because the basic size differs by grade, the same GSM value corresponds to a different lb figure depending on which grade you're working with — a 20# bond and a 50# text sheet can be nearly the same physical thickness.
The calculator also derives ream weight and M-weight (the weight of 1,000 sheets), and estimates caliper (sheet thickness) in mils and millimeters from GSM and a bulk factor you supply — bulk factor is a loose multiplier around 1.0 for standard density paper, higher for bulkier/loftier stocks, so treat the caliper output as an approximation rather than a lab-measured caliper. The most common error is picking the wrong grade for a basis weight figure pulled off a spec sheet, since the same number means very different things across grades.
Inputs
Results
GSM (g/m²)
88.81
How to Use This Calculator
- Select the paper grade (Bond, Book/Text, Cover, Bristol, Index, or Tag).
- Enter the US basis weight (lbs) for your paper as listed on the product spec.
- Set sheets per ream (typically 500) and the bulk factor if specified.
- Review GSM conversion, Caliper (mils and mm), Ream Weight, and M-weight.
- Use the GSM value when ordering from international suppliers or comparing global grades.
How the result changes with Sheets per ream
| Sheets per ream | GSM (g/m²) |
|---|---|
| 250 | 177.62 |
| 375 | 118.41 |
| 750 | 59.21 |
| 1,250 | 35.52 |
What each input means
- Paper grade
- Paper grade determines the basic size used for GSM conversion.
- Basis weight (lbs)
- US basis weight in pounds per ream at the grade's basic size.
- Sheets per ream
- Number of sheets per ream (standard is 500).
- Bulk factor
- Paper bulk multiplier (1.0 = standard; higher = thicker/loftier paper).
What each result means
- GSM (g/m²)
- Equivalent metric weight in grams per square meter.
- Basis weight (lbs)
- US basis weight in pounds per ream.
- Ream weight (lbs)
- Total weight of one ream at the basic size.
- Caliper (mils)
- Estimated sheet thickness in thousandths of an inch.
- Caliper (mm)
- Estimated sheet thickness in millimeters.
- M-weight (lbs)
- Weight of 1,000 sheets at the basic size.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersPaper grade = 1, Basis weight (lbs) = 60, Sheets per ream = 500, Bulk factor = 1 = 4 input(s) provided
- Calculate GSMGSM = (basisWeightLbs * 453.592 * SQ_IN_PER_SQ_M) / (basicSizeSqIn * sheetsPerReam)88.81 = 88.81
- Calculate Basis weightBasis weight60 = 60
- Calculate Ream weightReam weight = reamWeightGrams / 453.59260 = 60
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the same GSM value correspond to different basis weight numbers depending on grade?
Basis weight is the weight of one ream cut to a grade-specific basic size — 374 sq in for bond, 950 sq in for book/text, and different values for the other grades — not weight per unit area. GSM, by contrast, is always weight per square meter regardless of grade. Because the reference sheet size varies by grade, converting the same physical GSM into basis-weight pounds produces a different number for each grade, which is why the calculator requires you to select a grade before converting.
What is the bulk factor and how does it affect the caliper estimate?
Bulk factor is a multiplier you supply, defaulting to 1.0, representing how loosely or densely the paper's fibers are packed compared to a standard sheet; the calculator multiplies GSM by bulk factor and divides by 100 to estimate caliper in mils. A higher bulk factor for bulkier, loftier stock produces a thicker sheet estimate at the same GSM, but since it's a supplied approximation rather than a measured value, treat the caliper output as indicative rather than a lab-measured caliper.
What's the difference between ream weight and M-weight in the results?
Ream weight is the total weight of one ream, using whatever sheet count you set (default 500), at the grade's basic size — computed from GSM times the basic size area times sheet count. M-weight is a normalized figure: the weight of 1,000 sheets at that same basic size, which lets you compare weights across jobs printed with different sheet counts per ream.
Can I change sheets per ream from the standard 500?
Yes — the sheets-per-ream input defaults to 500, the traditional US ream, but accepts any value from 1 to 5,000. Changing it affects the GSM conversion, since sheets per ream is a divisor in that formula, as well as ream weight and M-weight, so enter the actual sheet count from your spec sheet if it differs from 500.
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